Vaccination and boosters reduce transmission: Study

Recent vaccination and boosters reduce infectiousness, but infection risk remains high, a study in California prisons found.

According to the researchers, 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 the University of California San Francisco, US, 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, it said.

The study also 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 per cent for each additional dose, it said.

“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, senior author of the study.

“Our findings are particularly relevant to improving health for the incarcerated population,” said Lo.

According to the study, the researchers analysed de-identified data collected by the California Department of Corrections and Rehabilitation (CDCR), US. The data included COVID-19 test results, vaccine status and housing locations for 111,687 residents, 97 per cent of whom were male, between 15th December, 2021 and 20th May, 2022.

The findings of the study have been published in the journal Nature Medicine.

Breakthrough infections were common, despite the residents’ relatively high vaccination rate of 81 per cent with the primary vaccine series. However, the rate of serious illness was low. In just over five months, there were 22,334 confirmed SARS-CoV-2 Omicron infections, 31 hospitalisations and no COVID-19 deaths, the study said.

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

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 per cent for someone who had never been infected, the study said.

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 study said.

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,” said Tan.

This includes making efforts to keep residents current with boosters and increasing the vaccination rate of the prison staff, only 73 per cent of whom had received the primary series at the time of the study, the study said.

The general rate of boosting could also be improved significantly. At the time of the study, just 59 per cent of residents and 41 per cent of staff had received all the doses recommended by the US Centres for Disease Control and Prevention (CDC), based on their age and health status, the study said.

“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,” said Lo.

“New ideas are needed since the risk of infection in this vulnerable population remains so great,” said Lo.

Edits by EP News Bureau