Effectiveness of COVID-19 Booster on the Risk of Hospitalization Among Medicare Beneficiaries

Abstract

Objective: To determine the effectiveness of booster vaccinations on the risk of hospitalization with coronavirus disease 2019 (COVID-19) and how it varies by enrollee characteristics and interval from the initial vaccination to receipt of a booster.

Patients and methods: This cohort study used 100% Medicare claims from January 1, 2020, through December 31, 2021, and matched 3,940,475 individuals who received boosters to 3,940,475 controls based on week and type of original COVID-19 vaccine and demographic and clinical characteristics. We compared the association of booster vs no booster with COVID-19 hospitalization using Cox proportional hazards regression models controlling for patient characteristics. We also determined the association of time from original vaccine to booster with COVID-19 hospitalization.

Results: Over a maximum of 130 days of follow-up, boosted enrollees had 8.20 (95% CI, 7.81 to 8.60) COVID-19 hospitalizations per million days vs 43.70 (95% CI, 42.79 to 44.64) for controls (81% effectiveness). Effectiveness varied by race, prior hospitalizations, and certain comorbidities, for example, leukemia/lymphoma (53% effectiveness), autoimmune disease (73%), and dementia (73%). Boosters received between 6 and 9 months after original vaccination varied between 81% and 85% effectiveness, while boosters received at 5 to 6 months (62%) or less than 5 months (58%) were less effective.

Conclusion: Boosters are highly effective in the Medicare population. Approximately 69,225 hospitalizations would be prevented by boosters in the 15 million individuals aged 65 years or older currently not boosted in a period similar to the September 2020 through January 2021 period studied. Boosters provided the greatest benefits if they were received between 6 and 9 months following original vaccinations. However, boosters were associated with substantial decreases in COVID-19 hospitalizations in all categories of enrollees.

Figure 1

Cumulative incidence curve of coronavirus disease 2019 (COVID-19) hospitalization among individuals who received boosters vs those who did not receive boosters. Joinpoint analyses of the curves found no significant changes in slope (P>0.30).

Figure 2

Adjusted vaccine effectiveness for boosted enrollees compared with those not boosted as a function of the months between primary vaccine and receipt of booster. The highest effectiveness was in boosters received 6 to 7, 7 to 8, or 8 to 9 months after the original vaccination, while intervals of less than 6 months or greater than 9 months had lower effectiveness. Error bars indicate 95% CIs. The number of enrollees, hospitalizations, hospitalization rates, and adjusted hazard ratios for each interval are given in Supplemental Table 2.