Covid boosters helped limit the spread of virus in California prisons during the first Omicron wave, according to an analysis by researchers at University of California, San Francisco (UCSF) that examined transmission between people living in the same cell.
The study, published in the journal Nature Medicine, demonstrated the benefits of vaccination and boosting, even in settings where many people are still getting infected, in reducing transmission.
It also showed the cumulative effects from boosting and the additional protection that vaccination gives to those who were previously infected. The likelihood of transmission fell by 11 per cent for each additional dose.
"A lot of the benefits of vaccines to reduce infectiousness were from people who had received boosters and people who had been recently vaccinated," said Nathan Lo, a faculty research fellow in the Division of HIV, Infectious Diseases and Global Medicine at UCSF.
The researchers analysed data collected by the California Department of Corrections and Rehabilitation (CDCR).
In just over five months, there were 22,334 confirmed SARS-CoV-2 Omicron infections, 31 hospitalisations and no COVID-19 deaths.
Vaccinated residents with breakthrough infections were significantly less likely to transmit them: 28 per cent versus 36 per cent for those who were unvaccinated.
However, the likelihood of transmission grew by 6 per cent for every five weeks that passed since someone's last vaccine shot.
Those with hybrid immunity, from both infection and vaccination, were 40 per cent less likely to transmit the virus.
Half of that protection came from the immunity that one acquires from fighting an infection and the other half came from being vaccinated.
The researchers said they were gratified to see that vaccination confers additional protection even for those who had already been infected, but they were surprised by how much the infection continued to spread, despite the residents' relatively high vaccination rates.
"Regardless of the benefits you see in vaccination and prior infection, there is still a high amount of transmission in this study," said Sophia Tan, the study's first author. "We hope these findings can support ongoing efforts to protect this vulnerable population."
"Within the two months following vaccination, people are the least infectious, which indicates that boosters and large timed vaccination campaigns may have a role to reduce transmission in surges," Lo added. (SJ/IANS)