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### References and key literature
- Chernozhukov, Victor, Hiroyuki Kasahara, and Paul Schrimpf (2021). “Causal impact of masks, policies, behavior on early Covid-19 pandemic in the US”. In: Journal of Econometrics 220.1, pp. 23–62.
- Dieterle, Steven, Otávio Bartalotti, and Quentin Brummet (2020). “Revisiting the Effects of Unemployment Insurance Extensions on Unemployment: A Measurement-Error-Corrected Regression Discontinuity Approach”. In: American Economic Journal: Economic Policy 12.2, pp. 84–114.
- Eichenbaum, Martin S, Sergio Rebelo, and Mathias Trabandt (Apr. 2021). “The Macroeconomics of Epidemics”. In: The Review of Financial Studies.
- Gollwitzer, Anton, et al. (2020). “Partisan differences in physical distancing are linked to health outcomes during the COVID-19 pandemic”. In: Nature human behaviour 4.11, pp. 1186–1197.
- Greenhalgh, Trisha, et al. (2020). “Face masks for the public during the covid-19 crisis”. In: Bmj 369.
- Hallas, Laura, et al. (Dec. 2020). Variation in US states’ responses to COVID-19. BSG Working Papers 2020/034. University of Oxford.
- Judson, Ruth A and Ann L Owen (1999). “Estimating dynamic panel data models: a guide for macroeconomists”. In: Economics letters 65.1, pp. 9–15.
- Karaivanov, Alexander, et al. (2020). Face masks, public policies and slowing the spread of COVID-19: evidence from Canada. Tech. rep. National Bureau of Economic Research.
- Lee, David S. and Thomas Lemieux (June 2010). “Regression Discontinuity Designs in Economics”. In: Journal of Economic Literature 48.2, pp. 281–355.
- Leech, Gavin, et al. (2021). “Mass mask-wearing notably reduces COVID-19 transmission”. In: medRxiv.
- Lyu, Wei and George L Wehby (2020). “Community Use Of Face Masks And COVID-19: Evidence From A Natural Experiment Of State Mandates In The US”. In: Health affairs 39.8, pp. 1419–1425.
- Milosh, Maria, et al. (2020). “Unmasking Partisanship: How Polarization Influences Public Responses to Collective Risk”. University of Chicago, Becker Friedman Institute for Economics Working Paper 2020-102.
- Mitze, Timo, et al. (2020). “Face masks considerably reduce COVID-19 cases in Germany”. In: Proceedings of the National Academy of Sciences 117.51, pp. 32293–32301.
- Pinkovskiy, Maxim L (2017). “Growth discontinuities at borders”. In: Journal of Economic Growth 22.2, pp. 145–192.
- Renne, Jean-Paul, Guillaume Roussellet, and Gustavo Schwenkler (2020). “Preventing COVID-19 Fatalities: State versus Federal Policies”. In: arXiv preprint arXiv:2010.15263.
- Simonov, Andrey, et al. (2020). The persuasive effect of fox news: non-compliance with social distancing during the covid-19 pandemic. Tech. rep. National Bureau of Economic Research.
- Welsch, David M (2020). “Do masks reduce COVID-19 deaths? A county-level analysis using IV”. In: Covid Economics (57).
- Wright, Austin L, et al. (2020). “Tracking mask mandates during the COVID-19 pandemic”. University of Chicago, Becker Friedman Institute for Economics Working Paper 2020-104.

### State-level mask mandate definitions and selected entries
- Definition: A state is classified as having a mask mandate if it at least is a requirement to wear a mask indoor if physical distance can not be maintained.
- Selected state entries (start dates and key order highlights):
  - Alabama
    - Start date: July 16, 2020. End date: Current, as of December 19, 2020.
    - Requirement: wear a mask or other facial covering when within six feet of a person from another household in specified public settings. State order.
  - Alaska
    - No Mask Mandate as of December 19, 2020. Recommendation: State strongly encourages mask wearing.
  - Arkansas
    - Start date: June 19, 2020. End date: Current, as of December 19, 2020. State order.
  - California
    - Start date: June 18, 2020. End date: Current, as of December 19, 2020. State order.
  - Colorado
    - Start date: July 16, 2020. End date: Current, as of December 19, 2020. State order; Executive Order amended January 6.
  - Connecticut
    - Start date: April 20, 2020. End date: Current, as of December 19, 2020. State order effective 8:00 p.m. on April 20, 2020.
  - Delaware
    - Start date: April 25, 2020. End date: Current, as of December 19, 2020. State order.
  - District of Columbia
    - Start date: July 22, 2020. End date: Current, as of December 19, 2020. Mayor’s Order 2020-080 July 22, 2020.
  - Florida
    - No Mask Mandate as of December 19, 2020.
  - Georgia
    - No Mask Mandate as of December 19, 2020.
  - Idaho
    - No Mask Mandate as of December 19, 2020.
  - Illinois
    - Start date: May 1, 2020. End date: Current, as of December 19, 2020. State order.
  - Indiana
    - Start date: July 27, 2020. End date: Current, as of December 19, 2020. State order effective 12:01 a.m. on July 27, 2020 for thirty (30) days unless changed.
  - Iowa
    - Start date: November 17, 2020. End date: Current, as of December 19, 2020. State order through 11:59 pm on February 6, 2021. [November 17, 2020]
  - Kansas
    - Start date: November 25, 2020. End date: Current, as of December 19, 2020. State order effective 12:01 a.m. on November 25, 2020.
  - Kentucky
    - Start date: July 9, 2020. End date: Current, as of December 19, 2020.
  - Louisiana
    - Start date: July 11, 2020. End date: Current, as of December 19, 2020. State order [July 11, 2021].
  - Maine
    - Start date: April 29, 2020. End date: Current, as of December 19, 2020. State order [April 29, 2021].
  - Maryland
    - Start date: April 18, 2020. End date: Current, as of December 19, 2020. State order effective 7:00 a.m. on April 18, 2020.
  - Massachusetts
    - Start date: May 6, 2020. End date: Current, as of December 19, 2020. State order.
  - Michigan
    - Start date: July 17, 2020. End date: Current, as of December 19, 2020. State order.
  - Minnesota
    - Start date: July 25, 2020. End date: Current, as of December 19, 2020. State order beginning July 24, 2020 at 11:59 p.m.
  - Mississippi
    - Start date: August 5, 2020. End date: September 30, 2020. Executive Order effective 8:00 a.m. on August 5, 2020.
  - Missouri
    - No Mask Mandate as of December 19, 2020.
  - Montana
    - Start date: July 15, 2020. End date: Current, as of December 19, 2020. State order.
  - Nebraska
    - No Mask Mandate as of December 19, 2020.
  - Nevada
    - Start date: June 26, 2020 End date: Current, as of December 19, 2020. State Order effective 11:59pm on June 25, 2020.
  - New Hampshire
    - Start date: November 20, 2020 End date: Current, as of December 19, 2020. All persons over the age of 5 must wear a mask in public spaces where unable to maintain six feet.
  - New Jersey
    - Start date: July 8, 2020  End date: Current, as of December 19, 2020.
  - New Mexico
    - Start date: May 15, 2020  End date: Current, as of December 19, 2020.
  - New York
    - Start date: April 18, 2020  End date: Current, as of December 19, 2020. Effective 8 p.m. on April 17, 2020 for individuals over age two.
  - North Carolina
    - Start date: June 27, 2020  End date: Current, as of December 19, 2020. Effective at 5:00 pm on June 26, 2020.
  - North Dakota
    - Start date: November 14, 2020  End date: January 18, 2021. Effective November 14, 2020 at 12:01 a.m. through December 14, 2020 at 12:01 a.m.
  - Ohio
    - Start date: July 24, 2020  End date: Current, as of December 19, 2020. Order effective at 6:00pm on July 23, 2020.
  - Oklahoma
    - No Mask Mandate as of December 19, 2020.
  - Oregon
    - Start date: July 1, 2020  End date: Current, as of December 19, 2020. Executive Order 20-25 effective July 1, 2020.
  - Pennsylvania
    - Start date: July 1, 2020  End date: Current, as of December 19, 2020. Order effective immediately on July 1, 2020.
  - Rhode Island
    - Start date: May 8, 2020  End date: Current, as of December 19, 2020. Effective May 8, 2020.
  - South Carolina
    - No Mask Mandate as of December 19, 2020.
  - South Dakota
    - No Mask Mandate as of December 19, 2020.
  - Tennessee
    - No Mask Mandate as of December 19, 2020.
  - Texas
    - Start date: July 3, 2020  End date: Current, as of December 19, 2020. Statewide order effective at 12:01 p.m. on July 3, 2020.
  - Utah
    - Start date: November 9, 2020  End date: Current, as of December 19, 2020. Order effective on November 9, 2020, at 1:00 p.m.
  - Vermont
    - Start date: August 1, 2020  End date: Current, as of December 19, 2020. Effective August 1, 2020.
  - Virginia
    - Start date: May 29, 2020  End date: Current, as of December 19, 2020. Order effective 12:00 a.m., Friday, May 29, 2020.
  - Washington
    - Start date: June 26, 2020  End date: Current, as of December 19, 2020. Order goes into effect on June 26, 2020.
  - West Virginia
    - Start date: July 7, 2020  End date: Current, as of December 19, 2020. Effective as of 12:01 AM on July 7, 2020 for individuals age 9 and over.
  - Wisconsin
    - Start date: August 1, 2020  End date: Current, as of December 19, 2020. Order enters into effect at 12:01am on Saturday, August 1, 2020.
  - Wyoming
    - Start date: December 9, 2020  End date: Current, as of December 19, 2020. Order effective on December 9, 2020.

