Association Between mRNA Vaccination and Infection From SARS-CoV-2 During the Delta and Omicron BA.1 Waves: A Population-Level Analysis

INTRODUCTION: COVID-19 mRNA vaccine protection against the Omicron variant of SARS-CoV-2 has been shown to be attenuated. Previous research in Shelby County, Tennessee found that vaccine effectiveness might differ by age in the Omicron surge, a finding not reported for other variants. To assess whether patterns in vaccine effectiveness by age group differed on the basis of the predominant strain of SARS-CoV2, we evaluated vaccine effectiveness in Shelby County, Tennessee by age group in the Delta wave and Omicron BA.1 (Omicron) wave. METHODS: Case and vaccination statuses of residents were assessed using COVID-19 surveillance data. Age was stratified as 18–34, 35–64, and ≥65 years. Vaccination groups included unvaccinated, fully vaccinated, and fully vaccinated + booster. Person time was counted in each wave by vaccination status until the time of a positive reported COVID-19 test or until the end of the study period. RESULTS: Incidence of COVID-19 was much higher during the Omicron wave than during the Delta wave across all vaccination groups. During the Delta wave, among adults, 79.2% fewer cases were identified in those fully vaccinated and 94.8% fewer in those fully vaccinated + booster, compared with 40.2% and 66.7%, respectively, in the Omicron wave, compared with those who were unvaccinated. CONCLUSIONS: This study found evidence that vaccine effectiveness differed by age group during the Omicron wave, where the same pattern was not prominent in the Delta wave. Further analysis investigating the influence of behavior patterns and other potential confounders on vaccine effectiveness would be useful in further understanding the relationship between age and vaccine effectiveness.