Stay-at-home orders and the closing of schools and nonessential businesses may be effective at slowing the spread of COVID-19 by limiting mobility and thus limiting interactions between people. However, such containment policies come with high social and economic costs. In contrast, public health measures aimed at reducing the infection rate when people do interact, particularly mandates that require them to wear masks, carry a much lower economic cost. But how effective are they?

“The Roles of Mobility and Masks in the Spread of COVID-19,” a new report from the Federal Reserve Bank of Boston, finds that that while mobility fosters the spread of COVID-19, mask mandates can dramatically mitigate the growth in case counts associated with increased mobility. The report, written by Boston Fed economists Daniel Cooper, Vaishali Garga, María José Luengo-Prado, and Jenny Tang, notes that a key determinant of the success of mask mandates is, unsurprisingly, compliance with those mandates.

For their analysis, the report’s authors use state-level differences in the numbers of COVID-19 cases and deaths, mobility, mask mandates, and mask wearing.

Mobility is measured by cellphone-based indexes of location-specific visits, time spent at home, and contacts between people. Mask mandates are measured by scope and date of implementation. Estimates for degrees of compliance with the mandates are based on survey responses and relevant characteristics of a state’s population, including political leanings.

The report’s estimates imply that if mobility had remained at the level it was on May 15, when large portions of the U.S. population were still in lockdown, the country’s cumulative number of COVID-19 cases would have been as much as 66% lower than it was on Nov. 15. This translates into roughly 7 million fewer people becoming infected during that six-month period.

The report also estimates that given the actual rate at which mobility grew – due largely to lockdown measures being lifted and businesses reopening – if a national mask mandate had been enacted on May 15, it could have prevented as many as 74% of the additional COVID-19 infections associated with the increase in mobility.

Furthermore, a national mask mandate with a high degree of compliance potentially could have completely offset the rise in the case count that accompanied the increase in mobility after May 15.

“Restricting mobility to its near-lockdown levels of mid-May certainly would have been effective at reducing the number of COVID-19 cases, and ultimately the number of deaths from the disease,” write the report’s authors. “But the costs of this intervention would have been substantial in terms of the reduced economic activity. Mask mandates, if they had been implemented earlier and across all states, would have been about half as effective as lockdowns at reducing the number of COVID-19 infections and deaths, but they are much less costly for the economy.”

The authors further write, “A good understanding of the effectiveness of public health measures such as mask mandates is important for policymakers trying to determine how much to limit mobility and social interactions – at the potential cost of reduced economic activity – given the substantial resurgence in the spread of the virus.”

Read the full report.

Daniel Cooper a senior economist and policy advisor in the research department at the Federal Reserve Bank of Boston. Full bio here.

Vaishali Garga is an economist in the Boston Fed research department. Full bio here.

María José Luengo-Prado is a senior economist and policy advisor in the Boston Fed research department. Full bio here.

Jenny Tang is a senior economist in the Boston Fed research department. Full bio here.