Effectiveness of the autumnal COVID-19 vaccine dose during the winter and summer waves of the 2023/24 season in Spain

Abstract

Background: We estimated vaccine effectiveness (VE) of the COVID-19 monovalent XBB.1.5 vaccine recommended in Spain in October 2023 for ≥60-year-olds against medically attended COVID-19 and COVID-19 hospitalization, during winter and summer epidemic waves. Research design and methods: Patients ≥60-year-olds consulting in primary-care or requiring hospitalization due to respiratory infection were systematically RT-PCR tested for SARS-CoV-2 and classified as cases if PCR-positive or controls if PCR-negative. Logistic regression compared autumnal vaccination ≥14 days before onset in cases and controls adjusting for sex, age group, comorbidities, region in Spain and week; VE=(1-Odds Ratio)x100. Results: We included 1,141/3,286 and 1,386/5,260 cases/controls from primary-care and hospitals. During the winter wave, VE (95% Confidence Interval) was 28% (5;47%) against medically attended COVID-19 [45% (−31;80%) during XBB.1.5 dominance; 26% (−2;46%) during BA.2.86] and 39% (24;51%) against hospitalization [73% (41;89%) during XBB.1.5; 37% (17;48%) during BA.2.86]. During the summer wave, VE decreased; −6% (−33;15%) against infection [21% (−11;44%) during BA.2.86; −39% (−91;-1%) during KP.3] and 6% (−14;22%) against hospitalization [8% (−20;30%) during BA.2.86; 5% (−35;21%) during KP.3]. Conclusions: COVID-19 VE was moderate during the 2023/24 winter wave, higher during XBB.1.5 dominance and against hospitalization. Effectiveness was null during the summer wave, likely reflecting waning immunity and SARS-CoV-2 variant substitution.