Jog on: why we shouldn’t worry about outdoor transmission

Didn’t that jogger come a bit close to me? Wasn’t he panting all over the place? Shouldn’t I have made more effort to step out of the way of that couple? We are all familiar with the park walk inner monologue. Indeed, during Lockdown 1.0, to go to one of London’s parks was to witness a weirdly dense new set of social norms in action – like warring cats, we dived and skipped and zigzagged our way about each other.

The debate over the extent of outside transmission of Coronavirus has intensified in the last few weeks – commentators have speculated whether the new variant is more transmissible outside. Indeed, with very few levers left to pull, the government may clamp down further on people perceived to be breaking one of the key exceptions to the general “stay at home” order – exercise. Mandatory mask wearing outside at all times is reported to be being considered.

And yet, a study conducted in northern Italy has concluded that outdoors transmission is rare except in very limited circumstances. Researchers at the Istituto di Scienze Dell’Atmosfera e Del Clima used models to work out how much virus would be emitted by infected individuals, what atmospheric conditions might aid transmission and how long the virus could be sustained under those conditions. The key finding is as follows: “assuming a number of infects equal to 10% of the population, the time necessary to inspire a quantum (i.e. the dose of airborne droplet nuclei required to cause infection in 63% of susceptible persons) would be 31.5 days in Milan (range 2.7–91 days) and 51.2 days in Bergamo (range 4.4–149 days).”

In layman’s terms, that means that if you had hundred people in the Piazza del Duomo in Milan, it would take 30 days for 63 of them to breathe in enough virus that they might then become infected. “The probability of airborne transmission due to respiratory aerosol is very low in outdoor conditions,” the researchers concluded.

Robert Dingwall, member of the New and Emerging Respiratory Virus Threats Advisory Group (NERVTAG), told me: “This is an important study, carried out during the peak of transmission in Northern Italy in March 2020. It is peer reviewed and published in a credible journal. As SAGE has observed, there is, at best, weak evidence for a very small benefit from face coverings.

“This study provides further evidence for the growing consensus that the risk of outdoor transmission is so small, in most circumstances, that face coverings cannot offer any benefit. As the authors note, more work is needed on crowded outdoor spaces, like Italian street markets, and indoor spaces. However, a recent SAGE review has also found that virus transmission is unlikely in large well-ventilated indoor spaces, like modern supermarkets and other public buildings.”

“We do seem to be trying to regulate people in a lot of places where the risks are zero to minimal, rather than concentrating on places where people really are packed in like sardines and the risks are clearer.”

The government should focus on communicating how much greater a risk of transmission in indoor settings with poor ventilation poses, rather than enforcing generalised mask wearing in settings where it makes no difference.