Effectiveness against severe COVID-19 of a second booster dose of the bivalent (original/BA.4–5) mRNA vaccine 7–90 days post-administration, relative to a first booster dose of an mRNA vaccine received ≥ 120 days earlier, was ca 60% both in persons ≥ 60 years never infected and in those infected > 6 months before. Relative effectiveness in those infected 4–6 months earlier indicated no significant additional protection (10%; 95% CI: −44 to 44). A second booster vaccination 6 months after the latest infection may be warranted.
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