Covid-19: Study suggests Delta subvariant less likely to cause symptoms

A subvariant of Delta that is growing in Britain is less likely to lead to symptomatic Covid-19 infection, a coronavirus prevalence survey found, adding that overall cases had dropped from a peak in October.

The Imperial College London REACT-1 study, released on Thursday, found that the subvariant, known as AY.4.2, had grown to be nearly 12 per cent of samples sequenced, but only a third had "classic" Covid symptoms, compared with nearly a half of those with the currently dominant Delta lineage AY.4.

Two-thirds of people with AY.4.2 had "any" symptom, compared with more than three-quarters with AY.4.

AY.4.2 is thought to be slightly more transmissible, but it has not been shown to cause more severe disease or evade vaccines more easily than Delta.

The researchers said that asymptomatic people might self-isolate less, but also that people with fewer symptoms might spread it less easily through coughing and also may be unlikely to get severely ill.

The REACT-1 study also found that booster doses reduced the risk of infection in adults by two-thirds compared with people who had two doses.