A variant of the Delta virus that is more infectious, but is less likely to cause symptoms, now accounts for more than one in 10 COVID cases in England, new research shows.
The latest results from the long-running REACT-1 study shows the prevalence of the AY.4.2 variant has been growing at a rate of 2.8% a day since September.
The variant is an evolutionary spin-off from the original Delta virus and is being closely tracked by the UK Health Security Agency.
Tests carried out between 19 October and 5 November on around 100,000 randomly selected people showed that 11.8% carried the variant.
But the data also shows that only 33% of those with the variant had the classic COVID symptoms of a fever, persistent cough, or a loss or change in taste or smell – compared to 46% of those with Delta.
They were also less likely to show any other symptoms.
Professor Paul Elliott, director of the REACT programme at Imperial College London, said the variant appeared to be even more infectious than the more common Delta version of the virus.
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"Why it is more transmissible we don't know. It does seem to be less symptomatic, which is a good thing," he said.
Professor Christl Donnelly, another of the researchers, said it wasn't clear how the variant would impact the pandemic.
"If it is less likely to be symptomatic, then it means it gets tested for less, and people may be out," she said.
"On the other hand, if they are not coughing it might be spreading less far in distance."
The UK Health Security Agency declared AY.4.2 a 'variant under investigation' on 20 October.
Research so far suggests it is associated with a slightly lower risk of hospitalisation and death than Delta. There does not appear to be a significant drop in vaccine effectiveness.
The latest REACT-1 results also show the booster dose is highly effective, reducing the risk of infection by two-thirds compared to people who have only had two doses.
A single dose of the vaccine given to school children reduces the risk of infection by 56% compared to those who have not had the jab.