Durability of SARS-CoV-2 mRNA Booster Vaccine Protection Against Omicron Among Health Care Workers with a Vaccine Mandate.

ABSTRACT

BACKGROUND: The SARS-CoV-2 Omicron Variant has spread rapidly throughout the world since being identified in South Africa in November 2021. Few studies have assessed primary series and booster vaccine effectiveness against Omicron among US health care workers. METHODS: We conducted a test-negative case-control design to estimate BNT162b2 and mRNA1273 primary vaccination and booster effectiveness against SARS-CoV-2 infection and symptomatic Covid-19 during an Omicron surge among employees of the University of Pennsylvania Health System. The study period was between 7/1/21-4/5/22. We defined the Delta period as 7/1/21-12/12/21 and the Omicron period as beginning 12/20/21. RESULTS: Our sample included 14,520 tests (2,776 [19%] positive)-7,422 (506 [7%] positive) during Delta, and 7,098 (2270 [32%] positive) during Omicron. Benchmarked against Delta, vaccine effectiveness of two vaccine doses was lower during Omicron, with no significant protection against infection. Booster doses added significant protection, although they also showed reduced effectiveness during Omicron. Compared to employees who had received two vaccine doses, three BNT162b2 doses had a relative effectiveness of 50% (95% CI 42-56%) during Omicron, relative to 78% (95% CI 63-87%) during Delta; three mRNA1273 doses had a relative effectiveness of 56% (95% CI 45-65%) during Omicron, relative to 96% (95% CI 82-99%) during Delta. Restricting the sample to symptomatic tests yielded similar results to our primary analysis. After initial waning in BNT162b2 booster protection against infection, it remained largely stable for at least 16 weeks after vaccination. DISCUSSION: Our findings provide a strong rationale for boosters among healthcare workers in the Omicron era.