LONDON — The delta subvariant recently identified in the U.K. is now twice as prevalent in England as it was two months ago, a government-backed study has found.

Published Thursday, an update to an ongoing study of Covid-19 in the U.K. found that the mutation — known as AY.4.2. — made up 12% of all new cases of the virus between Oct.19 and Nov. 5.

The study, which looked at swab samples from 100,000 people in England, was carried out by researchers at Imperial College London and Ipsos MORI, and was commissioned by the U.K.'s Department of Health and Social Care.

All of the strains identified in the analysis were delta or delta sub-lineages, with AY.4.2. making up 12% of all positive test samples.

That means the AY.4.2. subvariant grew by 2.8% every 10 days since late September. It is now the second most dominant strain of Covid, after its predecessor AY.4.

In the week beginning Sept. 27, AY.4.2. made up 6% of new Covid cases in the U.K., according to the country's Health Security Agency. By late October, the subvariant made up 10% of all new cases.

The subvariant — which is thought to have emerged in the U.K. over the summer — has two additional mutations affecting its spike protein, part of the virus's structure used to infiltrate cells. Questions are still hanging over exactly how, or if, those mutations will affect how quickly it spreads.

However, scientists have suggested that even though AY.4.2. may have a transmission advantage, it is not necessarily cause for alarm.

AY.4.2 causes milder disease

Thursday's findings also showed that the AY.4.2. mutation was less likely to cause symptomatic infection than other strains of the virus.

Two-thirds of people with AY.4.2. reported "any symptom," compared to more than 75% of those with the U.K.'s dominant AY.4 strain.

Just a third of those infected with the AY.4.2. subvariant had "classic Covid-19 symptoms" — such as a cough or a temperature — compared to almost half of the people with AY.4.

The U.K. recorded 38,263 new cases of Covid on Wednesday, and currently has one of the highest infection rates in the world, according to data compiled by Johns Hopkins University.