Chance & Necessity

The results of the first, or at least one of the first, studies where volunteers are deliberately infected with COVID-19, have just been published. In an interesting and important paper by Killingley et al. Thirty-four volunteers were exposed to a dose of COVID-19*. As a result half of them became infected, and of these 18, 16 showed symptoms at at least one time during the infection, while 2 (11% of those infected) never showed any symptoms. One of the big advantages of studies like this, is that you know exactly when the person was exposed to virus, and can track them from that time onwards. If someone is infected in the community, you usually don’t know when they were infected, and even if you do, then you don’t know at the time, so there is a delay in studying them.

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Above is a figure from a recent study published by the American government’s CDC (Centers for Disease Control and Prevention)*. Essentially, Andreko and coworkers studied two populations in California: one of 600 people who had just tested positive for COVID-19, and one of 1,100 who had not. They asked both sets of people about mask wearing while in public settings indoors (i.e., shops, workplaces, etc). Then they got the risk reduction from the numbers of mask wearers in the two populations. The idea being that if masks work you would expect a higher fraction of the 1,100 who don’t have COVD-19 to wear masks, than the 600 people who do. The way to read the figure is that they estimate that wearing an N95/KN95 (similar to FFP2, KF94, …) while indoors reduces your risk of catching COVID-19 by 83%.

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Above is a plot of deaths of children from flu in the USA, from 2017 to 2020. The data is from the USA’s CDC. I think the data in the UK will be similar, although the numbers will be lower as we have about a fifth the population of the USA. I just want to make three observations about this data.

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Google recommended to me this article in Pulse: a publication for GPs. It made me wince quite a bit. A representative of the UK’s “NHS England national infection prevention and control team” was suggesting GPs and others wear surgical masks. The evidence is that FFP2s would be better, offer more protection. I was particularly struck by:

However, she [a “NHS England national infection prevention and control team member”] added that the UK Health and Safety Executive recommends healthcare professionals ‘should not use’ FFP2 masks and that ‘the only requirement to use them’ would be ‘if you couldn’t get FFP3’.

GPs should not routinely use FFP2 as surgical mask gives ‘very good protection’, Pulse, 19th Jan 2022

That is kind of true, the UK’s Health and Safety Executive says:

HSE’s current stance is that where there is a respiratory risk of infection use of FFP3 devices represents best practice, and where these are not available then FFP2 may be an acceptable, pragmatic compromise.

Evaluating the protection afforded by surgical masks against influenza bioaerosols (Research Report 619), Prepared by the Health and Safety Laboratory for the Health and Safety Executive 2008

So, the HSE (who are people in the UK government who understand masks – unlike apparently NHS infection prevention and control) are indeed advising against FFP2s when there is significant risk of infection. They are advising upgrading to a better mask. FFP3s are better than FFP2s. They are not recommending downgrading to the much worse surgical masks, as NHS national infection prevention and control is doing. Odd.

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