(Bloomberg Opinion) -- We recently indulged in what felt like an illicit pleasure. As the novelty of Britain’s lockdown was fast turning to ennui, friends invited us to their garden for a drink. Boris Johnson had just moved the government’s messaging from “stay at home” to the more nebulous “stay alert.” Defcon 5 was lowered to Defcon 3ish. Nobody quite knew the boundaries. Could we? Should we?

Reader, we did. We entered decorously through a side gate rather than traipse through their home. They prepared a separate cheese plate for us and our own nibbles bowl. We left an empty seat between each of us around a large garden table and drank from separate bottles.

It worked great, but just don’t try to scale it. Even an impromptu drinks gathering required added space, fewer people, altered pathways and duplicate supplies. As businesses start retrofitting office spaces, removing every other restaurant table, installing screens and one-way systems, and imposing onerous limits on travel and the like, it will change how people work and consume. The capital stock of the modern economy is designed around large numbers of people coexisting in sometimes small spaces.

In the two meters (six feet) of separation world, the University of Cambridge has cancelled its in-person lectures for the next academic school year. Only 50 lawmakers can occupy a 650-person House of Commons. And with one week to go before some kids return to class in England, there are febrile debates over safety.

In the U.K.’s fabled and much-loved pub sector, some establishments are lucky enough to have big rooms and gardens. But many others feature cozier environments, with small spaces, narrow corridors and nooks with stools. Industry representatives say only about 30% of pubs will be able to reopen with two-meter distancing, while more than 80% could reopen with a one-meter rule. 

There is no question that social distancing has been critical to controlling the spread of the virus. But measures this extreme can only be undertaken in the name of saving lives and preventing a new spike in infections. All U.K. policy, the government keeps saying, is “guided by the science.” That sounds great until you remember that science often contains considerable uncertainty and requires judgments to be made.

Two meters, by the way, isn’t the rule everywhere. Britain and the U.S. (where guidance is provided by the Centers for Disease Control and Prevention) go by two meters. France and Italy follow the World Health Organization guidance that one meter is fine; Australia says 1.5 meters.

The principle originated in research in the 1930s that measured the distance traveled by droplets from a cough or sneeze before they evaporated or fell to the ground. Since the main transmission risk of SARS-CoV-2, the virus that causes Covid-19, was thought to be droplets sprayed in this manner by an infected person, two meters seemed a sensible way to go, as it would also protect from smaller droplets, or aerosols. (The other big transmission risk comes from droplets falling on surfaces that people touch — ergo the need to constantly sanitize.)

Only it’s more complicated than that. Research published in the Journal of the American Medical Association by Lydia Bourouiba, a fluid dynamicist at Massachusetts Institute of Technology, suggests that the classifications guiding WHO policy, and those of most governments, are antiquated. That approach focuses on the ballistic, missile-like trajectory of particles of at least five to 10 micrometers in diameter. But Bourouiba’s research shows that pathogen-bearing droplets can be carried seven or eight meters by “turbulent” gas clouds.

There are other considerations too, like the size of a shared space and its air flow. Air droplets can also hang around for hours. Viral RNA can sit on surfaces for up to a few days. And there’s a lot that must still be learned about the threat posed by smaller particles and those traveling longer distances with various viral payloads.

From this perspective, the two-meter rule is more sensible; most people will try to observe something close to it. But the MIT research raises the question of how much more protection it will offer than a one-meter limit. The U.K. and U.S. rule is based on a fair amount of educated guesswork about a virus whose behavior isn’t entirely understood. If anything, the research reinforces the case for more contact tracing, constant testing, better protective equipment and general face-mask wearing (though there is still little research on the effectiveness of homemade face masks against the coronavirus).

Yet for countries that failed to establish those prophylactic measures, extreme social distancing has become the fallback policy. In other words, it is a matter not of decided science but political choice. 

The question is not simply how much distancing we really need, but how much can we afford. Two meters is perfectly sensible guidance for individuals, but the two-meter city is going to feel like the worst of both worlds — a major dampener on the economic recovery, which doesn’t even offer enough real security from transmission.

Politically, this is a tricky call for Johnson. Scrapping the two-meter rule could undermine any future claims that U.K. policy is based on science, which has been the government’s mantra. Many will conclude that he is putting the economy before saving lives.

Much depends on how quickly and effectively the government can further ramp up its testing and roll out its new contact-tracing policy. The decline in new infections over recent days will also give the government some hope that perhaps the disease will burn itself out.

Johnson could argue that more conscientious mask-wearing by the general population will enable the distancing guideline to be reduced. Or, as the economic costs become more painful, the strong public support for lockdown measures could shift and Johnson may be able to make the case he has so far refused to make: that a two-meter world may be a little safer for some, but it will also be poorer and more destructive for others.

This column does not necessarily reflect the opinion of the editorial board or Bloomberg LP and its owners.

Therese Raphael is a columnist for Bloomberg Opinion. She was editorial page editor of the Wall Street Journal Europe.

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