A study of almost 10 million people in Wuhan, China, has found “no evidence of  asymptomatic transmission” according to an editorial  in the British Medical Journal. 

“A mass screening programme of more than 10 million residents of Wuhan, China, performed after SARS-CoV-2 was brought under control, has identified 300 asymptomatic cases of covid-19, none of which was infectious,” the BMJ also reports.

However, the authors of the Wuhan study, which was published in Nature Communications, cautioned that the findings may not be the same in countries where outbreaks have not been brought under control successfully – and did not show the virus “couldn’t be passed on by asymptomatic carriers”.

The researchers tested the close contacts of the 300 people who had tested positive for having had Covid-19 but had never developed any symptoms. Prof Fujian Song, from the University of East Anglia’s Norwich Medical School, who collaborated on the study, said “this screening programme identified 300 asymptomatic cases. But the virus cultures indicated no viable virus in the identified asymptomatic cases. This means that these people were not likely to infect anyone else.”

Researchers then swab tested 1,174 close contacts of those 300 asymptomatic cases – and all came back negative.

The findings appeared to echo those of a meta-analysis published in the Journal of the American

Medical Association on December 14th and which looked at the transmission of the virus in households.

That paper found that transmission rates were much higher among people who had symptoms than among those who didn’t, with an 18% rate for symptomatic individuals, and a much lower rate of 0.7% for those who showed no symptoms.

It has long been observed that people showing symptoms of illness, such as coughing or sneezing, are more likely to spread a virus. However, it is more difficult to  track and test asymptomatic carriers of a disease, and household transmission may not always extrapolate to the wider community.

However, some commentators believe that the findings should give policymakers pause in deciding lockdown measures.

Calling for a rethink of the strategy of mass testing, the British Medical Journal editorial says that scarce resources need to be put to the most effective use. “Mass testing risks the harmful diversion of scarce resources,” they warned, adding that “prioritising rapid testing of symptomatic people is likely to have a greater impact on identifying positive cases and reducing transmission than frequent testing of asymptomatic people in an outbreak area.”

“The absence of strong evidence that asymptomatic people are a driver of transmission is another good reason for pausing the roll out of mass testing in schools, universities, and communities,” the editors said.