### Data appendix — construction of "minimum distance to a mask border"
- Purpose: compute the “minimum distance to a mask border” for any county i as the shortest distance to a county with a different mask mandate for regression discontinuity.
- Computation steps:
  1. Obtain county-to-county distances from https://www.nber.org/research/data/county-distance-database. "The distance between counties are great-circle distances calculated using the Haversine formula based on internal points in the geographic area".
  2. Merge distances with state-level mask mandates for a given day, t.
  3. For county i compute the minimum distance to a county j that has a different mask mandate. If county i has a state-level mask mandate the distance is kept as a positive number; if county i does not have a state-level mask mandate, then the distance is kept as a negative number.
- Data restriction: dataset restricted to absolute minimum distances of at most 150 miles.
  - Footnote: After calculating the minimum distance to a different mask mandate, the maximum across such distances for counties at any state border is 148 miles.

### Other data & sources
- COVID-19 outcomes:
  - Cases and deaths from https://usafacts.org/.
  - Hospital admissions at facility-level aggregated to county-level from http://www.healthdata.org.
- Population: county data from https://usafacts.org/.
- 2020 General Election data: share of votes for President at county-level from https://github.com/tonmcg.
- Containment and health policy measures:
  - Index and individual measures from https://github.com/OxCGRT.
  - Index defined as “A containment and health index, showing how many and how forceful the measures to contain the virus and protect citizen health are (this combines ‘lockdown’ restrictions and closures with health measures such as testing policy and contact tracing)”.
- Mobility: county-level index = simple average of retail and recreation, grocery and pharmacy, transit stations and workplaces indices from https://www.google.com/covid19/mobility/.
- County-level mask mandates: data for counties in states without a statewide mask mandate taken from Wright et al., 2020.

### SIR model setup and identification
- SIR per-capita difference equations (lower case variables):
  - s_{t+1} − s_t = −β s_t i_t
  - i_{t+1} − i_t = β s_t i_t − δ i_t − α i_t
  - r_{t+1} − r_t = α i_t
  - d_{t+1} − d_t = δ i_t
  - Parameters: β is the contact rate, δ is the case fatality rate, α is the recovery rate.
- Change in ever infected:
  - i^{ever}_{t+1} − i^{ever}_t = β s_t i_t  (equation (B2))
- Simplifying assumptions:
  - s_t = 1 (approximate in early pandemic or if recovered become susceptible again).
  - New weekly COVID infections (newcases) reflect actual infections.
- County-level simplification:
  - newcases_{c,t} = β i_{c,t}  (equation (B3))
- Identification via regression discontinuity:
  - Design controls for local pandemic state across mask borders so i_{c,t} is roughly the same on either side; regression identifies effect of mask mandates on β up to a proportionality constant.
- Actively infected calculation:
  - i_{c,t+1} = i_{c,t} + newcases_{c,t+1} − newdeaths_{c,t+1} − α i_{c,t}, where α = 0.387 following Eichenbaum et al., 2021 and i_{c,0} equals the weekly cases that are first reported.
- Note on dynamics:
  - With T >30 the Nickell bias from lagged dependent variables is unlikely to be a serious problem.

### Key empirical findings (selected table estimates preserved exactly)
- Table B1 (Actively infected per 100,000 inhabitants)
  - Panel A, varying specification (columns 1–3):
    - State mask mandate: −0.21, −0.26, −0.14 [standard errors in brackets: [0.21][0.22][0.19]]
    - Distance: 0.01*, 0.01*, 0.00** [0.00][0.00][0.00]
    - Lagged Mobility (col 3): −0.02*** [0.00]
    - Constant: 5.24***, 5.46***, 5.38*** [0.14][0.13][0.09]
    - Observations: 469305, 585855, 585858
    - R^2: 0.728, 0.692, 0.692
  - Panel B, varying time since state mandates:
    - State mask mandate (col 1): 0.30** [0.14]
    - State mask mandate (cols 2–4): −0.29, −0.45, −0.39 [0.22][0.30][0.31]
    - Observations across columns: 30423, 31086, 30700, 30526
    - R^2: 0.736, 0.728, 0.717, 0.715
    - Time since mandate categories: 1-4, 4-8, 8-12, 12-16
  - Notes: estimates based on data for 3107 counties over 48 weeks; counties within 150 miles of a state border over which mask mandates vary are included. Standard errors clustered at specific pairwise state borders. ***, **, and * indicate statistical significance at 1, 5, and 10 percent, respectively.

