Researchers from the BSU and Department of Medicine, at the University of Cambridge; Imperial College London; and St George’s, University of London, published an editorial piece in the British Medical Journal today titled “Lightening the viral load to lessen COVID-19 severity”.
In this article, the researchers bring together evidence suggesting that severity of COVID-19 depends on the amount of virus that an individual is exposed to at the time of infection. This implies that separation measures such as mask wearing that lessen viral transmission will also reduce risk of severe disease.
The authors propose three main reasons for why rates of severe illness and mortality in those with COVID-19 have reduced as the pandemic has progressed. First, the age demographic of cases has changed, with proportionally fewer infections in over-60s. Second, treatment has improved. Randomized trials have helped us to identify which drugs are effective at treating COVID-19. Clinical management of patients has additionally improved through accumulated experience of managing the disease. The third proposed reason is that individuals have on average a lower intake of virus at the time of infection, due to increased adherence to separation measures, such as social distancing, mask wearing, and good ventilation.
While randomized trials comparing individuals exposed to a greater or lower dose of the virus for COVID-19 would be the “gold standard” evidence for demonstrating a link with disease severity, there are other valuable sources of evidence. Randomized trials that vary viral dose have been performed for similar diseases in humans, including influenza, and for COVID-19 in animals, including hamsters and ferrets. These all show reduced rates of severe disease in those exposed to lower viral dose. Masks are known to reduce spread of virus particles, and hence reduce exposure to the virus. Use of masks has been linked to lower rates of severe COVID-19 outcomes for humans in several observational studies, as well as for hamsters in laboratory conditions. While researchers did not attempt to make hamsters wear masks, they put up surgical masks across the partitions between groups of hamsters, resulting in lower rates of disease transmission and less severe outcomes compared to hamster groups having partitions with air circulating freely.
This evidence has clear public health implications, suggesting that separation measures will reduce severity of infection in addition to slowing the spread of the disease. This provides additional motivation for measures such as mask wearing, social distancing, and good ventilation, even if they are not implemented perfectly.
Stephen Burgess, group leader at the Medical Research Council Biostatistics Unit, University of Cambridge said:
“Wearing a mask and only meeting in well-ventilated areas will not only reduce the risk of catching coronavirus, but also reduce the chances of developing severe disease if you do get infected.”
Dipender Gill, a clinician scientist at Imperial College London and St George’s, University of London said:
“The available evidence supports the implementation of public health measures that are most effective for reducing levels of viral exposure. This will help create a balance between controlling the COVID-19 pandemic while allowing society to continue to operate.”
Burgess S, Smith D, Kenyon JC, Gill D. Lightening the viral load to lessen COVID-19 severity. BMJ 2020; 371 doi: https://doi.org/10.1136/bmj.m4763
A copy of the paper is available to download here: Lightening the viral load to lessen COVID-19 severity | The BMJ