In most age groups in England, breakthrough infections are higher now than they were in mid-August, according to data from the U.K. Health Security Agency, formerly Public Health England.

That rise has been especially stark in people in their 40s. In the four weeks to Oct. 31, 2.1% of fully vaccinated 40-to-49-year-olds tested positive for the virus. That is up around 90% from a four-week infection rate of 1.1% in mid-August. Other age groups have seen more modest increases—between 22% and 56%—in the rate of breakthrough infections. In under-30s, the rate is now lower than it was in mid-August.

Ajit Lalvani, chair of infectious diseases at Imperial College London and lead author of the household-transmission study, said people in their 40s were at higher risk of breakthrough infection for two reasons. “Waning immunity plus pools of unvaccinated people acting as vectors of infection into the household where it transmits effectively to vaccinated parents,” he said. “Both are happening.”