A derivative of the Delta variant of Covid-19 that is more infectious than the original strain has spread to account for one in 10 infections in England.
The AY.4.2 mutation has been increasing at a rate of 2.8 per cent per day since September, figures compiled by researchers in Imperial College London’s long-running React-1 study show.
British health officials are monitoring the strain as a “variant under investigation”.
Experts tested about 100,000 people selected randomly across most age groups and regions for coronavirus between October 19 and November 5 and said 11.8 per cent carried the variant.
Scientists said people infected with AY.4.2 are more likely to be asymptomatic compared to those who infected with other Covid-19 variants.
The study showed that one third of people who carry AY.4.2 displayed typical coronavirus symptoms such as a persistent cough, loss or change in taste and smell, and a fever. This compared to 46 per cent of people infected with the original Delta variant.
Prof Paul Elliott, director of the study, said the data suggested the strain was more infectious that the common Delta variant.
"Why it is more transmissible we don't know,” he said. “It does seem to be less symptomatic, which is a good thing.”
Christl Donnelly, professor of statistical epidemiology at Imperial College London, said the lower rate of infected people displaying symptoms could lead to an increase in transmission in England.
“It is absolutely the case that if people are waiting for symptoms to do a test and to therefore identify that they are infected and therefore cut back their contacts, being asymptomatic may facilitate transmission, for example,” she said.
“It is asymptomatic transmission that really can make the difference between what’s relatively easily containable and what needs vaccination.”
Researchers say the observational nature of survey data and the relatively small proportion of unvaccinated adults calls into question the comparability of vaccinated and unvaccinated groups.
However, they found that booster vaccines for eligible adults and the inoculation of children aged 12 and over were associated with a lower infection risk.
Experts at Imperial College London said these programmes should therefore remain a high priority for the UK government, which sets Covid-19 rules for England.
Devolved administrations in Scotland, Wales and Northern Ireland have the power to make their own decisions but often follow England in many areas.
The study has been released as a preprint that has not been through peer review and is not published in a journal.