- Table B2 (Ln(Infections per 100,000), controlling for prior actively infected)
  - Ln(Infections per 100’000): 0.59***, 0.59***, 0.59***, 0.51*** [0.03][0.03][0.03][0.02]
  - State mask mandate: −0.20**, −0.16**, −0.16**, −0.19** [0.10][0.07][0.07][0.07]
  - Distance and polynomial terms reported as significant at small magnitudes (0.00 and 0.01 levels) with standard errors [0.00].
  - Constant: 1.60***, 1.46***, 1.58***, 2.05*** [0.16][0.14][0.15][0.15]
  - Observations: 432664, 326643, 266503, 65365
  - R^2: 0.790, 0.790, 0.790, 0.760

- Robustness tables (selected highlights)
  - Table C2 (Cases per 100,000 controlling for statewide health and containment measures):
    - State mask mandate coefficients across specifications range from −57.38*** to −72.41*** to −54.55*** (standard errors e.g. [17.90][17.64][17.27]).
    - Observations vary (e.g., 4538, 1447, 4747).
  - Table C3 (Hospital admissions per 100,000 controlling for statewide measures):
    - State mask mandate coefficients around −10.56*** to −11.97*** across columns (example [3.96][5.07][3.90]).
    - Observations: 17588 across columns.
  - Table C4 (Deaths per 100,000 controlling for statewide measures):
    - State mask mandate coefficients around −0.81** to −1.18*** across columns ([0.35][0.34]).
    - Observations: 41034 across columns.
  - Table C5 (With and without controlling for mobility):
    - State mask mandate: −63.11***, −55.22*** (cases); −8.01*, −11.46*** (hospital admissions); −0.82**, −0.74** (deaths) with corresponding standard errors shown in table.
  - Tables C6–C8 (Sensitivity analyses):
    - Results across varying polynomial orders, bandwidths (180,165,150,135,120) and numbers of leads show core negative effects on cases and admissions persist; deaths negative in several specifications (exact coefficients and standard errors preserved in tables).
  - Table C9 (Controlling for county-level mask mandates):
    - State mask mandate (cases): −55.22***, −57.08*** [17.68][18.08]
    - County mandate coefficients e.g., −14.35 for cases with standard error [19.40]; county-level effects small and statistically insignificant for deaths and admissions in respective columns.

- Additional inference notes:
  - All specifications include time and county fixed effects.
  - Standard errors clustered at specific pairwise state borders.
  - Samples generally include 3107 counties over 48 weeks and restrict to counties within 150 miles of state border where mask mandates vary.

### Robustness and covariate continuity
- Regression discontinuity checks (Table C1) show distance and distance polynomial terms operate with small coefficients; state mask mandate coefficients on proxies for attitudes and many policy controls are often small or statistically insignificant.
- Inclusion of statewide health and containment measures does not overturn negative and statistically significant state mask mandate effects on cases, admissions, and deaths in many specifications (see Tables C2–C4).
- Sensitivity to bandwidth, polynomial order, leads, and inclusion of county-level mandates examined (Tables C6–C9); core negative effects on cases and admissions persist across many specifications.

### Lives saved calculation (state-by-state)
- Lives saved formula (Figure D1 note):
  - Lives saved calculated using Σ_c (ˆδ + ˆω Proxy_c) * avg(mask_{c,t}) * weeks * population_c / 100,000 where:
    - ˆδ, ˆω are estimated in column 3 of Table 1(b),
    - Proxy_c is the proxy for attitudes towards mask wearing in county c,
    - avg(mask_{c,t}) is the average of county c’s mask mandate for the period of April 18 and December 19, 2020,
    - weeks is the number of weeks in that period,
    - population_c is county c’s population.
  - Potentially saved lives use 1 − avg(mask_{c,t}) instead of avg(mask_{c,t}) in the formula.
- Categorical legend for lives saved by state:
  - Less than 500
  - Between 500 and 1000
  - Between 1000 and 3000
  - Between 3000 and 6000
  - More than 6000

*Source: wpiea2021205-print-pdf - References*

### References

### References and State-Level Mask Mandates

### Key References
- Chernozhukov, Victor, Hiroyuki Kasahara, and Paul Schrimpf (2021). “Causal impact of masks, policies, behavior on early Covid-19 pandemic in the US”. In:Journal of Econometrics220.1, pp. 23–62.
- Dieterle, Steven, Otávio Bartalotti, and Quentin Brummet (2020). “Revisiting the Effects of Unemployment Insurance Extensions on Unemployment: A Measurement-Error-Corrected Regression Discontinuity Approach”. In:American Economic Journal: Economic Policy12.2, pp. 84–114.
- Eichenbaum, Martin S, Sergio Rebelo, and Mathias Trabandt (Apr. 2021). “The Macroeconomics of Epidemics”. In:The Review of Financial Studies.
- Gollwitzer, Anton, Cameron Martel, William J Brady, Philip Pärnamets, Isaac G Freedman, Eric D Knowles, and Jay J Van Bavel (2020). “Partisan differences in physical distancing are linked to health outcomes during the COVID-19 pandemic”. In:Nature human behaviour4.11, pp. 1186–1197.
- Greenhalgh, Trisha, Manuel B Schmid, Thomas Czypionka, Dirk Bassler, and Laurence Gruer (2020). “Face masks for the public during the covid-19 crisis”. In:Bmj369.
- Hallas, Laura, Ariq Hatibie, Saptarshi Majumdar, Monika Pyarali, and Thomas Hale (Dec. 2020).Variation in US states’ responses to COVID-19. BSG Working Papers 2020/034. University of Oxford.
- Judson, Ruth A and Ann L Owen (1999). “Estimating dynamic panel data models: a guide for macroeconomists”. In:Economics letters65.1, pp. 9–15.
- Karaivanov, Alexander, Shih En Lu, Hitoshi Shigeoka, Cong Chen, and Stephanie Pamplona (2020).Face masks, public policies and slowing the spread of COVID-19: evidence from Canada. Tech. rep. National Bureau of Economic Research.
- Lee, David S. and Thomas Lemieux (June 2010). “Regression Discontinuity Designs in Economics”. In:Journal of Economic Literature48.2, pp. 281–355.
- Leech, Gavin, Charlie Rogers-Smith, Jonas Benjamin Sandbrink, Benedict Snodin, Robert Zinkov, Benjamin Rader, John S Brownstein, Yarin Gal, Samir Bhatt, Mrinank Sharma, et al. (2021). “Mass mask-wearing notably reduces COVID-19 transmission”. In:medRxiv.
- Lyu, Wei and George L Wehby (2020). “Community Use Of Face Masks And COVID-19: Evidence From A Natural Experiment Of State Mandates In The US: Study examines impact on COVID-19 growth rates associated with state government mandates requiring face mask use in public.” In:Health affairs39.8, pp. 1419–1425.
- Milosh, Maria, Marcus Painter, David Van Dijcke, and Austin L Wright (2020). “Unmasking Partisanship: How Polarization Influences Public Responses to Collective Risk”. In:University of Chicago, Becker Friedman Institute for Economics Working Paper 2020-102.
- Mitze, Timo, Reinhold Kosfeld, Johannes Rode, and Klaus Wälde (2020). “Face masks considerably reduce COVID-19 cases in Germany”. In:Proceedings of the National Academy of Sciences117.51, pp. 32293–32301.
- Pinkovskiy, Maxim L (2017). “Growth discontinuities at borders”. In:Journal of Economic Growth22.2, pp. 145–192.
- Renne, Jean-Paul, Guillaume Roussellet, and Gustavo Schwenkler (2020). “Preventing COVID-19 Fatalities: State versus Federal Policies”. In:arXiv preprint arXiv:2010.15263.
- Simonov, Andrey, Szymon K Sacher, Jean-Pierre H Dubé, and Shirsho Biswas (2020). The persuasive effect of fox news: non-compliance with social distancing during the covid-19 pandemic. Tech. rep. National Bureau of Economic Research.
- Welsch, David M (2020). “Do masks reduce COVID-19 deaths? A county-level analysis using IV”. In:Covid Economics(57).
- Wright, Austin L, Geet Chawla, Luke Chen, and Anthony Farmer (2020). “Tracking mask mandates during the COVID-19 pandemic”. In:University of Chicago, Becker Friedman Institute for Economics Working Paper2020-104.

