ANALYSIS: Studies on the positive effect of wearing masks in reducing coronavirus cases.

The weakening policies are out of step with those of many other countries. Germany strengthened its mask requirements at the end of April, for example. It was facing a slowdown in vaccination rates and a surge in cases. Spain tightened its requirements at the end of March.

The evidence is clear that masks cut down on COVID-19 deaths, but nearly a year and a half into the pandemic and with vaccination coverage climbing in many places, public-health scientists and officials are still struggling to get people — particularly unvaccinated people — to wear masks at appropriate times. Average mask use across the United States has been declining since mid-February. Meanwhile, infection rates in some places have increased. A patchwork of policies and mixed messages from both politicians and public-health officials has resulted in confusion, consternation and a mess of data to interpret. “We’re all over the map,” says Monica Gandhi, an infectious-disease physician at the University of California, San Francisco. “That’s been the problem this entire pandemic. We’ve been making it up as we go along.” ...

Mask use will continue for this pandemic, and it’s likely to become a common response to future outbreaks. So researchers are trying to get a handle on what the science says about how to encourage people to wear them. As the COVID-19 pandemic enters a new phase, scientists around the world are accessing the accumulated data and asking what makes some policies more effective than others, and probing when and how they need to change.  ...

Klaus Wälde, an economist at Johannes-Gutenberg University Mainz in Germany... and his colleagues used data from 401 regions in Germany to estimate the effect of mask mandates on SARS-CoV-2 transmission³. They took advantage of the regional variation to create artificial controls, and then estimated what would have happened had the intervention not been implemented. His team’s conclusion: requiring people to wear face masks decreases the daily growth rate of reported COVID-19 cases by more than 40%. ...

In a similar study in the United States, published this January⁵, researchers found that a national mandate for employees to wear face masks early in the pandemic could have reduced the weekly growth rate of cases and deaths by more than 10 percentage points in late April 2020. The study suggests that this could have reduced deaths by as much as 47% (or by nearly 50,000) across the country by the end of May last year. Another preprint, published in October, linked mask mandates with a 20–22% weekly reduction in COVID-19 cases in Canada⁶. ...