Posts mislead on study of mask effectiveness for Covid-19
Coronavirus skeptics are claiming a large-scale review of public health interventions proves that masks are useless against Covid-19. This is misleading; the Cochrane Library's editor-in-chief said these interpretations of the analysis are "inaccurate," and medical experts say other research shows face coverings offer some individual protection.
"Cochrane (gold standard of evidence-based medicine) says masks are ineffective," says one March 15, 2023 tweet shared hundreds of times.
Similar claims have spread across various platforms and languages, including German and Finnish, in the weeks since the January 30 publication of the review from Cochrane, a respected British non-profit that summarizes the results of medical research.
The posts drew new attention to impact of masks, which were the subject during the pandemic of a polarizing debate driven at times by misinformation.
But the review in question did not show masks to be futile, the organization's editor-in-chief said in a March 10 press statement responding to such claims.
"Many commentators have claimed that a recently-updated Cochrane Review shows that 'masks don't work', which is an inaccurate and misleading interpretation," Karla Soares-Weiser said.
"It would be accurate to say that the review examined whether interventions to promote mask-wearing help to slow the spread of respiratory viruses, and that the results were inconclusive. Given the limitations in the primary evidence, the review is not able to address the question of whether mask-wearing itself reduces people's risk of contracting or spreading respiratory viruses."
Language 'open to misinterpretation'
The review, titled "Physical interventions to interrupt or reduce the spread of respiratory viruses," examined 78 randomized controlled trials that looked at measures introduced to prevent the transmission of respiratory viruses. Those measures included not only mask-wearing, but also efforts to promote hand hygiene, quarantines and physical distancing.
A randomized controlled trial is a type of experiment in which participants are divided between two study groups, with one subject to the intervention and the other not.
In a summary of findings, the review's authors said: "We are uncertain whether wearing masks or N95/P2 respirators helps to slow the spread of respiratory viruses based on the studies we assessed."
But in her statement, Soares-Weiser said that wording was "open to misinterpretation" because it did not make clear that the review looked at policy measures to promote mask-wearing rather than the effectiveness of masks at an individual level.
"The review doesn't show that masks are useless but that the evidence from randomized controlled trials examined in the review shows no or no significant benefit," a spokesperson for Cochrane Germany told AFP on February 8. "The question in most of the studies wasn't whether masks help when they are worn correctly and consistently, but whether such interventions help."
The review's limitations
The review's authors acknowledged several limitations with the studies they analyzed -- chiefly, that telling people to put on masks does not guarantee they will wear them regularly.
"Relatively low numbers of people followed the guidance about wearing masks or about hand hygiene, which may have affected the results of the studies," the authors wrote.
Soares-Weiser said in her statement that in the most heavily weighted trial on community mask-wearing that the authors considered, 42.3 percent of people in the group directed to wear masks did so, compared with 13.3 percent in the control group.
"Evidence of effectiveness at the population level is not the same as evidence in individuals or in laboratories," said Jennifer Nuzzo, a professor of epidemiology at Brown University School of Public Health in Rhode Island, in a February 4 Twitter thread. "At the population level, compliance is a key variable that may reduce the effectiveness of masks."
"If you religiously wore a face mask any time you were around other people the effectiveness of that mask for you as an individual is likely to be different than what is measured at population level."
The review's authors also mentioned the quality of the masks and failures to use them properly as possible reasons for the low impact in some trials.
The majority of the studies included in the review also took place before the outbreak of the Covid-19 pandemic and concerned the transmission of influenza and other cold viruses. The authors said only two of the evaluated studies occurred during the pandemic, and that their addition "had minimal impact on the overall pooled estimate of effect."
One of the studies from the pandemic, from Bangladesh, concluded that "promoting community mask-wearing can improve public health." The authors wrote that "villages where in-person reinforcement of mask wearing occurred also showed a reduction in reporting Covid-like illness, particularly in high-risk individuals."
The other study conducted during the pandemic, from Denmark, said a recommendation to wear surgical masks outside the home did not lower the incidence of Covid-19 infections at a level that could be considered statistically significant, but the authors stopped short of saying no benefit would come from such a policy.
Scientists have also carried out observational and laboratory studies supporting the notion that masks are helpful, Nuzzo and others said on Twitter.
A November 2021 meta-analysis that included observational studies connected the use of several personal and social protective measures -- such as hand-washing, mask-wearing and physical distancing -- with a decrease in Covid-19 cases.
A February 2022 report from the US Centers for Disease Control and Prevention also said consistent use of a mask in indoor public settings is associated with lower chances of contracting Covid-19.
"Masks offer very good protection against illnesses that spread through droplet transmission," Samir Salameh, a professor of particle technology and recycling economy at FH Munster University of Applied Sciences, told AFP on July 29, 2022. "Masks don't offer 100 percent protection, but rather protect only with a certain degree of probability. This depends on the type of the mask, its age and the way it is worn. If the mask doesn't fit you, it offers significantly less protection."
AFP has fact-checked other false and misleading claims about Covid-19 here.