### State-Level Mask Mandates — Definition and Selected Entries
- Definition: A state is classified as having a mask mandate if itat leastis a requirement to wear a mask indoor if physical distance can not be maintained.
- Alabama
  - Start date: July 16, 2020. End date: Current, as of December 19, 2020.
  - Facial coverings for individuals. Effective July 16, 2020 at 5:00 P.M., each person shall wear a mask or other facial covering that covers his or her nostrils and mouth at all times when within six feet of a person from another household in any of the following places: an indoor space open to the general public, a vehicle operated by a transportation service, or an outdoor public space where ten or more people are gathered.
  - State order
- Alaska
  - No Mask Mandate as of December 19, 2020
  - The State of Alaska strongly encourages the wearing of masks in public but has not ordered it. The CDC and the World Health Organization also recommend that people wear a facial covering over their nose and mouth in community settings.
  - Recommendation
- Arizona
  - No Mask Mandate as of December 19, 2020
- Arkansas
  - Start date: June 19, 2020. End date: Current, as of December 19, 2020.
  - Whereas: On June 19, 2020, the Secretary of Health released guidance regarding the use of face covering by the general public, which asserts that ample evidence exists to support the efficacy of wearing face covering to prevent the transmission of COVID-19.
  - State order
- California
  - Start date: June 18, 2020. End date: Current, as of December 19, 2020.
  - “It mandates that face coverings be worn state-wide in the circumstances and with the exceptions outlined below. It does not substitute for existing guidance about social distancing and handwashing.” [Issued June 18]
  - Source: State order
- Colorado
  - Start date: July 16, 2020. End date: Current, as of December 19, 2020.
  - The mandatory statewide mask Executive Order went into effect on July 16, 2020, and Governor Polis has extended it repeatedly since then. The Governor’s Office released the most recent amendment to the Executive Order on January 6. An extension of the Executive Order takes effect for a maximum of 30 days. The Governor and his team review each Executive Order as it approaches expiration.
  - State order
- Connecticut
  - Start date: April 20, 2020. End date: Current, as of December 19, 2020.
  - Cloth Face Coverings or Higher Level of Protection Required in Public Wherever Close Contact is Unavoidable. Effective at 8:00 p.m. on Monday, April 20, 2020, any person in a public place in Connecticut who is unable to or does not maintain a safe social distance of approximately six feet from every other person shall cover their mouth and nose with a mask or cloth face-covering
  - State order
- Delaware
  - Start date: April 25, 2020. End date: Current, as of December 19, 2020.
  - WHEREAS, on April 25, 2020 at 4:00 p.m. E.D.T., I issued the Thirteenth Modification to the COVID-19 State of Emergency declaration to require the use of cloth face coverings in public settings where other social distancing measures are difficult to maintain, including in grocery stores and pharmacies, especially in areas of significant community-based transmission;” “Obligations for Individuals to Wear Face Coverings. i. While visiting a business, indoor or outdoor space open to the public, individuals are required to wear a face covering, except as follows: [September 3, 2020]
  - State order
- District of Columbia
  - Start date: July 22, 2020. End date: Current, as of December 19, 2020.
  - II. INDOOR WEARING OF MASKS
    - Except as specified in Section IV of this Order:
    - 1. Persons must wear a mask in the common areas of apartments, condominiums and cooperatives.
    - 2. Businesses, office buildings, and other establishments open to members of the public shall post signage on their exterior doors stating that a person may not enter unless the person is wearing a mask. In addition, the business, office building, or other establishment shall exclude or attempt to eject persons who are not wearing masks or who remove their required masks.
    - 3. Employers shall provide masks to their employees.
  - III. OUTDOOR AND TRANSIT-RELATED WEARING OF MASKS
    - Except as specified in Section IV of this Order:
    - 1. Persons leaving their residences shall wear a mask when they are likely to come into contact with another person, such as being within six feet of another person for more than a fleeting time; and
    - 2. Persons who are operating or a passenger in a taxi or a vehicle that is part of a Transportation Network Company, or who are a passenger on or operator of any form of public transit in the District, including a bus, subway, streetcar, shuttle bus or van, or school bus, must wear a mask at all such times. [Mayor’s Order 2020-080 July 22, 2020]
  - State order
- Florida
  - No Mask Mandate as of December 19, 2020
- Georgia
  - No Mask Mandate as of December 19, 2020
- Idaho
  - No Mask Mandate as of December 19, 2020
- Illinois
  - Start date: May 1, 2020. End date: Current, as of December 19, 2020.
  - I hereby order the following, effective May 1, 2020 (...)  Section 1.  Public Health Requirements for Individuals Leaving Home and for Businesses Wearing a face covering in public places or when working.  Any individual who is over age two and able to medically tolerate a face-covering (a mask or cloth face-covering) shall be required to cover their nose and mouth with a face-covering when in a public place and unable to maintain a six-foot social distance. Face-coverings are required in public indoor spaces such as stores.
  - State order
- Indiana
  - Start date: July 27, 2020. End date: Current, as of December 19, 2020.
  - This directive becomes effective at 12:01 a.m. on Monday, July 27, 2020, and continues for a period of thirty (30) days until 11:59 p.m.  on Wednesday, August 26, 2020 unless rescinded, modified or extended by me. Any recission, modification or extension of this directive will be contingent on the status of our state’s COVID-19 data which may be directly impacted and influenced by Hoosiers’ compliance with this directive. During this time period, every individual within the state of Indiana shall wear a face covering over the nose and mouth when:
    - a. inside a business, public building, or other indoor place open to the public. This does not extend, however, to private offices, private workspaces or meeting in which six feet of social distancing can be achieved and maintained between people not in the same household;
    - b. in an outdoor public space whereever it is not feasible to maintain six feet of social distancing from another person not in the same household; or
    - c. using public transportation or while in a taxi, private car sharing, or ride-sharing vehicle.
  - State order
- Iowa
  - Start date: November 17, 2020. End date: Current, as of December 19, 2020.
  - Persuant to Iowa Code paragraph 135.144 (3), and in conjuction with the Iowa Department of Public Health, unless otherwise modified by subsequent proclamation or order of the Iowa Department of public health, I continue to order that until 11:59 pm on February 6, 2021, all people two or older must wear a mask or other face covering when inside an indoor space that is open to the public and within six feet of individuals who are not members of their household for 15 minutes or longer [...]
  - [November 17, 2020]
  - State order
- Kansas
  - Start date: November 25, 2020. End date: Current, as of December 19, 2020.
