An article published online in Lancet last week provides the most complete assessment of the evidence regarding the effectiveness of various strategies to prevent transmission of SARS viruses. It's open access, and here's the link: https://www.thelancet.com/journals/lancet/article/PIIS0140-6736(20)31142-9/fulltext?dgcid=raven_jbs_etoc_email
The authors carried out a search of all relevant published and unpublished studies and "...identified 172 observational studies across 16 countries and six continents, with no randomised controlled trials and 44 relevant comparative studies in health-care and non-health-care settings (n=25 697 patients)."
The key findings were:
1. "Transmission of viruses was lower with physical distancing of 1 m or more, compared with a distance of less than 1 m.......; protection was increased as distance was lengthened......"
2. "Face mask use could result in a large reduction in risk of infection......., with stronger associations with N95 or similar respirators compared with disposable surgical masks or similar......"
3. "Eye protection also was associated with less infection.........”
The data indicated that all three strategies were highly effective, although the evidence was assessed as strongest for physical distancing. Given the varying opinions about masks, it's worth noting that the analyses indicated that risk of infection in people wearing masks was reduced by up to 85% relative to people not using masks. The analyses focused on the reduction in infection risk associated with using a mask yourself; the possible ancillary benefits for others in the community were not measured. Also, the possible effectiveness of strategies, such as hand washing, avoiding public indoor environments, etc. were not included. Nevertheless, this combined analysis appears to be the most careful and comprehensive assessment of the effectiveness of three major preventive measures. Although the report findings are limited by the absence of randomized studies, given the many practical and ethical challenges of performing such investigations, it probably represents the best information that we're likely to have for the foreseeable future.