Covid infection rates roughly doubled in the over-65s between September and October, according to researchers at Imperial College London who warn the virus is spreading from schoolchildren into more vulnerable age groups.

Scientists on the React-1 study analysed about 67,000 swabs taken across England between 19 and 29 October, and found higher rates of infection in every region apart from Yorkshire and the Humber when compared to September.

While infection rates were highest among schoolchildren, with nearly 6% of 5 to 17-year-olds testing positive, the study found rises in every age group, including the oldest and most vulnerable where rates doubled to 0.8% in 65- to 74-year-olds and 0.67% in the 75s and over.

“Although the rates are much lower in these older and more vulnerable people, we did see a doubling of rates in that group and clearly that’s a worry,” said Prof Paul Elliott, director of the React study. “It’s driven from young school age but going right across the population.”

The study, which analyses swabs from a representative group of people in the community whether they have symptoms or not, recorded its highest overall infection rate ever for England, at 1.72%. The study launched in May 2020, however, after the spring wave that year, and because of a pause in data taking, missed the peak of last winter’s wave.

By far the sharpest rise in cases between September and October was seen in the south-west, home to the 10 lower tier local authorities with the highest rates in the country. According to the study, prevalence of the infection almost quadrupled from 0.59% in September to 2.18% in October in areas around Bristol, Swindon and Gloucester.

Elliott said the study could not explain why cases had surged in the south-west, but added that the rise might be related to issues at the Immensa lab which issued tens of thousands of false positive test results in the region, leading infected people to believe they were safe to mingle. The UK Health Security Agency is investigating how the lab failed to spot the problem before the public did.

Genome sequencing of some of the virus samples revealed that the newly emerged relative of the Delta variant known as AY.4.2 is spreading faster than the original Delta variant. The AY.4.2 variant made up only 4.6% of cases in the September round of the React study, but rose to 10.3% in the latest October analysis. “It is slowly increasing and the UK Health Security Agency is keeping a watching brief on that,” said Prof Graham Cooke, a member of the team.

Towards the end of October, the study shows that infection rates began to fall across the country, including in the south-west, but the researchers stress that the decline coincides with the school half-term break when children mix far less than during term time. Further rises were still “very possible”, said Cooke.