Medical journal The Lancet has conducted what it claims to be its most comprehensive study to date on the effects of optimal physical distancing, face masks, and eye protection to prevent the spread of Covid-19.
The journal found that physical distancing of at least 1 metre lowers the risk of Covid-19 transmission, but a distance of 2 metres could be more effective.
Face coverings and masks might protect both healthcare workers and the general public against infection with Covid-19, and protective eye covering may also provide an additional benefit – although the certainty of the evidence is low for both forms of protection.
“Importantly, even when properly used and combined, none of these interventions offers complete protection and other basic protective measures (such as hand hygiene) are essential to reduce transmission,” The Lancet said.
The study, conducted to inform WHO guidance documents, is the first time researchers have systematically examined the optimum use of these protective measures in both community and healthcare settings for Covid-19, The Lancet said.
It noted that many countries and regions have issued conflicting advice about physical distancing to reduce transmission of Covid-19, based on limited information.
“Our findings are the first to synthesise all direct information on Covid-19, SARS, and MERS, and provide the currently best available evidence on the optimum use of these common and simple interventions to help flatten the curve and inform pandemic response efforts in the community,” said Professor Holger Schünemann from McMaster University in Canada, who co-led the research.
“Governments and the public health community can use our results to give clear advice for community settings and healthcare workers on these protective measures to reduce infection risk.”
The best available evidence suggests that Covid-19 is most commonly spread by respiratory droplets, especially when people cough and sneeze, entering through the eyes, nose, and mouth, either directly or by touching a contaminated surface.
At the moment, although there is consensus that SARS-CoV-2 mainly spreads through large droplets and contact, the debate continues about the role of aerosol spreading.
For the current analysis, an international team of researchers did a systematic review of 172 observational studies assessing distance measures, face masks, and eye protection to prevent transmission between patients with confirmed or probable Covid-19, SARS, or MERS infection and individuals close to them (eg, caregivers, family, healthcare workers), up to 3 May 2020.
Pooled estimates from 44 comparative studies involving 25,697 participants were included in the meta-analysis. Of these, 7 studies focused on Covid-19 (6,674 participants), 26 on SARS (15,928), and 11 on MERS (3,095).
The Covid-19 studies included in the analysis consistently reported a benefit for the three interventions and had similar findings to studies of SARS and MERS.
Analysis of data from nine studies (across SARS, MERS and Covid-19, including 7,782 participants) looking at the physical distance and virus transmission found that keeping a distance of over one metre from other people was associated with a much lower risk of infection compared with less than one metre (risk of infection when individuals stand more than a metre away from the infected individual was 3% vs 13% if within a metre).
However, the modelling suggests for every extra metre further away up to three metres, the risk of infection or transmission may halve.
The authors noted that the certainty of their evidence on physical distancing is moderate and that none of the studies quantitatively evaluated whether distances of more than 2 metres were more effective, although meta-analyses provided estimates of risk.
Thirteen studies (across all three viruses, including 3,713 participants) focusing on eye protection found that face shields, goggles, and glasses were associated with a lower risk of infection, compared with no eye covering. The authors noted that the certainty of the evidence for eye coverings is low.
Evidence from 10 studies (across all three viruses, including 2,647 participants) found similar benefits for face masks in general (risk of infection or transmission when wearing a mask was 3% vs 17% when not wearing a mask).
For healthcare workers, N95 and other respirator-type masks might be associated with greater protection from viral transmission than surgical masks or similar.
For the general public, face masks are also probably associated with protection, even in non-health-care settings, with either disposable surgical masks or reusable 12-16 layer cotton ones.
However, the authors noted that there are concerns that mass face mask use risks diverting supplies from health-care workers and other caregivers at the highest risk for infection.
Lead author, professor Raina MacIntyre from the Kirby Institute, University of New South Wales in Australia, noted that the report found that respirators and multilayer masks are more protective than single layer masks.
“This finding is vital to inform the proliferation of homemade cloth mask designs, many of which are single-layered.
“A well-designed cloth mask should have water-resistant fabric, multiple layers, and good facial fi. Universal face mask use might enable safe lifting of restrictions in communities seeking to resume normal activities and could protect people in crowded public settings and within households.”