Protection against Omicron from Vaccination and Previous Infection in a Prison System

Abstract

Background: Information regarding the protection conferred by vaccination and previous infection against infection with the B.1.1.529 (omicron) variant of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is limited.

Methods: We evaluated the protection conferred by mRNA vaccines and previous infection against infection with the omicron variant in two high-risk populations: residents and staff in the California state prison system. We used a retrospective cohort design to analyze the risk of infection during the omicron wave using data collected from December 24, 2021, through April 14, 2022. Weighted Cox models were used to compare the effectiveness (measured as 1 minus the hazard ratio) of vaccination and previous infection across combinations of vaccination history (stratified according to the number of mRNA doses received) and infection history (none or infection before or during the period of B.1.617.2 [delta]-variant predominance). A secondary analysis used a rolling matched-cohort design to evaluate the effectiveness of three vaccine doses as compared with two doses.

Results: Among 59,794 residents and 16,572 staff, the estimated effectiveness of previous infection against omicron infection among unvaccinated persons who had been infected before or during the period of delta predominance ranged from 16.3% (95% confidence interval [CI], 8.1 to 23.7) to 48.9% (95% CI, 41.6 to 55.3). Depending on previous infection status, the estimated effectiveness of vaccination (relative to being unvaccinated and without previous documented infection) ranged from 18.6% (95% CI, 7.7 to 28.1) to 83.2% (95% CI, 77.7 to 87.4) with two vaccine doses and from 40.9% (95% CI, 31.9 to 48.7) to 87.9% (95% CI, 76.0 to 93.9) with three vaccine doses. Incremental effectiveness estimates of a third (booster) dose (relative to two doses) ranged from 25.0% (95% CI, 16.6 to 32.5) to 57.9% (95% CI, 48.4 to 65.7) among persons who either had not had previous documented infection or had been infected before the period of delta predominance.

Conclusions: Our findings in two high-risk populations suggest that mRNA vaccination and previous infection were effective against omicron infection, with lower estimates among those infected before the period of delta predominance. Three vaccine doses offered significantly more protection than two doses, including among previously infected persons.

Figure 1. Study Population.

The Ad26.COV2.S vaccine, an adenovirus vector–based vaccine, was developed by Johnson & Johnson–Janssen. The delta variant is also known as B.1.617.2.

Figure 2. Testing and Cases in the Study Cohort.

The upper graphs of Panels A and B show the daily numbers and percentages of residents and staff who underwent testing for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and who had a positive test result. There was reduced staff testing on federal holidays. The lower graphs of Panels A and B show the daily numbers of positive cases of coronavirus disease 2019 (Covid-19). Testing and case series were extended over the historical period that began 2.5 months before the start of the study period.

Figure 3. Vaccination and Previous Infection Status of the Study Cohort over Time.

The California Department of Corrections and Rehabilitation began their vaccination program for the mRNA primary series at the end of 2020 and began offering boosters at the end of August 2021. A high incidence of previous infection was detected in the study cohort during periods of high incidence of Covid-19 in the general population in California.

Figure 4. Adjusted Estimates of Effectiveness of Vaccination and Previous Infection against Omicron Infection among Residents and Staff in California State Prisons.

Shown are unadjusted case counts, total counts, and adjusted estimates of the effectiveness of vaccination and previous infection against omicron infection outbreaks that occurred between December 24, 2021, and April 14, 2022. Persons who had been vaccinated had received two or three doses of mRNA vaccines only. The red squares denote residents, and the blue squares denote staff. 𝙸 bars denote 95% confidence intervals.