Recent COVID-19 vaccination and previous infections independently lowered the odds of Omicron variant transmission from infected California prison inmates to their cellmates, but the risk remained high, suggests a study published earlier this week in Nature Medicine.
University of California at San Francisco (UCSF) researchers led the study of 111,687 inmates in 35 prisons populated mostly by men (97%) from Dec 15, 2021, to May 20, 2022. The California Department of Corrections and Rehabilitation supplied the data.
Despite 81% uptake of the primary COVID-19 vaccine series, breakthrough infections were common. The rate of severe disease was low, however; of the 22,334 inmates tested positive, 31 were hospitalized, and none died.
Unvaccinated, infected inmates had an estimated 36% risk of spreading the virus, compared with 28% among infected vaccinees. After adjustment, any vaccination, previous infection alone, and both vaccination and previous infection cut the risk of SARS-CoV-2 transmission by 22%, 23%, and 40%, respectively.
Booster doses and more recent vaccination further lowered infectiousness among vaccinated inmates, with each dose conferring an 11% risk reduction; the risk of transmission rose 6% for every 5 weeks that had elapsed since the last shot.
Vaccinating more inmates, staff
"A lot of the benefits of vaccines to reduce infectiousness were from people who had received boosters and people who had been recently vaccinated," senior author Nathan Lo, MD, PhD, said in a UCSF news release. "Our findings are particularly relevant to improving health for the incarcerated population."
A lot of the benefits of vaccines to reduce infectiousness were from people who had received boosters and people who had been recently vaccinated.
The researchers urged prisons to keep inmates up to date with boosters, as only 59% of inmates and 41% of staff had done so, and to increase vaccination among staff, as only 73% of them had completed the primary series.
"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 said. "New ideas are needed, since the risk of infection in this vulnerable population remains so great."