By Sola Ogundipe

Vaccination and boosting, especially when recent, helped to limit the spread of COVID-19 in California prisons during the first Omicron wave, according to an analysis by researchers at UC San Francisco that examined transmission between people living in the same cell.

The study demonstrates the benefits of vaccination and boosting, even in settings where many people are still getting infected, in reducing transmission. And it shows 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 percent for each additional dose.

The researchers analysed deidentified data collected by the California Department of Corrections and Rehabilitation (CDCR).

Breakthrough infections were common, despite the residents’ relatively high vaccination rate of 81 percent with the primary vaccine series. But the rate of serious illness was low.

In just over five months, there were 22,334 confirmed SARS-CoV-2 Omicron infections, 31 hospitalizations and no COVID-19 deaths.

Vaccinated residents with breakthrough infections were significantly less likely to transmit them: 28 per cent versus 36 percent for those who were unvaccinated. But the likelihood of transmission grew by 6 percent for every five weeks that passed since someone’s last vaccine shot.

Natural immunity from a prior infection also had a protective effect, and the risk of transmitting the virus was 23 per cent for someone with a reinfection compared to 33 percent for someone who had never been infected.

Those with hybrid immunity, from both infection and vaccination, were 40% 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 addition 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.