Households are the site of most SARS-CoV-2 transmission globally. The study, which focused on densely sampled household contacts exposed to the delta variant, found that even with no or few symptoms, the chance of transmitting the virus to other unvaccinated housemates is about two in five, or 38 per cent.

This drops to one in four, or 25 per cent, if housemates are also fully vaccinated.

“This finding indicates that breakthrough infections in fully vaccinated people can efficiently transmit infection in the household setting,” said corresponding author Prof Ajit Lalvani, National Heart and Lung Institute, Imperial College London, UK.

The study included 440 households in London and Bolton doing PCR Covid tests between September 2020 and September 2021.

The results reveal that even after two doses of Covid vaccines people can appear to be just as infectious. In other words, although vaccinated people can clear the infection more quickly, their peak viral load — when people are most infectious — remains similar to that seen in unvaccinated people, meaning they can still readily pass on the virus in household settings.