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Evaluation Video

Oral Presentation

Session: Assessing COVID-19 Vaccine Effectiveness

Added Benefit and Waning Effectiveness of a Third-Dose mRNA COVID-19 Vaccine in Community-Dwelling Medicare Beneficiaries Aged ≥65 Years in Omicron Period

Tuesday, August 27, 2024
8:15 AM – 8:30 AM CEST
Location: Conv Hall I C
Background: U.S. older adults (people aged ≥65 years) were disproportionately impacted by the COVID-19 pandemic. It is important to evaluate real-world effectiveness of mRNA COVID-19 vaccines among this vulnerable population.

Objectives: We evaluated the added benefit of third doses versus second doses of mRNA COVID-19 vaccines and if effectiveness wanes over time among community-dwelling older adults during an Omicron-dominant period.

Methods: We conducted a real-world retrospective cohort study using Medicare Fee-for-Service claims data among U.S. community-dwelling beneficiaries aged ≥65 years. Beneficiaries were followed during high COVID-19 circulation periods from December 19, 2021-March 26, 2022. Time-varying exposures included second and third doses of Moderna or Pfizer-BioNTech COVID-19 vaccines. We used inverse probability of treatment weighting and Cox proportional hazard models to estimate relative vaccine effectiveness (RVE) against COVID-19-related outcomes (e.g., hospitalization and death) by vaccine dose and brand.

Results: 8,135,020 two-dose mRNA COVID-19 vaccinated community-dwelling beneficiaries were eligible for the study, of whom 73.3% received a third dose by March 26, 2022. Third doses provided substantial added benefit against COVID-19-related outcomes throughout the observed follow-up time. While the added benefit was greatest against COVID-19-related death, COVID-19-related hospitalization had higher outcome counts that allowed for more precise RVE estimates. Against COVID-19 hospitalization, the third-dose added benefit was consistent for 120 days since third-dose vaccination (e.g., RVE at 14-60 days was 77.2% [95% confidence interval (CI): 76.0%-78.4%] for Moderna and 72.5% [95% CI: 70.8%-74.0%] for Pfizer-BioNTech), and was still significant but substantially lower afterwards (RVE at >120 days was 42.1% [95% CI: 37.5%-46.3%] for Moderna and 51.0% [95% CI: 48.0%-53.7%] for Pfizer-BioNTech). For those with prior medically-attended COVID-19 diagnosis, the added benefit against COVID-19 hospitalization was statistically significant for both brands until 120 days, and remained significant for Moderna after 120 days. Moderna three-dose vaccines were more effective against COVID-19 hospitalization than Pfizer-BioNTech three-dose vaccines for 14-120 days, but after 120 days there was no evidence of a difference in effectiveness between brands in the overall population.

Conclusions: A third dose of an mRNA COVID-19 vaccine provided substantial added benefit against severe COVID-19-related outcomes in this vulnerable population—even among those with prior COVID-19. The added benefit of a third dose remained high for at least 4 months during high COVID-19 circulation periods.