Can face masks reduce COVID severity and improve herd immunity?

‘[I]t’s possible that one of the pillars of COVID-19 pandemic control – universal facial masking – might help reduce the severity of disease and ensure that a greater proportion of new infections are asymptomatic.
 
‘[A]ny public health strategy that could reduce the severity of disease should increase population-wide immunity as well.’
 
That is the hypothesis of the authors of a new perspective piece recently published in the New England Journal of Medicine.
 
This idea is based on recent ‘virologic, epidemiologic, and ecologic’ data.
 
‘This possibility is consistent with a long-standing theory of viral pathogenesis, which holds that the severity of disease is proportionate to the viral inoculum received,’ the authors wrote.
 
‘Since masks can filter out some virus-containing droplets (with filtering capacity determined by mask type), masking might reduce the inoculum that an exposed person inhales.’
 
According to Associate Professor Hassan Vally, an epidemiologist with a special interest in infectious diseases at La Trobe University, this hypothesis seems sound.
 
‘It’s an interesting theory and it’s plausible, based on what we know about the viral dose or the infectious dose being important in determining your illness severity,’ he told newsGP.
 
‘We know that masks protect people from getting infected, but this is saying that even if you don’t completely prevent that happening – so if you only let a small number of virus particles in – then potentially it’s exposing you to enough of the virus for you to mount an immune response which is going to protect you in the future.’
 
Associate Professor Vally says this is not a new concept.
 
Indeed, the authors say mask-wearing could become a form of ‘variolation’ to generate immunity and slow spread of the virus while waiting for an effective vaccine.
 
The concept of variolation, the authors note, was utilised during the time of smallpox whereby susceptible people were inoculated with material taken from a vesicle of an infected patient.
 
The aim was to cause mild infection and subsequent immunity in the recipient.
 
Stephen Kleid, an ENT and head and neck surgeon at Peter MacCallum Cancer Centre and Masada and Epworth Freemasons hospitals in Melbourne, told newsGP that he did not think the idea held much merit when he first read through this new paper.
 
‘But it makes sense that if the size of the inoculum makes a difference, then those who do catch [SARS-CoV-2] with a mask on will get less [exposure to the virus],’ he said.
 
‘They’ll get less severe illness and they’ll get immunity.’
 
Increasing the number of people exposed to SARS-CoV-2 while wearing face masks may then increase the asymptomatic rate.
 
The authors say there is observational evidence for this hypothesis – the typical rate of asymptomatic infection with SARS-CoV-2 was estimated by the US Centers for Disease Control and Prevention (CDC) to be 40% in mid-July, but the rate of asymptomatic infection is reported to be higher than 80% in settings with universal facial masking.
 
‘Countries that have adopted population-wide masking have fared better in terms of rates of severe COVID-related illnesses and death, which, in environments with limited testing, suggests a shift from symptomatic to asymptomatic infections,’ the authors wrote.
 
The paper also offers examples to back the notion that mask-wearing increases the asymptomatic rate.
 
‘In an outbreak on a closed Argentinian cruise ship, for example, where passengers were provided with surgical masks and staff with N95 masks, the rate of asymptomatic infection was 81% (as compared with 20% in earlier cruise ship outbreaks without universal masking),’ the authors wrote.
 
‘In two recent outbreaks in US food-processing plants, where all workers were issued masks each day and were required to wear them, the proportion of asymptomatic infections among the more than 500 people who became infected was 95%, with only 5% in each outbreak experiencing mild-to-moderate symptoms.’


 Associate Professor Hassan Vally believes the theory is interesting, but is only hypothetical at this stage.

However, Associate Professor Vally says these examples do not prove that mask-wearing leads to a higher asymptomatic rate.
 
‘It’s very weak evidence scientifically at this stage, because there could be a whole lot of other differences between the examples that have been described to prove their point,’ he said.
 
‘So it needs to be tested in a more rigorous [manner] before anyone gets too excited.’
 
While the authors say reducing disease severity is one hopeful outcome associated with universal mask-wearing, another potential benefit may be its effect on herd immunity.
 
‘Promising data have been emerging in recent weeks suggesting that strong cell-mediated immunity results from even mild or asymptomatic SARS-CoV-2 infection,’ they wrote.
 
‘So any public health strategy that could reduce the severity of disease should increase population-wide immunity as well.’
 
Associate Professor Vally is cautious about linking this with the concept of herd immunity.
 
‘To get herd immunity you need a certain percentage of people who are going to get sick, and we’ve all agreed that we’re nowhere near that,’ he said ‘And if we were to get anywhere near that, that would be devastating in terms of the amount of viral spread we would have in the community.
 
‘I can see what they’re getting at, but the implications of this on developing herd immunity in a population in the real world needs more consideration.
 
However, Mr Kleid believes universal mask-wearing is theoretically likely to boost the number of cases with asymptomatic disease. But whether that will then have an impact on herd immunity is another question.
 
‘There are just so many unknowns,’ he said.
 
Even if these hypothetical additional benefits of face masks outlined in this paper are proven, Associate Professor Vally does not think it will alter current practices relating to their use. 
 
‘This is the bit I struggle with,’ he said.
 
In places with high transmission rates of SARS-CoV-2, Associate Professor Vally believes masks should be a standard measure for their established ability to reduce spread.
 
‘So I don’t think it’s going to change anything,’ he said.
 
The authors also stress that their ideas remain hypothetical and further studies are required to test their validity.
 
‘This is where science has to step in and investigate it further,’ Associate Professor Vally said.
 
‘And then the question is, what does it mean? What would we do differently if this is shown to be likely to be true or possibly to be true?
 
‘I think all those things need to be worked through.’
 
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