Relative Effectiveness and Waning of a Third Dose of mRNA COVID-19 Vaccine in Medicare Beneficiaries Aged 65 Years and Older During the BA.1/BA.2 Omicron Period.

Abstract

We assessed the added benefit and waning effectiveness of a third COVID-19 vaccine dose (original formula) for preventing COVID-19-related outcomes. We used Medicare claims data to conduct a retrospective cohort study in US community-dwelling Medicare Fee-for-Service beneficiaries aged ≥65 years during the BA.1/BA.2 Omicron period (19 December 2021-26 March 2022). We estimated relative vaccine effectiveness (RVE) of 3 vs 2 doses of mRNA COVID-19 vaccines using marginal structural Cox regression models. Among 8 129 355 eligible beneficiaries, 73.4% were 3 dose vaccinated by 26 March 2022. At 14 to 60 days since vaccination, a third dose provided significant added benefit against COVID-19-related hospitalization for Moderna (RVE, 77.0%; 95% CI, 75.7%-78.2%) and Pfizer-BioNTech (RVE, 72.3%; 95% CI, 70.6%-73.8%). Added benefit was lower at >120 days. For those with prior medically attended COVID-19 diagnoses, Pfizer-BioNTech provided an added benefit for 120 days, while Moderna provided some added benefit at >120 days. Added benefit for either vaccine was higher against death as compared with less severe outcomes, which still decreased at >120 days. A third-dose COVID-19 vaccine provided significant added benefit against COVID-19-related hospitalization and death, even for beneficiaries with prior medically attended COVID-19 diagnoses. This added benefit decreased after 4 months.