Effectiveness of mRNA Booster Vaccine Against Coronavirus Disease 2019 Infection and Severe Outcomes Among Persons With and Without Immune Dysfunction: A Retrospective Cohort Study of National Electronic Medical Record Data in the United States.

The risk reduction of 4 doses versus 3 doses of mRNA vaccine was somewhat lower during the Omicron compared to Delta period, but remained highly significant (aORs for breakthrough infection, hospitalization, and severe outcomes were 0.14 [95% CI, .09–.20], 0.16 [95% CI, .09–.27], and 0.10 [95% CI, .03–.32], respectively).

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