Relative effectiveness of a 2nd booster dose of COVID-19 mRNA vaccine up to four months post administration in individuals aged 80 years or more in Italy: A retrospective matched cohort study

•Evidence on relative effectiveness (RVE) of a 2nd booster of COVID-19 vaccine is scarce.•We estimated RVE of a 2nd booster as compared to a 1st booster given ≥ 120 days earlier in elders ≥ 80 years.•RVE against severe COVID-19 was about 30 % 2–4 months after the 2nd booster dose.•However, RVE against infection was negligible in the same time interval.•Cost-benefit of a 3rd booster four months after the 2nd booster should be evaluated.