Can face masks protect you from Covid-19? New study says they may offer wearers only limited protection
But researchers say the findings should not be used to argue against their widespread use to prevent people infecting others as they did not test the role of masks in source control, that is, reducing the spread to others from an infected person
Public health experts have been recommending the use of face masks to reduce coronavirus transmission. The US Centers for Disease Control and Prevention (CDC) recently suggested that wearing masks provide a two-way benefit: It protects others from the spread of Covid-19, and it offers protection to the wearer as well. However, Danish researchers report in a new analysis that face masks may only provide limited protection to the wearer against coronavirus.
The randomized trial of more than 6,000 participants in Denmark adds new evidence to what is known about whether masks protect the wearer from coronavirus infection in a setting where public health measures, including social distancing, are in effect but others are not wearing masks. The DANMASK-19 (Danish study to assess face masks for the protection against Covid-19 infection) trial randomized participants to follow those public health measures with or without an additional recommendation to wear a surgical mask when outside the home. Mask use outside of hospitals was uncommon in Denmark at the time. After 1 month of follow-up, 1.8% of participants in the mask group and 2.1% in the control group developed coronavirus infection.
According to the authors, the findings do not confirm the expected halving of the risk of infection for people wearing the mask, but the results could indicate a more moderate degree of protection of 15-20%.
The study did not examine the function of the masks as source control, that is, to limit Covid-19 transmission from an infected person wearing a mask to others. The CDC argues that masks are primarily intended to reduce the emission of virus-laden droplets (source control), which is especially relevant for asymptomatic or presymptomatic infected wearers who feel well and may be unaware of their infectiousness to others, and who are estimated to account for more than 50% of Covid-19 transmissions.
The report also did not elucidate the effect of masks in situations where it is not possible to maintain social distance. Accordingly, the team emphasizes that results should not be used to raise doubts that the widespread use of masks outside of healthcare can be an effective means of reducing coronavirus infections.
“The recommendation to wear surgical masks to supplement other public health measures did not reduce the SARS-CoV-2 (the virus that causes Covid-19) infection rate among wearers by more than 50% in a community with modest infection rates, some degree of social distancing, and uncommon general mask use. The data were compatible with lesser degrees of self-protection,” write authors in the report published in the Annals of Internal Medicine.
They caution, “The findings, however, should not be used to conclude that a recommendation for everyone to wear masks in the community would not be effective in reducing SARS-CoV-2 infections, because the trial did not test the role of masks in source control of SARS-CoV-2 infection. During the study period, authorities did not recommend face mask use outside hospital settings and mask use was rare in community settings. This means that study participants’ exposure was overwhelmingly to persons not wearing masks.”
In a scientific brief updated on November 10, the CDC emphasized that adopting universal masking policies can help avert future lockdowns, especially if combined with other non-pharmaceutical interventions such as social distancing, hand hygiene, and adequate ventilation.
Researchers from Copenhagen University Hospital recruited 6,024 adults who spent at least three hours daily outside their homes, whose occupations did not require masks, and who did not have a previously known diagnosis of coronavirus. Participants were randomized into the mask group or the control group and those in the mask group were given a supply of surgical masks. “Based on the lowest adherence reported in the mask group during follow-up, 46% of participants wore the mask as recommended, 47% predominantly as recommended, and 7% not as recommended,” the findings state.
The study took place between April and May 2020. Public transportation and shops remained open and recommended public health measures included quarantine of persons with Covid-19 infection, social distancing, limiting the number of people seen, frequent hand hygiene and cleaning, and limited visitors to hospitals and nursing homes.
At the conclusion of the trial, infection rates were similar between the two groups. “The primary outcome was Covid-19 infection in the mask wearer at 1 month by antibody testing, polymerase chain reaction (PCR), or hospital diagnosis. The primary outcome occurred in 42 participants (1.8%) in the mask group and 53 (2.1%) in the control group,” reveals analysis.
According to the investigators, these findings offer evidence about the degree of protection mask wearers can anticipate in a setting where others are not wearing masks and where other public health measures, including social distancing, are in effect.