BACKGROUND: Covid-19 vaccines are updated to match circulating strains based on reasoning that better strain-matched immunogenicity should provide better protection. Randomized evidence with disease endpoints to support strain matching is lacking. We evaluated COVID-19 incidence among adults randomized to a second booster of Prototype or Omicron-based vaccines.
METHODS: COVAIL was a four-stage Phase 2 clinical trial; results from Stages 1 (mRNA-1273 [Moderna]) and 2 (BNT162b2 [Pfizer/BioNTech]) are described here. Adults who had received a primary series and one booster of an authorized COVID-19 vaccine were eligible. Participants received one dose of either Prototype vaccine or a monovalent or bivalent Omicron BA.1 vaccine. SARS-CoV-2 neutralization titers (ID) were measured pre- and post-vaccination. Covariate-adjusted cumulative COVID-19 incidence and Cox regression analyses were conducted separately for each stage. 50
RESULTS: 706 participants with pre- and day 15 post-vaccination IDtiters (n = 503 in Stage 1, n = 203 in Stage 2) were included. Within stages, participant characteristics and baseline IDtiters were similar between Prototype and Omicron-based arms. There was no difference in cumulative COVID-19 incidence for Prototype vs. Omicron-based vaccine in Stage 1 (RR 1.04, 95 % CI 0.73-1.48), while incidence was higher among Prototype recipients in Stage 2 (RR 2.56, 1.44-4.52). Cox regression analysis showed no difference in Stage 1 (HR 1.04, 0.68-1.58), but higher incidence for Prototype recipients in Stage 2 (HR 2.95, 1.52-5.72). 50 50
CONCLUSIONS: Omicron-based vaccines as second boosters were more protective against COVID-19 relative to Prototype among those receiving BNT162b2 but not mRNA-1273. Differences between stages such as force of infection, antigen matching, and vaccine differences may explain this finding.
CLINICALTRIALS: govRegistry Number: NCT05289037.
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