Double jabbed people are catching Covid and passing it on to those they live with, warn experts who have studied UK household cases.
Individuals who have had two vaccine doses can be just as infectious as those who have not been jabbed.
Even if they have no or few symptoms, the chance of them transmitting the virus to other unvaccinated housemates is about two in five, or 38%.
This drops to one in four, or 25%, if housemates are also fully vaccinated.
The Lancet Infectious Diseases work shows why getting even more people vaccinated and protected is important, they say.
Unvaccinated people cannot rely on those around them being jabbed to remove their risk of getting infected, they warn.
Vaccines do an excellent job of preventing serious Covid illness and deaths, but are less good at stopping infections, particularly since the emergence of the more infectious Delta variant which is dominant in the UK.
And over time, the protection offered by vaccines wanes and needs boosting with further doses.
Since households are where most Covid transmission occurs, making sure every member who is eligible for a vaccine has had one and is up to date with their doses makes sense, say experts.
According to the study, which ran from September 2020 to September 2021 and included 440 households in London and Bolton doing PCR Covid tests:
- People who are double jabbed have a lower, but still appreciable, risk of becoming infected with the Delta variant compared with unvaccinated people
- They also appear to be just as infectious
- Vaccinated people clear the infection more quickly, but their peak viral load - when people are most infectious - is similar to that seen in unvaccinated people
- This may explain why they can still readily pass on the virus in household settings
Prof Ajit Lalvani, of Imperial College London, UK, who co-led the study, said: "The ongoing transmission we are seeing between vaccinated people makes it essential for unvaccinated people to get vaccinated to protect themselves from acquiring infection and severe Covid-19, especially as more people will be spending time inside in close proximity during the winter months.
"We found that susceptibility to infection increased already within a few months after the second vaccine dose - so those eligible for booster shots should get them promptly."
Co-author Dr Anika Singanayagam, also from Imperial, said: "Our findings provide important insights into the effect of vaccination in the face of new variants, and specifically, why the Delta variant is continuing to cause high Covid case numbers around the world, even in countries with high vaccination rates.
"Continued public health and social measures to curb transmission - such as mask wearing, social distancing, and testing - thus remain important, even in vaccinated individuals."