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- Fully vaccinated people can catch and pass on the Delta variant at home, real-world data shows.
- Vaccines largely protect against severe COVID-19 but don't stop it spreading entirely, the study authors said.
- Experts urged people to get vaccinated, have boosters when eligible, and take extra precautions.
Fully vaccinated people can catch and pass on the highly infectious Delta variant at home, including to other vaccinated people – but unvaccinated people remain most at risk, real-world data shows.
A study, led by Imperial College London and published in Lancet Infectious Diseases on Thursday, identified 71 people who had caught COVID-19 caused by the Delta variant.
Out of these people's fully vaccinated household contacts, 25% caught COVID-19, compared to 38% of their unvaccinated household contacts, the study found.
Ajit Lalvani, chair in infectious diseases at Imperial College London, who co-led the study, said in a statement that vaccines were "not enough" to stop people getting infected with the Delta variant and spreading it at home. "This is likely to be the case for other indoor settings where people spend extended periods of time in close proximity," he said.
Dr. Anika Singanayagam, a researcher at Imperial College London who co-lead the study, said in a statement that it was "essential" for unvaccinated people, who remain at risk of severe illness, to get a COVID-19 shot. About three-quarters of people in the UK are fully vaccinated, official data shows.
Singanayagam said that fully vaccinated people become more susceptible to COVID-19 "within a few months" after the second vaccine dose. "So those offered a booster should get it promptly," she said.
Singanayagam added that the study provided "important insights" into why the Delta variant was "causing high COVID-19 case numbers around the world, even in countries with high vaccination rates."
"Continued public health and social measures to curb transmission thus remain important, even in vaccinated individuals," she said.
Peter Openshaw, professor of experimental medicine at Imperial College London, who wasn't involved in the research, said in a statement that the results show "you have a good chance of not getting infected if you're living with someone with COVID-19 and your chances are better if you have been recently double-vaccinated."
But, "if you want to avoid being infected you still need to do everything you can to avoid close contact, wear a mask and wash your hands even if you have been vaccinated," he said.
Dr. Simon Clarke, associate professor of cellular microbiology at University of Reading, said in a statement that vaccines "drive down" COVID-19 infections, but were "not a silver bullet."
"Infection in the wider community can still be amplified by transmission at home," he said. Clarke cautioned that it would be a "grave mistake" to assume that households were the only place where transmission occurs.
The researchers from Imperial College London, Manchester NHS Foundation Trust, and the UK Health Security Agency used the UK's central contact tracing system to identify 621 participants and tested them for COVID-19 regardless of symptoms with a lab test. Of the 621 participants, 163 had COVID-19, and 71 of these were caused by the Delta variant.
Of those infected with Delta, 54% were fully vaccinated, 32% were unvaccinated and 14% had received one vaccine dose, the study authors said.
They then looked at the number of people infected by the participants at home and the vaccination status of those that caught the virus. The study, funded by the National Institute for Health Research, ran between September 2020 to September 2021.