With some public health officials warning of an upcoming surge in cases of COVID-19 as the weather turns colder and people spend more time indoors, new Stanford-led research suggests mRNA vaccination and prior infection continued to provide protection against infection as the Omicron variant rose to dominance.
The researchers found that vaccination increased levels of protection even among those with previous infections, and three vaccine doses offered significantly more protection than two. They also found that while previous infection with pre-Omicron strains protected against reinfection, levels of protection were far lower for infections that occurred before the emergence of the Delta variant in the summer of 2021.
Using data from the California Department of Corrections and Rehabilitation, Elizabeth Chin, lead author of a new study published in The New England Journal of Medicine on Oct. 26, and collaborators noted prisons are a high-risk setting for COVID-19 and provide insight into how other at-risk or underserved populations could fare.
Prisons have been the sites of some of the largest outbreaks during the COVID-19 pandemic. Despite differences in living situations, testing programs, and population structure, the researchers found relatively similar estimates between residents and staff, Chin said.
“This similarity in two distinct, high-risk populations is particularly noteworthy as we try to understand the impacts of COVID-19 on other vulnerable populations, such as health care workers and residents of nursing facilities or homeless shelters,” said Chin, a Stanford PhD student in biomedical data science and a member of the Stanford COVID modeling consortium launched by Stanford Health Policy faculty and trainees at the beginning of the pandemic.
The consortium has worked closely with the state’s prison system to evaluate best practices and policies. California operates the country’s second-largest state prison system, with 35 prisons across the state. Prisons are particularly risky congregate settings for COVID-19 and many large outbreaks occurred in US prisons during the pandemic.
The Stanford-led team conducted an earlier study that looked at how many residents and prison staff were willing to be vaccinated. Another study from the group focused on one prison and learned that vaccination continued to provide powerful protection from the Delta variant — even among residents who had been infected before — but that relying on vaccination alone is insufficient in preventing outbreaks in the vulnerable population.
In this new study, the team examined the protection against infection with the SARS-CoV-2 Omicron variant conferred from vaccination and prior infection with pre-Omicron strains. The researchers used a retrospective cohort design to analyze infection risk during the Omicron wave — December 2021 through April 2022 — to compare protection against infection across combinations of vaccination history and infection history. A secondary analysis compared the effectiveness of three vaccination doses versus two.
Among the 59,794 CA prison residents who met the criteria to be included in the study cohort, 16.7% tested positive during the study period, and among 16,572 prison staff who met the inclusion criteria, 30.3% tested positive during the same period. The estimated effectiveness of vaccination against confirmed SARS-CoV-2 infection during the period of Omicron predominance among persons without known previous infections was 18.6% for those who had received only two vaccine doses and 40.9% for those who had received three doses.
“This new study is relatively unique in the way that we were able to evaluate protection across detailed categories of prior exposure in two distinct populations that are both highly tested but vary in terms of the mix of prior infection and vaccination history,” said Joshua Salomon, a professor of health policy and senior author of the paper.
Chin noted decreased protection against reinfection, particularly among those infected before the emergence of the Delta-variant, was cause for concern.
“But the good news is that boosters remain effective and enhance protection among those with previous infections. As this study also shows, vaccination is not enough to prevent infection — we must simultaneously invest in other strategies, including testing, masking, and reducing the size and density of prisons, to protect these two vulnerable populations.”
As the possibility of another infection wave looms, an important finding from the study is that vaccination and boosters provided strong benefits even among people with prior infection, which now includes the majority of Americans.
“As we’re seeing relatively modest uptake of the new boosters so far this fall, we hope this study provides a useful reminder that protective immunity gained through either infection or vaccination will wane over time, but that boosters can help to restore some of that protection,” said Salomon.
Other co-authors of the paper are Stanford Health Policy's David Studdert and Jeremy Goldhaber-Fiebert.