Effectiveness of the COVID-19 Vaccine in One California Prison
The latest study by the Stanford Health Policy COVID-19 modeling team shows that vaccination continues to provide powerful protection from the delta variant, even among people who have been infected before.
When the COVID-19 delta variant of hit the Sierra Conservation Center — a low-to-medium security prison for men in central California — the Moderna vaccine remained effective against the more virulent strain of the virus, according to a new Stanford-led study.
The study, published in The New England Journal of Medicine on Oct. 20, also found that two doses of the Moderna vaccine resulted in additional, substantial protection against the delta infection in men who had previous confirmed COVID-19 infections.
This is one of the first studies to analyze the effectiveness of vaccination during an outbreak dominated by transmission of the delta variant — and the first in a congregate setting. The researchers found that although the estimated vaccine effectiveness against infection (56.6%) was substantially lower than estimated in studies conducted before the emergence of the delta variant, protection against symptomatic infection remained high (84.2%.) And in men who already had the coronavirus, the vaccine reduced the risk of subsequent infections by 80.5%, highlighting the benefits of getting vaccinated for those with prior history of infection.
Still, it’s important to note that during the height of the pandemic, incarcerated people were infected by the coronavirus at a rate five times higher than the nation’s general population.
“Even though we detected high vaccine effectiveness in our study, the outbreak underscores that incarcerated people continue to face risks from the COVID-19 pandemic,” said Elizabeth Chin, lead author of the study and a PhD student in biomedical data science. “Relying on vaccination alone is insufficient in preventing outbreaks in this vulnerable population.”
Chin emphasized that prison officials must prioritize not only efforts to increase vaccine coverage among their residents and staff, but also to step up non-pharmaceutical strategies such as masking, testing, and reducing the population size and density of their correctional facilities, given highly tranmissible variants.
Chin is a member of a team of researchers who have been using mathematical modeling and data analyses throughout the pandemic as part of the Prevention Policy Modeling Lab and SC-COSMO consortium. They have been modeling and analyzing COVID-19 data to inform prevention and mitigation strategies for decision makers around the county, including the state of California and California prisons. On this study, the researchers collaborated with colleagues at the California Department of Corrections and Rehabilitation and the California Correctional Health Care Services.
One-quarter of the men in the Sierra Conservation Center in Jamestown, CA — 827 of 3,221 residents — were included in the 30-day study launched in mid-July of 2021. Of those residents, 468 were fully vaccinated with Moderna and 359 were unvaccinated. Vaccination for both staff and residents is voluntary in Californian correctional facilities, though state attempts to mandate vaccines for staff is currently being challenged in court.
Though there were no COVID-19 infections detection among the residents between April 23 and July 15, 2021, a substantial outbreak began on July 16, which resulted in 15% of the population becoming infected by August 15.
So the team tracked infections over a 30-day period from July 16 to August 15, 2021, among those SCC residents who had close exposure during that time, or residence in a housing unit that had at least one detected case of COVID-19, and were unvaccinated or vaccinated with Moderna. Residents in the cohort underwent frequent testing throughout the study period. There were 122 confirmed infections, including 27 symptomatic infections and one hospitalization.
“Our study shows that vaccination continues to provide powerful protection, even with the delta variant, and even among people who have been infected before,” said Joshua Salomon, senior author of the study. “There are many prison systems around the country that have much lower vaccination coverage than California, so this outbreak reminds us just how important it is to continue to work to increase those coverage rates, alongside other mitigation measures.”
The other co-authors of the NEJM correspondence are David Leidner of the California Department of Corrections and Rehabilitation; Yifan Zhang, Elizabeth Long and Lea Prince — all Stanford Health Policy researchers; Ying Li of the California Correctional Health Care Services; and SHP faculty members David Studdert and Jeremy Goldhaber-Fiebert. Jason Andrews is another member of the modeling team.
