The majority of SARS-CoV-2 transmission occurs in households around the world. The study, which focused on densely sampled household contacts exposed to the delta variant, discovered that even with no or few symptoms, the likelihood of transmitting the virus to other unvaccinated housemates is about two in five, or 38%.
If housemates are also fully vaccinated, this drops to one in four, or 25%.
While Covid vaccines do help to prevent serious Covid illness and death, they are less effective at spreading infections, especially since the emergence of the more infectious Delta variant.
Several studies have also shown that the protection provided by these vaccines is eroding, necessitating the use of 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.
Between September 2020 and September 2021, 440 households in London and Bolton participated in the study, which included PCR Covid tests.
The findings show that even after two Covid vaccine doses, people can appear to be just as infectious. In other words, while vaccinated people recover faster from infection, their peak viral load (when they are most infectious) remains similar to that of unvaccinated people, implying that they can still easily spread 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.
Furthermore, the study found that susceptibility to infection increases just a few months after the second vaccine dose, emphasising the importance of booster shots.
Continued public health and social measures to prevent transmission, such as mask wear, social distancing, and testing, are thus important, even in vaccinated individuals, according to the study.