Vaccination alone is not enough to stop the household transmission of the delta variant of Covid-19, which now accounts for 99.8 per cent of infections in the UK.

One in four fully vaccinated people who have exposure to the delta variant at home are likely to become infected themselves, according to a recent paper in The Lancet Infectious Diseases.

It found that while vaccination reduces the risk of delta variant infection and accelerates viral clearance, fully vaccinated individuals with breakthrough infections have peak viral load similar to unvaccinated cases and can efficiently transmit infection in household settings, including to fully vaccinated contacts.

Researchers from Imperial College London, the UK Health Security Agency, and Manchester Foundation NHS Trust collaborated to carry out this “real life” study of household transmission, the setting where most Covid transmission occurs.

In the 12 months to September, 2021, 621 participants (602 contacts identified via the UK Test and Trace, and 19 index cases)  from 471 index notifications were recruited to the Assessment of Transmission and Contagiousness of Covid-19 in Contacts (ATACCC) study. The subjects contributed 8,145 upper respiratory tract samples from daily sampling for up to 20 days. Transmission risk by vaccination status was analysed for 231 contacts exposed to 162 epidemiologically linked delta variant-infected index cases.

The study found that the ‘secondary attack rate’ in household contacts exposed to the delta variant was 25 per cent for fully vaccinated individuals compared with 38 per cent in unvaccinated individuals. This is higher than estimates of UK national surveillance data (10.8 per cent), which probably underestimates the secondary attack rate, the researchers note.