Asking people to wear masks is linked to reduced coronavirus transmission
Richard B. Levine/Alamy
A review of the studies done so far has concluded that wearing masks really does help prevent the spread of covid-19. It is not the first paper to come to this finding, so why is the issue still controversial? The problem is that conducting individual studies of the highest standard during a pandemic is challenging.
The gold standard for research is a randomised controlled trial (RCT), where people are randomly assigned to either receive a treatment or intervention, such as wearing a mask, or not. Due to practical difficulties, there have only been two RCTs that investigated whether wearing masks prevents the spread of covid-19 outside of healthcare settings.
One study conducted in Denmark was too small to produce a statistically significant result. The other study, conducted in Bangladesh, found that villages supplied with masks had 35% fewer symptomatic infections in people aged over 60 and 10% fewer infections overall compared to villages without masks.
Therefore, the much-criticised Cochrane review published in January, which looked only at RCTs, stated that it could not draw “firm conclusions” about the efficacy of masks. However, some people mistakenly claimed that this review found that masks do not work.
Many highly effective policies, such as speed limits and wearing seatbelts, have never been assessed through RCTs, as pointed out by Shama Cash-Goldwasser at the Resolve to Save Lives initiative in New York. When there is a lack of RCTs, it is appropriate to consider other types of evidence, as mentioned in their review.
Laboratory studies demonstrate that masks help block larger droplets and aerosols that can carry viruses. Higher-quality N95 masks have better filtration capabilities than lower-quality masks.
Observational studies, which retrospectively analyze events to determine the impact of certain measures, also provide evidence. For example, during an outbreak on a ship called the USS Theodore Roosevelt early in the pandemic, crew members who reported wearing masks were 30% less likely to be infected compared to those who did not wear masks.
The question of whether mask-wearers are less likely to be infected is different from whether mask mandates, where authorities require people to wear masks, reduce infection rates among the wider population, including those who do not wear masks themselves. Studies conducted in Germany and the US have found reductions in infections after the implementation of mask mandates.
The review concludes that the available evidence strongly suggests that community-wide masking can reduce the spread of SARS-CoV-2. Christopher Dye from the University of Oxford, whose team has conducted an even more comprehensive review as part of a Royal Society report, agrees with these findings.
Although observational studies have limitations in terms of bias, when multiple studies consistently reach the same conclusion, it provides more confidence in the findings.
Cash-Goldwasser and her colleagues also acknowledge that no public health intervention is 100% effective, and the benefits of masks should be weighed against any potential adverse effects. For example, concerns have been raised about the impact of wearing masks on the development of young children.
Clear communication of the evidence is crucial for authorities to make informed decisions in future pandemics. The review concludes that “masking with the highest-quality masks that can be made widely available should play an important role in controlling whatever pandemic caused by a respiratory pathogen awaits us.”
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