  - Effective at 12:01 a.m.  on Wednesday, November 25, 2020, any person in Kansas shall wear a face covering when they are in the following situations: a. Inside, or in line to enter, any indoor public space; b. Obtaining services from the healthcare sector in settings including, but not limited to, a hospital, pharmacy, medical clinic, laboratory, physician or dental office, veterinary clinic, or blood bank; 1 c. Waiting for or riding on public transportation or while in a taxi, private car service, or ride-sharing vehicle; d. While outdoors in public spaces and unable to maintain a 6-foot distance between individuals (not including individuals who reside together) with only infrequent or incidental moments of closer proximity. 3. Also effective at 12:01 a.m. on Wednesday, November 25, 2020, all businesses or organizations in Kansas must require all employees, customers, visitors, members, or members of the public to wear a face covering when:  a.  Employees are working in any space visited by customers or members of the public, regardless of whether anyone from the public is present at the time; b. Employees are working in any space where food is prepared or packaged for sale or distribution to others; c. Employees are working in or walking through common areas, such as hallways, stairways, elevators, and parking facilities; d. Customers, members, visitors, or members of the public are in a facility managed by the business or organization; or e. Employees are in any room or enclosed area where other people (except for individuals who reside together) are present and are unable to maintain a 6-foot distance except for infrequent or incidental moments of closer proximity.
  - State order
- Kentucky
  - Start date: July 9, 2020. End date: Current, as of December 19, 2020.
  - WHEREAS, I declared on July 9, 2020, through Executive Order 2020-586 that Kentuckians were required to wear face covering in many public places to fight the continued spread of COVID-19.
- Louisiana
  - Start date: July 11, 2020. End date: Current, as of December 19, 2020.
  - Every individual in Louisiana shall wear a face covering over the nose and mouth when inside a commercial establishment or any other building or space open to the public, whether indoor or outdoor. This shall include public or commercial modes of transportation. This order shall apply statewide. [July 11, 2021]
  - State order
- Maine
  - Start date: April 29, 2020. End date: Current, as of December 19, 2020.
  - Consistent with guidance from the United States Centers for Disease Control and Prevention individuals must wear cloth face coverings in public settings where other physical distancing measures are difficult to maintain. [April 29, 2021]
  - State order
- Maryland
  - Start date: April 18, 2020. End date: Current, as of December 19, 2020.
  - Effective as of 7:00 a.m. on April 18, 2020: i. all riders and operators on any Public Transportation are required to wear Face Coverings (excluding any operator in a separate compartment that is off-limits to riders); ii. all customers over the age of nine are required to wear Face Coverings while inside the enclosed area of any Retail Establishment or Foodservice Establishment; iii. adult customers accompanying children age two through nine shall use reasonable efforts to cause those children to wear Face Coverings while inside the enclosed area of any Retail Establishment or Foodservice Establishment; iv. all Retail Establishments shall require staff to wear, and those staff shall wear, Face Coverings while working in areas open to the general public and areas in which interactions with other staff are likely; and v. all Foodservice Establishments shall require staff who interact with customers (including, without limitation, delivery personnel) to wear, and those staff shall wear, Face Coverings while working. b. Single-use Face Coverings shall be properly discarded in trash receptacles. It is recommended that all reusable Face Coverings be cleaned frequently (at least once per day). c. Wearing a Medical-Grade Mask satisfies any requirement in paragraph II.a to wear a Face Covering, but all Marylanders are urged to reserve Medical-Grade Masks for use by health care workers and first responders.
  - State order
- Massachusetts
  - Start date: May 6, 2020. End date: Current, as of December 19, 2020.
  - Effective Wednesday, May 6, 2020, any person over age two who is in a place open to the public in the Commonwealth, whether indoor or outdoor, and is unable to or does not maintain a distance of approximately six feet from every other person shall cover their mouth and nose with a mask or cloth face covering, except where a person is unable to wear a mask or face covering due to a medical condition or the person is otherwise exempted by Department of Public Health guidance. A person who declines to wear a mask or cloth face covering because of a medical condition shall not be required to produce documentation verifying the condition. This requirement applies to all workers and customers of businesses and other organizations open to the public that are permitted to operate as COVID-19 Essential Businesses as defined in Appendix A ofCOVID-19Order13, as extended by COVID-19 Orders 21and30. All persons are required to wear masks or cloth face coverings at all times when inside grocery stores, pharmacies, and other retail stores. All persons are also required to wear masks or cloth face coverings when providing or using the services of any taxi, car, livery, ride-sharing, or similar service or any means of mass public transit, or while within an enclosed or semienclosed transit stop or waiting area.
  - State order
- Michigan
  - Start date: July 17, 2020. End date: Current, as of December 19, 2020.
  - Acting under the Michigan Constitution of 1963 and Michigan law, I order the following: 1. Any individual who leaves their home or place of residence must wear a face covering over their nose and mouth: (a) When in any indoor public space; (b) When outdoors and unable to consistently maintain a distance of six feet or more from individuals who are not members of their household; and (c) When waiting for or riding on public transportation, while in a taxi or ridesharing vehicle, or when using a private car service as a means of hired transportation.
  - State order
- Minnesota
  - Start date: July 25, 2020. End date: Current, as of December 19, 2020.
  - Beginning on Friday, July 24, 2020 at 11:59 p.m., Minnesotans must wear a face covering in indoor businesses and indoor public settings, as described in this order and the related industry guidance, available at the Stay Safe Minnesota website (https://staysafe.mn.gov), as well as any other guidance referenced in this order.  Workers must also wear face coverings outdoors when it is not possible to maintain social distancing.  When leaving home, Minnesotans are strongly encouraged to have a face covering with them at all times to be prepared to comply with the requirements of this Executive Order
  - State order
- Mississippi
  - Start date: August 5, 2020. End date: September 30, 2020.
  - Every person in Mississippi shall wear a face covering, covering the nose and mouth, while inside a business, school, or other building or space open to the public, or when in an outdoor public space whenever it is not possible to maintain a minimum of six feet social distancing from another person not in the same househould [....]. The Executive Order shall we be effective at 8:00 a.m. on Wednesday, August 5, 2020.
  - Source: State order
- Missouri
  - No Mask Mandate as of December 19, 2020
- Montana
  - Start date: July 15, 2020. End date: Current, as of December 19, 2020.
  - I hereby direct the following measures be in place in the State of Montana, effective immediately: [...] Except as provided in section 4 of this Directive, all businesses, government offices, or other persons responsible for indoor spaces open to the public shall require and take reasonable measures to ensure that all employees, contractors, volunteers, customers, or other members of the public wear a face covering that covers their mouth and nose at all times while entering or remaining in any indoor spaces open to the public.
  - Source: State order
- Nebraska
  - No Mask Mandate as of December 19, 2020
- Nevada
  - Start date: June 26, 2020 End date: Current, as of December 19, 2020.
  - When and where am I required to wear a face covering? Whenever you leave the house. To keep businesses open and help slow the spread, face coverings are required.  Face coverings should be worn at all times in the following circumstances: o Public spaces: o Inside of, or standing in line waiting to enter, any indoor public space. o While outside in a public space when 6 feet of social distancing from those not in your same household isn’t possible. o Public or Private Transportation or paratransit that others HAVE used or WILL use o While at work o While working out indoors at a gym, fitness center, dance studio, or boutique fitness facility This Directive is effective at 11:59pm on Thursday, June 25, 2020...
  - State Order
- New Hampshire
  - Start date: November 20, 2020 End date: Current, as of December 19, 2020.

