The researchers looked at 172 observational studies in 16 countries and six continents, including studies in health care and community settings. They do not see randomized controlled trials, considered the gold standard but it is almost impossible to apply to study transmission of infections in people.

Researchers at universities around the world analyzed studies of the Covid-19 outbreak, the acute SARS virus and the Middle Eastern respiratory syndrome virus (MERS), which came from the same virus family.

‘A big step forward’

The review results support a physical distance policy of at least 1 meter (3.3 feet) and, if feasible, 2 meters (6.6 feet) or more, the researchers said. The information can also be used to inform models that predict the spread of disease and help with contact tracking schemes, the research team, led by Holger Schünemann of McMaster University in Canada, added.

Trish Greenhalgh, professor of Primary Care Health Services at Oxford University who was not involved in the study, said that while all of these results were subject to uncertainty, the extraordinary message was that physical measures were effective in preventing Covid-19.

“This is a big step forward in our knowledge, because the previous meta-analysis is mainly based on the prevention of influenza and other diseases, which do not behave the same as the Covid-19 virus,” he told the Science Media Center in London.

“In all three questions, the evidence seems to support the steps. For example, on average, staying 1 meter from someone else seems to reduce your chance of capturing Covid-19 by 80%. Wearing a mask or facepiece seems to reduce your risk by 85%. And wearing glasses or face shields seems to reduce the risk by 78%. “

Pull Jašarević, a WHO spokesman, said the international health agency was in the process of updating its advice on wearing masks, with changes likely to be announced this week.

That change “tends to touch the best material that you can make yourself mask, based on the latest data from the US, “he told CNN via email.

“WHO has maintained its focus on the use of masks where there is evidence that they can limit transmission, such as in health care settings. In fact, new guidelines can expand their use in these settings,” he added.

The next step for government leaders

The key question now is how the government and society should interpret this result, said Linda Bauld, a professor of public health at the University of Edinburgh.

“The first and perhaps most useful finding is the problem of physical distance. There are many complaints that guidance in the UK at 2 meters is excessive because it is more than in other countries,” he told the Science Media Center.

“But this review supports it. Maintaining this distance is likely to reduce risk compared to 1 meter. Thus, if possible, this is the distance retailers and employers should use because more places and workplaces are reopened in the future.”

Bauld added that while the certainty of evidence was low on face masks, making it more difficult for the government to make this mandatory, reviews added to the evidence that leaders must ask the public to wear face masks on public transportation, in shops and other indoor spaces even when physical distance occurs, he said.

For healthcare workers, this study found that N95 masks and other respirator-type masks might provide greater protection from virus transmission compared with surgical masks or cotton masks or multiple layers of gauze.

However, the review did not see the effect of wearing masks to protect others, said Greenhalgh of Oxford. He said the general public must wear standard cloth masks or surgical masks, leaving respirator masks for health care workers.

Even when used and combined correctly, long distance, masks and eye protection do not offer complete protection, this research emphasizes, and basic steps such as hand washing are very important to reduce transmission.