Households are the site of most SARS-CoV-2 transmission globally. The study, which focussed 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.
While Covid vaccines do prevent serious Covid illness and deaths, they are less effective at spreading infections, particularly since the emergence of the more infectious Delta variant.
Various studies have also shown the waning protection of these vaccines, effectively pressing the need for boosters.
"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.
"Vaccination reduces the risk of delta variant infection and accelerates viral clearance. Nonetheless, 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. Host-virus interactions early in infection may shape the entire viral trajectory," Lalvani said.
Further, the study showed that the susceptibility to infection increases already within a few months after the second vaccine dose, pressing the need for booster shots.
Continued public health and social measures to curb transmission -- such as mask wearing, social distancing, and testing -- thus remain important, even in vaccinated individuals, the study suggested.