Booster vaccination provided additional protection for residents and staff of California state prisons against SARS-CoV-2 Omicron


In a recent study published in PLOS ONEresearchers assessed the extent to which prior infection and vaccination protected residents and staff of the U.S. state of California prison system from breakthrough infection with severe acute respiratory syndrome coronavirus 2 (SARS-CoV- 2) Omicron subvariant B.1.1 .529.

Study: Omicron protection against vaccination and previous infections in a prison system. Image Credit: Tobias Arhelger/Shutterstock


Prisons and jails are risky environments where many outbreaks of coronavirus disease 2019 (COVID-19) have occurred during the ongoing pandemic. The California Department of Corrections and Rehabilitation (CDCR) operates the second largest US state prison system. They vaccinated their residents and staff so that 77.9% and 40.3% of their residents and staff had received a third (booster) dose of messenger ribonucleic acid (mRNA) vaccine by the end of August 2021.

Yet, they identified a case of Omicron within the CDCR system in December 2021. Shortly thereafter, there was an outbreak among their residents and staff, coinciding with the time of the global Omicron surge. Examination of these two high-risk populations ensured reliable determination of infection during the study period due to their high levels of testing. CDCR operates a voluntary multi-layered reverse transcriptase-polymerase chain reaction (RT-PCR) testing program in response to detected COVID-19 outbreaks. As part of this program, they are mandatorily testing all staff at least once a week and staff who work in health facilities at least twice a week.

About the study

In the current study, researchers collected data from approximately 60,000 incarcerated individuals and 17,000 prison staff in California during the Omicron outbreak period between December 24, 2021 and April 14, 2022, for the retrospective analysis of the risk of infection during the Omicron wave. They used weighted Cox models to compare vaccine efficacy (VE) and previous infection against the Omicron subvariant, B.1.1.529.

In particular, they took into account previous infections that had occurred in the study population before and during the period of predominance of the Delta variant. The models allowed baseline risk to vary by prison, and inverse probability weighting reduced confounding effects due to differences in baseline characteristics among cohort members.

The team calculated propensity scores to balance covariates by demographic, clinical, and prison characteristics. They weighted residents by prison, COVID-19 risk score (0, 1, or ≥2), room type, gender, and age. Similarly, they weighted staff by prison, position (custody or health care), age (18 to 55 or older), and gender. The team estimated how well the mRNA vaccines protected against infection stratified by the number of doses people had received and whether they had had a previously documented infection before the observation period began.

Another arm of the study included a rolling matched cohort design that assessed the effectiveness of three doses of vaccine in eligible inmates compared to those who received only two doses. The researchers only included residents of the study cohort who were imprisoned in a CDCR prison before January 1, 2021. Similarly, they covered correctional staff employed before January 1, 2021 who had worked all throughout 2021. Notably, as of January 1, 2022, more than 50% of prisons had entered an epidemic phase, with the epidemic in the last prison having started on January 18, 2022.

Study results

Among 59,794 residents who met the study inclusion criteria, 16.7% tested positive for COVID-19 during the study period. Similarly, 30.3% of 16,572 staff members tested positive for COVID-19 during the study period. The median interval from the start of their last infection to the prison outbreak was 393 days for residents; among staff, the same was 367 days.

The effectiveness of previous infection against Omicron infection in unvaccinated individuals ranged from 16.3% to 48.9%. Similarly, VE estimates ranged from 18.6% to 83.2% with two doses of vaccine and from 40.9% to 87.9% with three doses of vaccine. A booster dose increased VE estimates to 25% for people with no history of SARS-CoV-2 infection and 57.9% for people infected before the Delta-predominance period.

The secondary analysis used an alternative design that likely corrected for biases that would have biased the VE estimates downward. Thus, it more efficiently provided estimates among people who became eligible for a third dose of vaccine. Nevertheless, conservative estimates from both analyzes indicated additional protection of a booster dose of mRNA against confirmed infection, regardless of previous infection history.

Another notable observation from the study was that the resident population had lower VE estimates than the staff population for most combinations of exposure history. As testing was neither mandatory nor routinely done for residents, previous undiagnosed infections could have diluted estimates of VE among residents.


To conclude, although mRNA vaccines and earlier infections provided less protection against Omicron infectious breakthroughs compared to earlier variants of SARS-CoV-2, boosters continue to provide additional protection even in people previously infected. Thus, booster vaccination remains important for highly vulnerable populations bearing a disproportionate burden of COVID-19.

Journal reference:

  • Omicron protection against vaccination and prior infections in a prison system, Elizabeth T. Chin, David Leidner, Lauren Lamson, MS, Kimberley Lucas, David M. Studdert, Jeremy D. Goldhaber-Fiebert, Jason R. Andrews, Joshua A Solomon, NEJM 2022, do I: 10.1056/NEJMoa2207082

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