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

Area of Science:

• Epidemiology
• Immunology
• Public Health

Background:

• Limited data exists on protection against the Omicron variant (B.1.529) from vaccination and prior infection.
• Assessing protection in high-risk populations is crucial for public health strategies.

Purpose of the Study:

• To evaluate the effectiveness of mRNA vaccines and previous infection against Omicron variant infection.
• To compare protection levels across different vaccination and infection histories in a high-risk cohort.

Main Methods:

• Retrospective cohort study of California state prison residents and staff during the Omicron wave (Dec 2021-Apr 2022).
• Weighted Cox models used to assess vaccine and infection effectiveness.
• Secondary analysis compared two vs. three vaccine doses using a rolling matched-cohort design.

Main Results:

• Previous infection provided 16.3% to 48.9% protection against Omicron in unvaccinated individuals.
• Vaccine effectiveness ranged from 18.6% to 87.9% depending on dose number and infection history.
• A third vaccine dose (booster) significantly increased protection by 25.0% to 57.9% compared to two doses.

Conclusions:

• mRNA vaccination and prior infection demonstrate effectiveness against Omicron infection.
• Three vaccine doses provide enhanced protection compared to two doses, even in previously infected individuals.
• Protection estimates were lower for individuals infected before the Delta variant predominance period.