*Source: wpiea2021205-print-pdf - References*

### 1. Beginning on November 20, 2020, all persons over the age of 5 within the

### wpiea2021205-print-pdf - 1. Beginning on November 20, 2020, all persons over the age of 5 within the

### State mask orders — summary of mandates and effective dates
- New Hampshire
  - Beginning on November 20, 2020, all persons over the age of 5 within the State of New Hampshire shall wear a mask or cloth face covering over their noses and mouths any time they are in public spaces, indoors or outdoors, where they are unable to or do not consistently maintain a physical distance of at least six feet from persons outside their own households.
  - Definition: "public spaces" includes any part of private or public property that is generally open or accessible to members of the general public, including lobbies, waiting areas, outside plazas or patios, restaurants, retail businesses, streets, sidewalks, parks, beaches, elevators, restrooms, stairways, parking garages, etc.

- New Jersey
  - Start date: July 8, 2020  End date: Current, as of December 19, 2020.
  - Order highlights: When it is not practicable for individuals in outdoor public spaces to socially distance and keep a six-foot distance from others, all individuals shall wear face coverings. Requirement does not apply where the individual is under two years of age.
  - Indoor spaces accessible to the public (retail, recreational, entertainment, areas of government buildings open to the public, mass transit) continue to require face coverings, with health and safety exceptions including high intensity aerobic or anaerobic activities and being in the water.
  - Indoor commercial spaces not open to the public (e.g., office buildings) must have policies that at a minimum require face coverings when in prolonged proximity to others.

- New Mexico
  - Start date: May 15, 2020  End date: Current, as of December 19, 2020.
  - Order highlights: "(7) Unless a healthcare provider instructs otherwise, all individuals shall wear a mask or multilayer cloth face covering in public settings except when eating, drinking, or exercising."

- New York
  - Start date: April 18, 2020  End date: Current, as of December 19, 2020.
  - Effective at 8 p.m. on Friday, April 17, 2020 any individual who is over age two and able to medically tolerate a face-covering shall be required to cover their nose and mouth with a mask or cloth face-covering when in a public place and unable to maintain, or when not maintaining, social distance.

- North Carolina
  - Start date: June 27, 2020  End date: Current, as of December 19, 2020.
  - Where Face Coverings Are Required: Detailed sectoral requirements including Retail Businesses, Restaurants, Personal Care/Grooming/Tattoo Businesses, Child Care/Day/Overnight Camps (children eleven (11) years or older), State Government, Transportation, high-density occupational settings (NAICS sectors 311 to 339, 236 to 238, and 111, 112, 1151, and 1152), Meat or Poultry Processing Plants (Face Coverings must be Surgical Masks, as long as Surgical Mask supplies are available), Long Term Care facilities (Surgical Masks required while supplies available), and other healthcare settings per CDC guidance.
  - Effective at 5:00 pm on June 26, 2020.

- North Dakota
  - Start date: November 14, 2020  End date: January 18, 2021
  - Effective November 14, 2020, at 12:01 a.m. through December 14, 2020, at 12:01 a.m., face coverings are required in indoor businesses and indoor public settings; must be worn in outdoor business and public settings when it is not possible to maintain physical distancing. Applies to all workers, patrons, customers, visitors or guests unless exempt under Paragraph 4.

- Ohio
  - Start date: July 24, 2020  End date: Current, as of December 19, 2020.
  - Facial Coverings (Masks): All individuals in the State of Ohio shall wear face coverings at all times when: a. in any indoor location that is not a residence; b. outdoors and unable to consistently maintain a distance of six feet or more from individuals who are not members of their family/household; or c. waiting for, riding, driving, or operating public transportation, a taxi, a car service, or a ride sharing vehicle.
  - Order effective at 6:00pm on July 23, 2020.

- Oklahoma
  - Start date: No Mask Mandate as of December 19, 2020

- Oregon
  - Start date: July 1, 2020  End date: Current, as of December 19, 2020.
  - Executive Order 20-25 provides framework; Governor can modify aspects to be expressed through Oregon Health Authority guidelines.
  - Applicability: guidance applies statewide to all businesses and to the general public when visiting these businesses and to the general public when visiting indoor spaces open to the public.
  - Effective date: July 1, 2020.

- Pennsylvania
  - Start date: July 1, 2020  End date: Current, as of December 19, 2020.
  - Individuals required to wear face coverings if: A. outdoors and unable to consistently maintain a distance of six feet from individuals who are not members of their household; B. in any indoor location where members of the public are generally permitted; E. engaged in work, whether at the workplace or performing work off-site, when interacting in-person with any member of the public, working in any space visited by members of the public, working in any space where food is prepared or packaged for sale or distribution to others.
  - Order effective immediately on July 1, 2020.

