**Infographics available—see notes to editors**
- Simulations of COVID-19 spread in the USA show maintaining face mask use in public indoor spaces for at least two weeks after achieving population vaccination targets (with a minimum of 70% coverage) is cost-effective and usually cost-saving, and reduces hospitalisations and deaths.
- The emergence of more transmissible COVID-19 variants, waning immunity, and increased social mixing all increase the benefits of continuing face mask use for a defined, short period of time.
- The study authors emphasise that mask wearing will continue to be an important part of non-pharmaceutical interventions, in addition to vaccines, to slow the spread of COVID-19 and suggest that providing subsidies for face masks, along with subsidies for tests and treatments, will be essential tools as the world enters the next phase of the pandemic.
Maintaining face mask use in public indoor spaces and on public transportation for at least two weeks after COVID-19 vaccination targets (with a minimum of 70%) are met is cost-effective, typically cost-saving, and saves lives, according to a modelling study published in The Lancet Public Health journal. The lower the level of the final population vaccination coverage, the greater the economic and health benefits of maintaining face mask use.
Simulations of the US population indicate that continuing to use facemasks at the levels seen from March to July 2020 for between two and 10 weeks after reaching vaccination targets would save billions of dollars in societal and healthcare costs and greatly reduce hospitalisations and deaths due to COVID-19.
Face masks were used widely in the USA earlier in the pandemic; however, many mask requirements put in place during delta and omicron surges have recently been relaxed, despite vaccination rates still being below potential herd immunity thresholds. While precise herd immunity thresholds for COVID-19 remain unknown, the World Health Organization (WHO) has set a global vaccination coverage target of 70% by mid-2022 . The US Centers for Disease Control and Prevention (CDC) recently launched a tool offering guidance on face mask use based on COVID-19 levels in each of the USA’s more than 3,000 counties. These new recommendations are not mandates, but instead intended to inform indoor mask use policies for states and municipalities. Policies related to mask use in settings under federal jurisdictions, such as airports, and in healthcare settings, will remain in place .
Senior author Dr Bruce Y. Lee, of the CUNY Graduate School of Public Health and Health Policy, USA, says, “Our findings emphasise that vaccination alone is not enough to control the pandemic and that overlapping layers of protective measures are needed to limit economic impacts and deaths. The newly released recommendations from CDC come at a time when many places in the USA are also beginning to consider easing mask requirements, and the choice to wear a mask will come down to individual decisions or decisions of private businesses. Our model represents the US population, but the breadth and scale of the simulated scenarios mean the results are also applicable to other countries.” 
Previous studies have shown that face masks decrease the spread of severe COVID-19. However, the economic value of maintaining face mask use – especially at different levels of population vaccination coverage – has not been clear.
The authors of the new study developed a computational model representing the spread and impact of COVID-19 among the more than 327 million people in the USA and simulated the use of maintaining face mask use before and after achieving different vaccination coverage levels under a wide variety of circumstances. Scenarios varied the final population COVID-19 vaccination levels and the date these targets were reached, as well as levels of facemask use and the date facemask use ended.
In all simulated scenarios, it was cost-effective to maintain face mask use for somewhere between two and 10 weeks after hitting population vaccination targets. Maintaining mask use was always cost-effective, and usually cost-saving when the cost of mask-wearing per person per day was less than $1.25. The authors determined that the combination of N95, surgical, and cloth masks used in the US from March to July 2020 resulted in a cost of $0.32 per person per day.
The lower the level of final population vaccination coverage, the greater the benefits of continued face mask use. For example, if the USA achieved 90% vaccine coverage by May 1 2022, maintaining facemask use until then would avert $13.3 billion in societal costs and $2.4 billion direct medical costs, as well as 6.29 million COVID-19 cases, 136,700 hospitalizations, and 16,000 deaths. By comparison, achieving an 80% coverage would increase savings to $16.7 billion in societal costs, $2.9 billion in direct medical costs, and avert 7.66 million cases, 174,900 hospitalizations, 20,500 deaths.
For a given final vaccination level, the longer it takes to reach that level, the greater the benefits of maintaining face mask use. For instance, reaching 80% vaccination coverage by July 1st would avert $18.7 billion in societal costs, $3.3 billion direct medical costs, 8.57 million cases, and 23,200 deaths.
The emergence of more transmissible variants further boosts the benefits of face mask use, as does reduced vaccine effectiveness, either through the emergence of variants or waning immunity.
Increasing social mixing of people of different ages by relaxing social distancing measures also increases the benefits of face mask use. In each case, using face masks saves billions of dollars in societal and healthcare costs and greatly reduces hospitalisations and deaths from COVID-19.
“Our findings offer some light at the end of the tunnel, suggesting that face mask use doesn’t have to continue forever, but that it remains an important tool to stop the spread of COVID-19 as we enter the next phase of the pandemic,” says Dr Peter Hotez of the National School of Tropical Medicine at Baylor College of Medicine (USA) and co-author of the study. 
“Virus transmission does not stop as soon as population vaccination targets are met but maintaining mask use for just a few short weeks after reaching these targets can deliver tremendous economic and health benefits,” adds Dr Maria Elena Bottazzi, of the National School of Tropical Medicine at Baylor College of Medicine (USA). 
The authors acknowledge some limitations regarding the use of models, which are simplifications of real-life and cannot account for every possible outcome. The effectiveness of face masks against COVID-19 transmission was fixed for the duration of simulations; however this may vary from day to day and over time, and with local policies. The simulations assume vaccine coverage of the entire population, however, some groups – such as children under five years old – are not yet eligible.
NOTES TO EDITORS
This work was supported by the Agency for Healthcare Research and Quality (AHRQ), the National Institute of General Medical Sciences (NIGMS) as part of the Models of Infectious Disease Agent Study, the National Science Foundation (NSF), the National Center for Advancing Translational Sciences (NCATS) of the National Institutes of Health, and the City University of New York (CUNY) in support of the Pandemic Response Institute (PRI). It was conducted by researchers from the CUNY Graduate School of Public Health and Health Policy and Baylor College of Medicine.
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PJH and MEB codirect the Texas Children’s Center for Vaccine Development and with US are codevelopers of vaccines against emerging and neglected diseases including coronaviruses such as COVID-19. Baylor College of Medicine non-exclusively licensed a COVID-19 vaccine construct to Biological E, an India-based manufacturing company. These authors have no financial stakes in any COVID-19 vaccine candidates under development. All other authors declare no competing interests.