People who received a new COVID-19 vaccine booster were more likely to contract COVID-19 than people who received no COVID-19 vaccine doses, according to a new study of prisons in California.

Researchers analyzed data from 33 state prisons from January to July 2023 to try to assess the effectiveness of the bivalent shots, which were introduced in the fall of 2022.

Among 96,201 inmates with data on COVID-19 testing and vaccination, researchers identified 2,835 cases.

They found that 1,187 of the cases were among people who had received a bivalent vaccine, versus just 568 cases among the unvaccinated.

The rest were among people who received only monovalent, or old vaccines. That group was excluded from further analysis.

While the population of bivalent recipients was higher than the unvaccinated—36,609 compared to 20,889—the rate of infection was still elevated in the bivalent group owing to nearly double the number of infections, the researchers found.

Infection rates in the group that received bivalent shots was 3.2 percent, over the 2.7 percent in the unvaccinated.

“The bivalent-vaccinated group had a slightly but statistically significantly higher infection rate than the unvaccinated group,” Dr. Robert Mayes of the California Correctional Healthcare Services and the other authors wrote.