Several studies published this month indicate that wearing face masks or coverings can effectively curb transmission of the novel coronavirus, lending support for policies requiring individuals to wear masks or coverings in public—but some scientists contend that the studies' results should be viewed with caution.

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Mask requirements vary by state

Although CDC recommends that Americans wear face masks or covering in instances when social distancing may not be possible as a means to prevent transmission of the new coronavirus, requirements on whether Americans must wear masks in public varies from state to state.

According to an Axios-Ipsos poll of U.S. residents conducted from May 29 to June 1, about half of respondents said they were wearing masks "at all times" when they left their homes.

Research shows wearing masks can significantly reduce risk of coronavirus transmission

However, new research could support some experts' and officials' calls for a more sweeping policy regarding face masks and coverings to curb the virus' spread, the Washington Post reports.

For instance, a study published Thursday in the Proceedings of the National Academy of Sciences (PNAS) found that requiring people to wear masks in epicenters of new coronavirus cases may have prevented tens of thousands of infections from the virus.

For the study, researchers examined how the new coronavirus is transmitted by reviewing infection trends in Wuhan, China; Italy; and New York City—all of which were early epicenters of the virus' transmission. The researchers also observed the precautions implicated to curb the virus' spread in those epicenters and compared the rates of coronavirus infection in Italy and New York City before and after rules regarding face masks and covering were put in place.

The researchers found that "airborne transmission" appears to be "the dominant route for infection" from the new coronavirus, according to the study. In addition, they found that coronavirus infection trends changed once governments enforced mask-wearing rules in Italy in April 6 and in New York City on April 17.

In New York, for instance, the daily new infection rate dropped by 3% per day after a policy requiring that people wear face masks or coverings in public took effect, the researchers found.  Overall, the researchers estimated that requirements related to face masks and coverings "significantly reduced the number of infections … by over 78,000 in Italy from April 6 to May 9 and over 66,000 in New York City from April 17 to May 9."

Further, the researchers concluded that "[f]ace covering prevents both airborne transmission by blocking atomization and inhalation of virus-bearing aerosols and contact transmission by blocking viral shedding of droplets." Based on their findings, they wrote that CDC and the World Health Organization should enforce stricter policies on wearing face masks or coverings to further curb the novel coronavirus' spread. "The current mitigation measures, such as social distancing, quarantine, and isolation implemented in the United States, are insufficient by themselves in protecting the public," the researchers wrote.

Separately, Richard Stutt, a researcher at the University of Cambridge, on Wednesday published a model in the Proceedings of the Royal Society A that showed widespread use of face masks and coverings can help to reduce the new coronavirus' spread—even if the masks or coverings don't provide complete protection against droplets that may contain the pathogen. Stutt said wearing face masks or coverings can help to significantly curb the coronavirus' transmission when paired with lockdown orders.

"You can do lockdown, you can do masks, but you get the best result when you combine them," Stutt said.

In addition, a review of 172 observational studies published earlier this month in The Lancet also concluded that wearing face masks or coverings can help curb the risk of coronavirus infection and transmission, the Post reports. Holger Schünemann, a co-author of the review and an epidemiologist and physician at McMaster University, said the review indicated that, "[i]n multiple ways … the use of masks is highly protective in health care and community settings."

Should the findings be viewed with caution?

Some scientists say the new findings should be viewed with caution.

Schünemann said he and his colleagues have "low certainty" in their findings, because their observational review did not provide results as robust those that would come from a randomized controlled trial.

Separately, some scientists have criticized the PNAS study, claiming that the researchers used "poor methods" to estimate the number of infections that were avoided because of face masks and coverings, the Post reports.

In addition, the new studies do not make clear whether transmission of the new coronavirus declined directly because of face masks' or coverings' physical barriers or if other changes in behavior, such as people's tendency to touch their faces less often when wearing masks or coverings, drove down transmission, according to the Post.

Still, Jeffrey Shaman, an epidemiologist at Columbia University said, "Anecdotally, it appears that face-mask use is an important control against multiple modes of [new coronavirus] transmission." He added, "[S]tudies support this anecdotal observation and provide some quantification of the effects of face masks" (Lapid, Reuters, 6/12; Kane/Kim, CNN, 6/12; Guarino et al., Washington Post, 6/13).