- Rhode Island
  - Start date: May 8, 2020  End date: Current, as of December 19, 2020.
  - Effective on Friday, May 8, 2020, any person who is in a place open to the public, whether indoors or outdoors, shall cover their mouth and nose with a mask or cloth face covering unless doing so would damage the person’s health.
  - Exemptions: No person under two years of age or any person whose health would be damaged thereby; face coverings not required for people who can easily, continuously, and measurably maintain at least six (6) feet of distance; not required of those developmentally unable to comply, including young children.
  - Additional: Masks required at all times inside grocery stores, pharmacies or other retail stores; required when providing or using taxi, car, livery, ride-sharing, or similar services or mass public transit, or within enclosed or semi-enclosed transit stops or waiting areas. Nothing requires a store to refuse entry to a customer not wearing a face covering.

- South Carolina
  - Start date: No Mask Mandate as of December 19, 2020

- South Dakota
  - Start date: No Mask Mandate as of December 19, 2020

- Tennessee
  - Start date: No Mask Mandate as of December 19, 2020

- Texas
  - Start date: July 3, 2020  End date: Current, as of December 19, 2020.
  - Every person in Texas shall wear a face covering over the nose and mouth when inside a commercial entity or other building or space open to the public, or when in an outdoor public space, wherever it is not feasible to maintain six feet of social distancing from another person not in the same household.
  - Statewide order effective at 12:01 p.m. on July 3, 2020.

- Utah
  - Start date: November 9, 2020  End date: Current, as of December 19, 2020.
  - Restrictions: An individual shall wear a face mask while within six feet of any individual from a separate household.
  - Order effective on November 9, 2020, at 1:00 p.m.

- Vermont
  - Start date: August 1, 2020  End date: Current, as of December 19, 2020.
  - As of Saturday, August 1, 2020, Vermonters shall wear masks or cloth facial coverings over their nose and mouth any time they are in public spaces, indoors or outdoors, where they come in contact with others from outside their households, especially in congregate settings, and where it is not possible to maintain a physical distance of at least six feet.

- Virginia
  - Start date: May 29, 2020  End date: Current, as of December 19, 2020.
  - All patrons aged ten and over shall when entering, exiting, traveling through, and spending time inside listed settings cover their mouth and nose with a face covering as described by the CDC.
  - Settings include personal care and grooming businesses; all brick and mortar retail businesses; food and beverage establishments when permitted to reopen for indoor dining; entertainment or recreation businesses when permitted to reopen; train stations, bus stations, and intrastate public transportation; any other indoor place shared by groups in close proximity; state or local government buildings when accessed for public services (with certain education-related exceptions).
  - Order effective 12:00 a.m., Friday, May 29, 2020.

- Washington
  - Start date: June 26, 2020  End date: Current, as of December 19, 2020.
  - Order of the Secretary of Health 20-03 directed every person in Washington State to wear a face covering when in any indoor or outdoor public setting, subject to certain exceptions.
  - Order goes into effect on Friday, June 26, 2020.

- West Virginia
  - Start date: July 7, 2020  End date: Current, as of December 19, 2020.
  - Effective as of 12:01 AM, Eastern Standard Time, on July 7, 2020: All individuals age 9 and over shall wear an adequate face covering when in confined, indoor spaces, other than their residence or when actively engaged in consumption of food and/or beverage, and when not able to adequately social distance from other individuals who do not reside in the same household.

- Wisconsin
  - Start date: August 1, 2020  End date: Current, as of December 19, 2020.
  - FACE COVERING REQUIRED: Every individual, age five and older, in Wisconsin shall wear a face covering if both of the following apply: a. The individual is indoors or in an enclosed space, other than at a private residence; and b. Another person or persons who are not members of individual’s household or living unit are present in the same room or enclosed space.
  - Face coverings strongly recommended in all other settings, including outdoors when it is not possible to maintain physical distancing.
  - Order enters into effect at 12:01am on Saturday, August 1, 2020.

- Wyoming
  - Start date: December 9, 2020  End date: Current, as of December 19, 2020.
  - All members of the public must wear a Face Covering outside their home in specified situations: a. inside or in line to enter any business or government facility open to the general public (excluding federal buildings); b. obtaining services at or visiting healthcare operations; c. waiting for or riding public transportation or paratransit, or riding in taxi/private care service/shuttle/tour/ride-sharing vehicle (driver must also wear a Face Covering when passengers are in vehicle).
  - Order effective on December 9, 2020.

### Data Appendix — construction of "minimum distance to a mask border" variable
- Purpose: Regression discontinuity design requires computing the “minimum distance to a mask border” for any county i, defined as the shortest distance to a county with a different mask mandate.
- Computation steps:
  1. Obtain the distance of each county to any other in the U.S. from https://www.nber.org/research/data/county-distance-database. "The distance between counties are great-circle distances calculated using the Haversine formula based on internal points in the geographic area".
  2. Merge the information on distances with the state-level mask mandates (see Appendix A) for a given day, t.
  3. For a given countyi compute the minimum distance to another county, j, that has a different mask mandate. If countyi has a state-level mask mandate the distance is kept as a positive number; if countyi does not have a state-level mask mandate, then the distance is kept as a negative number.

*Italic: wpiea2021205-print-pdf - 1. Beginning on November 20, 2020, all persons over the age of 5 within the*

### 4. Fourth, the process is repeated for all dates in our dataset.

### 4. Fourth, the process is repeated for all dates in our dataset.

### B.2 Other Data & Sources
- COVID-19 outcomes:
  - Cases and deaths from https://usafacts.org/.
  - Hospital admissions at facility-level aggregated to county-level from http://www.healthdata.org.
- Population: data for all counties retrieved from https://usafacts.org/.
- 2020 General Election data: the share of votes for President at the county-level taken from https://github.com/tonmcg.
- Containment and health policy measures:
  - Index and individual measures (including categorical degree of mandated face coverings) taken from https://github.com/OxCGRT.
  - Index defined as “A containment and health index, showing how many and how forceful the measures to contain the virus and protect citizen health are (this combines ‘lockdown’ restrictions and closures with health measures such as testing policy and contact tracing)”.
- Mobility: overall county-level index constructed as the simple average of individual indices for retail and recreation, grocery and pharmacy, transit stations and workplaces from https://www.google.com/covid19/mobility/.
- County-level mask mandates: data for counties in states without a statewide mask mandate taken from Wright et al., 2020.
- Data restriction: dataset restricted to absolute minimum distances of at most 150 miles. Footnote: After calculating the minimum distance to a different mask mandate, the maximum across such distances for counties at any state border is 148 miles.

