Person-to-person transmission of SARS-CoV-2: a systematic review
This briefing note was distilled by Dr. John Hollins, past Chair Canadian Club of Rome, from a science assessment by six authors, including Dr. Derek Chu of McMaster University, on behalf of the World Health Organisation’s SURGE1 group. It was published in The Lancet on 2020 June 1 (The Lancet).
This note also draws on an article in CBC News on 2020 June 24.
Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) causes COVID-19 and is spread person-to-person through close contact. This study investigated the effects of physical distance, face masks, and eye protection on virus transmission in health-care and other settings.
Funding: World Health Organisation. Approach
The researchers obtained data from 21 sources for SARS-CoV-2 and the betacoronaviruses that cause Severe Acute Respiratory Syndrome and Middle East Respiratory Syndrome. The research considered 172 observational studies from 16 countries on six continents, mainly in health-care settings (n=25,697 patients). The assessment cites 87 scientific papers. It screened records, extracted data and assessed the risk of bias. It rated the statistical certainty of the evidence that was assessed.
The study focussed on what transmission actually happened at zero to two metres, rather than measurements of how far aerosols and
droplets travel. Transmission of viruses was lower with physical distancing of 1 metre or more, compared with a distance of less than
1 metre (moderate certainty2). Protection increased as distance was lengthened (moderate certainty3).
The use of face masks could result in a large reduction in risk of infection (low certainty4), with stronger associations with N95 or similar respirators compared with disposable surgical masks or similar (e.g., reusable 12–16-layer cotton masks; (low certainty5). Eye protection was associated with less infection (low certainty6).
The findings of this systematic review and meta-analysis support physical distancing of 1 metre or more and provide quantitative estimates for models and contact tracing to inform policy. Optimum use of face masks, respirators, and eye protection in public and health-care settings should be informed by these findings and contextual factors.
Robust randomised trials are needed to better inform the evidence for these interventions, but this systematic appraisal of currently best available evidence might inform interim guidance.
This review, conducted in short order by a large multi-national team provides a substantial overview of the current scientific understanding of the transmission of a virus between persons. The understanding is modest. The level of statistical certainty is between low and moderate. The authors’ interpretation is presented in the conditional tense.
Public health officials and the decision makers they advise have been flying blind. With this review and the practical experience accumulated in many countries during the past six months, the pilots have a somewhat better view, but they are not yet out of the
On CBC, author Dr. Chu said “The Catch-22 is the available information is less than ideal. So, what to do?” I observe that human beings generally are not well equipped to deal with probabilities and that communication with a public audience has to be straightforward, simple, and transparently honest. It would be constructive, in my opinion, if all public health officials and political figures openly acknowledged that a rule like two metres physical separation is inherently arbitrary, rather than lay down inflexible rules as a truth from on high.
The authors’ final point in this summary—
Robust randomised trials are needed —is salient. The question it begs is why in 2020 is understanding of the transmission between
persons of viruses like SARS-CoV-2 as limited as it is? The Medical Research Council of Canada and the Natural Sciences and Engineering Research Council together spend more than $2 billion a year on research. What would it take to put this issue high on their agenda?
1 COVID-19 Systematic Urgent Review Group Effort, set up by the World Health Organisation.
2 (n=10 736, pooled adjusted odds ratio [aOR] 0·18, 95% CI 0·09 to 0·38; risk difference [RD] -10·2%, 95% CI -11·5 to -7·5)
3 (change in relative risk [RR] 2·02 per m; pinteraction=0·041)
4 (n=2647; aOR 0·15, 95% CI 0·07 to 0·34, RD -14·3%, -15·9 to -10·7;
5 (pinteraction=0·090; posterior probability >95%),
6 (n=3713; aOR 0·22, 95% CI 0·12 to 0·39, RD -10·6%, 95% CI -12·5 to -7·7;
2020 June 24