CDC study: N95, KN95 masks best reduce odds of COVID-19 infection

People who said they always wore some type of face mask indoors were less likely to test positive for COVID-19 compared to those who did not, though high-quality N95 respirators and KN95 were better at stopping infections than other masks, the Centers for Disease Control and Prevention (CDC) said Friday. 

According to the study, surgical masks reduced the odds of testing positive for COVID-19 by about 66 percent compared to no masks at all. People who reported wearing N95 respirators while indoors were about 83 percent less likely to test positive compared with those who said they never wore a mask inside, the CDC found.

The odds of infection were 56 percent lower for people wearing a cloth mask, but the findings were not statistically significant.

“These findings reinforce that in addition to being up to date with recommended COVID-19 vaccinations, consistently wearing a face mask or respirator in indoor public settings reduces the risk of acquiring SARS-CoV-2 infection,” the CDC wrote in the study, which was conducted by researchers at the California Department of Public Health.

“Using a respirator offers the highest level of personal protection against acquiring infection, although it is most important to wear a mask or respirator that is comfortable and can be used consistently,” the CDC added.

The study was conducted from Feb. 18 through Dec. 1, 2021, before the omicron variant became widespread.

Researchers surveyed 652 people who had recently tested positive for the virus and matched them with 1,176 people who had tested negative and self reported being in indoor public settings during the two weeks preceding testing.

Mask use has been one of the most consistent points of contention throughout the pandemic, as state, local and federal lawmakers argue over the authority to require masks indoors, as well as their effectiveness.

The CDC noted well-fitting face masks and respirators are known to be effective in laboratory conditions, though few studies have assessed their real-world effectiveness in preventing acquisition of SARS-CoV-2 infection.

The researchers said relying on self-reporting was a limitation of the data, especially as it was limited to persons who sought out a COVID-19 test and were willing to participate in a telephone interview. The study also did not account for other preventive behaviors that could influence risk for acquiring infection, such as physical distancing. 

Still, the study was conclusive in showing real-world data that backed up laboratory testing. The researchers also said the differences between mask types shows the importance of making sure access is not an issue.

“This highlights the importance of improving access to high-quality masks to ensure access is not a barrier to use,” the agency wrote. 

The federal government in January began sending millions of free N95 masks to retail pharmacies and community health centers nationwide. Some states have also been providing residents with free KN95 masks.