A group of researchers from Sweden's Karolinska Institute and Canada's McMaster University, led by nephrologist Catherine Clase, reviewed 25 published studies into cloth face masks, and their filtration ability.
“We were concerned about the debate created worldwide on the effectiveness of face masks,” Karolinska Institute epidemiology professor Juan Jesus Carrero, one of the authors of the study which was led by McMaster University professor Catherine Clase, told The Local.
“We settled to study, interpret and summarise the evidence there is to date evaluating whether face masks can protect from spreading and from being infected by viruses. We identified 25 articles that dealt with this problem, some of them almost 100 years old. We took a great amount of time in studying them, and in some cases re-analysing their data with modern or more appropriate statistics.”
Taken as a whole, the researchers say the findings are convincing enough to recommend wearing face masks as a disease control measure.
“We observe that the efficacy of filtration varies, sometimes less good, sometimes as good as a medical mask. However, they always offer some non-negligible degree of protection,” Carrero explained.
The virus causing Covid-19 has a similar diameter to many other viruses, he said, and although cloth cannot stop isolated virions, it can stop the larger particles caused by speaking, sneezing or eating, which the epidemiologist said were responsible for most virus transmission.
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Sweden's Public Health Agency has not recommended mask-wearing by the general public, even while the majority of countries around the world have done so as a measure to curb the spread of the coronavirus. Swedish authorities have said the lack of conclusive scientific proof is one reason.
The World Health Organisation (WHO) and other national governments have stressed the importance of wearing masks as a complementary measure to other actions, such as hand hygiene and social distancing.
State epidemiologist Anders Tegnell told The Local on Tuesday: “That report is saying the same thing as numerous reports from earlier on, that cloth masks will stop droplets from coming through to a certain extent. If that can be translated into some kind of effect in the community, the authors say they don't know.”
He said Sweden was not considering reviewing its guidelines on face masks in the current situation, but would look at them again if the development of the epidemic worsens in Sweden.
Another concern that Tegnell has raised is that mask-wearing may encourage people to relax on other more crucial measures, for example keeping distance to a lesser extent or touching their faces more.
“Face masks are just one piece of the puzzle that should never substitute hand hygiene, social distancing or disinfection of public areas. It is true that an incorrect use of face masks may diminish their efficiency, but that is something that can be corrected by educating the population on how to put face masks on, how to wear them and put them off, how to wash them or dispose of them,” Carrero said.
The researchers behind the literature review emphasised that their findings show the efficacy of the masks is also affected by how they are made, cleaned and used. Among the most protective materials, according to the study, were muslin, cotton and flannel masks, especially those in three or four layers and with a thread count of at least 100 threads per inch.