Background Containment of the COVID-19 pandemic requires evidence-based strategies to reduce transmission. Because COVID-19 can spread via respired droplets, many states have mandated mask use in public settings. Randomized control trials have not clearly demonstrated mask efficacy against respiratory viruses, and observational studies conflict on whether mask use predicts lower infection rates. We hypothesized that statewide mask mandates and mask use are associated with lower COVID-19 case growth rates in the United States.
Methods We calculated total COVID-19 case growth and mask use for the continental United States with data from the Centers for Disease Control and Prevention and Institute for Health Metrics and Evaluation. We estimated post-mask mandate case growth in non-mandate states using median issuance dates of neighboring states with mandates.
Results Case growth was not significantly different between mandate and non-mandate states at low or high transmission rates, and surges were equivocal. Mask use predicted lower case growth at low, but not high transmission rates. Growth rates were comparable between states in the first and last mask use quintiles adjusted for normalized total cases early in the pandemic and unadjusted after peak Fall-Winter infections. Mask use did not predict Summer 2020 case growth for non-Northeast states or Fall-Winter 2020 growth for all continental states.
Conclusions Mask mandates and use are not associated with slower state-level COVID-19 spread during COVID-19 growth surges. Containment requires future research and implementation of existing efficacious strategies.
Competing Interest Statement
The authors have declared no competing interest.
Start-up funds from the University of Louisville College of Arts and Sciences supported this project. No external funding supported this work.
I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained.
The details of the IRB/oversight body that provided approval or exemption for the research described are given below:
No IRB approval was sought because the research did not concern human subjects or identifiable patient data. All source data were publicly available from either the CDC or the IHME.
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All numeric data is is contained within the manuscript as supplemental Excel tables. Raw data are available from the Centers for Disease Control and Prevention (link: https://data.cdc.gov/Case-Surveillance/United-States-COVID-19-Cases-and-Deaths-by-State-o/9mfq-cb36) or the Institute for Health Metrics and Evaluation (link: https://covid19.healthdata.org/). GraphPad Prism files are available upon request.
Our main finding is that mask mandates and use are not associated with lower SARS-CoV-2 spread among US states. 80% of US states mandated masks during the COVID-19 pandemic. Mandates induced greater mask compliance but did not predict lower growth rates when community spread was low (minima) or high (maxima). We infer that mandates likely did not affect COVID-19 case growth , as growth rates were similar on all days between actual or modeled issuance dates and 6 March 2021. Higher mask use (rather than mandates per se) has been argued to decrease COVID-19 growth rates . While compliance varies by location and time, IHME estimates are robust (derived from multiple sources ) and densely sampled (day-level precision). Higher mask use did not predict lower maximum growth rates, smaller surges, or less Fall-Winter growth among continental states. Mask-growth rate correlation was only evident at minima. This may be an artifact of faster growth at fewer normalized cases, as well as regional differences in case prevalence early in the pandemic. States in the high mask quintile grew at similar rates as states in the low mask quintile after maxima (when interstate total case differences were smaller than before minima). In addition, mask use did not predict normalized cases at minima, and low mask growth curves trailed those of high mask (particularly Northeast) states before minima. Growth maxima and Fall-Winter surges did not differ between Northeast and other states. Northeast states exhibited the highest seroprevalence up to at least July 2020  and constituted 80% of the top quintile of mask use, which may explain their comparatively lower Summer growth. Overall, mask use appears to be an intra-state lagging indicator of case growth.
There is inferential but not demonstrable evidence that masks reduce SARS-CoV-2 transmission. Animal models , small case studies , and growth curves for mandate-only states  suggest that mask efficacy increases with mask use . However, we did not observe lower growth rates over a range of compliance at maximum Fall-Winter growth (45-83% between South Dakota and Massachusetts during maxima)  when growth rates were high. This complements a Danish RCT from 3 April to 2 June 2020, when growth rates were low, which found no association between mask use and lower COVID-19 rates either for all participants in the masked arm (47% strong compliance) or for strongly compliant participants only . Masks have generally not protected against other respiratory viruses. Higher self-reported mask use protected against SARS-CoV-1 in Beijing residents , but RCTs found no differences in PCR confirmed influenza among Hong Kong households assigned to hand hygiene with or without masks (mask use 31% and 49%, respectively) . Medical and cloth masks did not reduce viral respiratory infections among clinicians in Vietnam  or China , and rhinovirus transmission increased among universally masked Hong Kong students and teachers in 2020 compared with prior years . These findings are consistent with a 2020 CDC meta-analysis  and a 2020 Cochrane review update .
Our study has implications for respiratory virus mitigation. Public health measures should ethically promote behaviors that prevent communicable diseases. The sudden onset of COVID-19 compelled adoption of mask mandates before efficacy could be evaluated. Our findings do not support the hypothesis that SARS-CoV-2 transmission rates decrease with greater public mask use. As masks are required in public in many US states, it is prudent to weigh potential benefits with harms. Masks may promote social cohesion as rallying symbols during a pandemic , but risk compensation can also occur . Prolonged mask use (>4 hours per day) promotes facial alkalinization and inadvertently encourages dehydration, which in turn can enhance barrier breakdown and bacterial infection risk . British clinicians have reported masks to increase headaches and sweating and decrease cognitive precision . Survey bias notwithstanding, these sequelae are associated with medical errors . By obscuring nonverbal communication, masks interfere with social learning in children . Likewise, masks can distort verbal speech and remove visual cues to the detriment of individuals with hearing loss; clear face-shields improve visual integration, but there is a corresponding loss of sound quality [37, 38]. Future research is necessary to better understand the risks of long-term daily mask use . Conversely, it is appropriate to emphasize interventions with demonstrated or probable efficacy against COVID-19 such as vaccination  and Vitamin D repletion .
In summary, mask mandates and use were poor predictors of COVID-19 spread in US states. Case growth was independent of mandates at low and high rates of community spread, and mask use did not predict case growth during the Summer or Fall-Winter waves. Strengths of our study include using two mask metrics to evaluate association with COVID-19 growth rates; measuring normalized case growth in mandate and non-mandate states at comparable times to quantify the likely effect of mandates; and deconvolving the effect of mask use by examining case growth in states with variable mask use. Our study also has key limitations. We did not assess counties or localities, which may trend independently of state averages. While dense sampling promotes convergence, IHME masking estimates are subject to survey bias. We only assessed one biological quantity (confirmed and probable COVID-19 infections), but the ongoing pandemic warrants assessment of other factors such as hospitalizations and mortality. Future work is necessary to elucidate better predictors of COVID-19 spread. A recent study found that at typical respiratory fluence rates, medical masks decrease airway deposition of 10-20µm SARS-CoV-2 particles but not 1-5µm SARS-CoV-2 aerosols . Aerosol expulsion increases with COVID-19 disease severity in non-human primates, as well as with age and BMI in humans without COVID-19 . Aerosol treatment by enhanced ventilation and air purification could help reduce the size of COVID-19 outbreaks.