Why Not Face Shields Too?
Last March, I wrote a post suggesting that enough informal evidence was emerging that masks might provide protection against COVID-19 that states might consider enacting mask mandates, requiring at least cloth-type masks. I also suggested that RCTs of masks could be helpful in helping ascertain the effectiveness of masks. The rest is history. Mask mandates followed. RCTs did not.
This post asks a simple question: Why has there been no movement to require face shields in addition to masks in public places? My instinctive cost-benefit analysis suggests that such a regulation might be worthwhile, but that’s not the point of my post. After all, I recognize that there was no causal effect of my first post on the adoption of mask mandates, and I assess the plausibility of shield-plus-mask mandates at close to zero. Rather, I’m interested in the inquiry as a puzzle and as a window into how the public thinks about COVID risk-reduction measures.
Let me start with the affirmative case. Face shields are cheap, less than $1 each for reusable ones. They can be worn concurrently with other personal protective equipment. By themselves, they are not as effective as masks by themselves at protecting wearers from aerosol-sized droplets, and so the CDC does not recommend them “as a substitute for masks.” Still face shields at least “block the initial forward motion of the jet” of air from a cough, potentially reducing inhalation of virus particles. A study found that “wearing a face shield reduced the inhalational exposure of the worker by 96% in the period immediately after a cough.” A study suggests that a type of shield provides extra protection for ophthalmologists. Perhaps most impressively, after 19% of a group of community health workers in India working with Covid-19 patients themselves tested positive, face shields were added to their existing PPE, and there were no further positive tests.
None of these studies is a gold standard randomized controlled trial. But the studies of face masks generally aren’t RCTs either. One face mask study randomized some people to receiving a recommendation to wear a mask, and it found only statistically insignificant benefits of receiving the recommendation. The medical community has shrugged at the absence of evidence from controlled trials, pointing out that there are other forms of evidence and claiming that randomized controlled trials would be unethical.
The failure to run effective RCTs for both masks and face shields strikes me as a major failure in global efforts against this pandemic. Even if one believes the anecdotal evidence that masks are beneficial is clear, there are many seemingly intelligent people (not just Alex Berenson) who point to anecdotal evidence that mask-making doesn’t make a difference. Consider, for example, the comment from “Oleg” that Covid is spreading r
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