Enlarge /. If only part of the public wears protective clothing, is this helpful?
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How can you best protect yourself if you are at risk of being exposed to SARS-CoV-2? It seems an easy question, but many of the options – face masks, locks, social distancing, etc. – were politically controversial. In addition, it has been difficult for health authorities to maintain a unified message as our level of knowledge has changed and they need to reconcile things such as maintaining protective equipment for healthcare workers.
But a few months after the pandemic, we got clear evidence that rules of social isolation help and support that policy. So where do we stand when using masks?
Two recent events indicate where the evidence is going. The first involves withdrawing paper that appeared to show that the use of masks was ineffective. And the second is a meta-analysis of all recent studies on the use of protective equipment against SARS-CoV-2 and its relatives SARS and MERS. There is support for mask protection – as well as eye protection – although the underlying evidence is not as strong as we might want it to be.
How do you test that?
It turns out that testing the effectiveness of masks is more difficult than expected. A recent study published in the Annals of Internal Medicine seemed like a well-designed experiment that you might think would be crucial. The researchers took patients with SARS-CoV-2 infection, put masks on them, asked them to cough, and collected any material that went through the masks.
The paper concluded that all masks were ineffective, but has since been withdrawn because the authors disregarded the sensitivity of the devices used to detect the virus. (Retraction Watch contains more details.) It is also noteworthy that the paper contains only four infected people and no control coughers, so it shouldn't be considered critical anyway. In an environment where there is so little quality information, the study had appeared in dozens of news reports.
To work around the problem of small, underserved studies like this, the World Health Organization asked a group of researchers at McMaster University to do a thorough review of the medical literature. The team included studies on the related coronaviruses SARS and MERS, as many studies had been completed with these previous viruses.
Despite these standards, researchers had difficulty finding detailed studies on the use of protective equipment. Despite the identification of results from a total of over 25,000 people involved in various studies, there were no randomized controlled trials among the studies they identified. Some of the studies did not even use WHO standards to determine who was infected.
While a meta-analysis can give a better idea of what is going on, although it is based on smaller studies that may not be conclusive in themselves, it is important to acknowledge that the source material here is not of a high quality.
In total, the authors found 172 observational studies that dealt with issues related to preventing the transmission of coronaviruses. Sixty-six of them focused on the distance at which viruses could be transmitted, providing information about the effectiveness of social distancing. Another 30 dealt with different types of face masks; 13 focused specifically on eye protection. Others either dealt with several topics or did not respond to any of the protective measures mentioned here. Less than 10 of these studies looked at COVID-19 cases; The rest focused on SARS or MERS caused by related coronaviruses.
Various measures of distance and infection were used in the underlying studies for the effects of distancing on transmission. The authors explained this by running over 10,000 randomized models to determine what was required for the results of previous work. These indicated that there was strong evidence that significant protection of at least one meter from infected people provided significant protection. There was weaker evidence that an even greater distance was more effective.
Overall, this is what we learn at the population level, where there is strong evidence that various socially distancing rules are effective.
For face masks, the researchers found that the overall protective effect was significant, but the underlying evidence was weak. In other words, the data matches a variety of possible degrees of protection, but the most likely answer is that masks are very protective. One reason for this is that N95 masks provide better protection for multi-layer masks than single-layer masks.
This also affected the results in terms of the context in which the masks were effective. Since medical personnel had better access to N95 masks, the use of face masks seemed to be more effective there. However, when this was adjusted, the use of masks by the public also appeared to be protective. However, given the severe lack of N95 masks in many locations, it is not clear when the public can use this information to protect them.
The last piece of protective equipment they look at is glasses that also reduce the transmission of corona viruses. This is something that has not been emphasized much, at least if the medical staff had sufficient access to face protection. But eye protection is something that a large majority of the public probably already has access to.
Not the last word
The study has some obvious limitations: it tries to integrate a large amount of individual research results that can use different methods and standards of success. One thing that the authors acknowledge as not being taken into account is a measure of exposure duration that undoubtedly affects the effectiveness of various forms of protection. They also recognize that the context of exposure – for example in hospitals or in local public transport – can affect the effectiveness of various forms of protection.
The other big problem is that all three major emerging corona viruses – SARS, MERS, and SARS-CoV-2 – are treated as equivalent. Although they are all members of the same family of coronaviruses (the beta coronaviruses), they differ significantly. Above all, one of them binds to a completely different protein in the cells in order to gain access and trigger an infection.
Despite these limitations, it will likely be some time before we get better information as the focus has been on providing possible protection in the early stages of the outbreak. And at least some of the relevant factors, such as For example, how well protection limits the spread of particles that are expelled from a cough is not specific to a virus.
However, one important fact that the authors found is that none of these protective agents are complete. Any contact with infected people carries a risk. Therefore, our focus should be on reducing the overall risk of infection spreading, hoping that at some point we can reach the point where contact tracking and quarantine can severely limit further infections. To this end, a less effective form of protection that is used by a large number of people could have a greater impact than a highly effective form that is available to only a few.
Finding that out would be beyond the scope of this study. But at least people now have some information to inform the models that could test it.
The Lancet, 2020. DOI: 10.1016 / S0140-6736 (20) 31142-9 (Via DOIs).