### C SIR model — setup and identification
- SIR per-capita difference equations (lower case variables):
  - s_{t+1} − s_t = −β s_t i_t
  - i_{t+1} − i_t = β s_t i_t − δ i_t − α i_t
  - r_{t+1} − r_t = α i_t
  - d_{t+1} − d_t = δ i_t
  - β is the contact rate, δ is the case fatality rate, α is the recovery rate.
- Change in ever infected:
  - i^{ever}_{t+1} − i^{ever}_t = β s_t i_t  (equation (B2))
- Simplifying assumptions:
  - s_t = 1 (approximate in early pandemic or if recovered become susceptible again).
  - New weekly COVID infections (newcases) reflect actual infections.
- County-level simplification:
  - newcases_{c,t} = β i_{c,t}  (equation (B3))
- Identification via regression discontinuity:
  - Design attempts to control for local state of the pandemic across mask borders so i_{c,t} is roughly the same on either side of the border; then regression identifies effect of mask mandates on β up to a proportionality constant.
- Actively infected calculation:
  - i_{c,t+1} = i_{c,t} + newcases_{c,t+1} − newdeaths_{c,t+1} − α i_{c,t}, where α = 0.387 following Eichenbaum et al., 2021 and i_{c,0} equals the weekly cases that are first reported.
- Note on dynamic specification:
  - Authors do not use a specification that corrects for using a lagged dependent variable strictly speaking; with T >30 the Nickell bias is unlikely to be a serious problem.

### Key empirical findings (selected table estimates preserved exactly)
- Table B1 (Actively infected per 100,000 inhabitants)
  - Panel A, varying specification (columns 1–3):
    - State mask mandate: −0.21, −0.26, −0.14 [standard errors in brackets: [0.21][0.22][0.19]]
    - Distance: 0.01*, 0.01*, 0.00** [0.00][0.00][0.00]
    - Lagged Mobility (col 3): −0.02*** [0.00]
    - Constant: 5.24***, 5.46***, 5.38*** [0.14][0.13][0.09]
    - Observations: 469305, 585855, 585858
    - R^2: 0.728, 0.692, 0.692
  - Panel B, varying time since state mandates:
    - State mask mandate (col 1): 0.30** [0.14]
    - State mask mandate (cols 2–4): −0.29, −0.45, −0.39 [0.22][0.30][0.31]
    - Observations across columns: 30423, 31086, 30700, 30526
    - R^2: 0.736, 0.728, 0.717, 0.715
    - Time since mandate categories: 1-4, 4-8, 8-12, 12-16
  - Notes: estimates based on data for 3107 counties over 48 weeks; counties within 150 miles of a state border over which mask mandates vary are included. Standard errors clustered at specific pairwise state borders. ***, **, and * indicate statistical significance at 1, 5, and 10 percent, respectively.
- Table B2 (Ln(Infections per 100,000), controlling for prior actively infected)
  - Ln(Infections per 100’000): 0.59***, 0.59***, 0.59***, 0.51*** [0.03][0.03][0.03][0.02]
  - State mask mandate: −0.20**, −0.16**, −0.16**, −0.19** [0.10][0.07][0.07][0.07]
  - Distance and polynomial terms reported as significant at small magnitudes (0.00 and 0.01 levels) with standard errors [0.00].
  - Constant: 1.60***, 1.46***, 1.58***, 2.05*** [0.16][0.14][0.15][0.15]
  - Observations: 432664, 326643, 266503, 65365
  - R^2: 0.790, 0.790, 0.790, 0.760
- Robustness tables (selected highlights)
  - Table C2 (Cases per 100,000 controlling for statewide health and containment measures):
    - State mask mandate coefficients across specifications range from −57.38*** to −72.41*** to −54.55*** (standard errors e.g. [17.90][17.64][17.27] depending on column).
    - Observations vary (e.g., 4538, 1447, 4747, etc. as reported in the table).
  - Table C3 (Hospital admissions per 100,000 controlling for statewide measures):
    - State mask mandate coefficients reported around −10.56*** to −11.97*** across columns (example [3.96][5.07][3.90] standard errors).
    - Observations: 17588 series across columns.
  - Table C4 (Deaths per 100,000 controlling for statewide measures):
    - State mask mandate coefficients reported around −0.81** to −1.18*** across columns ([0.35][0.34] etc.).
    - Observations: 41034 series across columns.
  - Table C5 (With and without controlling for mobility):
    - State mask mandate: −63.11***, −55.22*** (cases); −8.01*, −11.46*** (hospital admissions); −0.82**, −0.74** (deaths) with corresponding standard errors shown in table.
  - Table C6–C8 (Sensitivity analyses):
    - Results reported across varying polynomial orders, bandwidths (e.g., bandwidths 180,165,150,135,120) and numbers of leads; state mask mandate estimates remain negative and often statistically significant for cases and admissions, and for deaths in several specifications (exact coefficients and standard errors preserved in the tables).
  - Table C9 (Controlling for county-level mask mandates):
    - State mask mandate (cases): −55.22***, −57.08*** [17.68][18.08]
    - County mandates coefficients reported (e.g., −14.35 for cases with large standard error [19.40]; small and statistically insignificant for deaths and admissions in respective columns).
- Additional notes on inference:
  - All specifications include time and county fixed effects.
  - Standard errors clustered at specific pairwise state borders.
  - Samples generally include 3107 counties over 48 weeks and restrict to counties within 150 miles of state border where mask mandates vary.

### D Robustness (covariate continuity and policy controls)
- Regression discontinuity checks on covariates (Table C1) show distance and distance polynomial terms operating with small coefficients; state mask mandate coefficients on proxies for attitudes and many policy controls are often small or statistically insignificant (exact values provided in table).
- Inclusion of statewide health and containment measures (broad policy index and individual policy dummies) does not overturn negative and statistically significant state mask mandate effects on cases, admissions, and deaths in many specifications (see Tables C2–C4 for exact coefficients and standard errors).
- Sensitivity to bandwidth, polynomial order, leads, and inclusion of county-level mandates examined across Tables C6–C9; core negative effects on cases and admissions persist across many specifications.

### E Extra Figures (state-by-state lives saved calculation)
- Figure D1 note formula for Lives Saved:
  - Lives saved calculated using Σ_c (ˆδ + ˆω Proxy_c) * avg(mask_{c,t}) * weeks * population_c / 100,000 where ˆδ, ˆω are estimated in column 3 of Table 1(b), Proxy_c is the proxy for attitudes towards mask wearing in county c, avg(mask_{c,t}) is the average of county c’s mask mandate for the period of April 18 and December 19, 2020, weeks is the number of weeks in that period, and population_c is county c’s population.
  - Potentially saved lives use 1 − avg(mask_{c,t}) instead of avg(mask_{c,t}) in the formula.
- Categorical legend for lives saved by state: Less than 500; Between 500 and 1000; Between 1000 and 3000; Between 3000 and 6000; More than 6000.

*Source: Authors’ calculations.*

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