BackgroundCOVID-19 remains a major infectious disease with substantial implications for individual and public health including the risk of a post-infection syndrome, long COVID. The continuous changes in dominant variants of SARS-CoV-2 necessitate a careful study of the effect of preventative strategies.AimWe aimed to estimate the effectiveness of post-vaccination, post-infection and hybrid immunity against severe cases requiring oxygen support caused by infections with SARS-CoV-2 variants BA1/2 and BA4/5+, and against long COVID in the infected population and their changes over time.MethodsWe used a Cox regression analysis with time-varying covariates and calendar time and logistic regression applied to national-level data from Czechia from December 2021 until August 2023.ResultsRecently boosted vaccination, post-infection and hybrid immunity provide significant protection against a severe course of COVID-19, while unboosted vaccination more than 10 months ago has a negligible protective effect. The post-vaccination immunity against the BA1/2 or BA4/5+ variants, especially based on the original vaccine types, appears to wane rapidly compared with post-infection and hybrid immunity. Once infected, however, previous immunity plays only a small protective role against long COVID.ConclusionVaccination remains an effective preventative measure against a severe course of COVID-19 but its effectiveness wanes over time thus highlighting the importance of booster doses. Once infected, vaccines may have a small protective effect against the development of long COVID.

All authors contributed equally to this work

Centre for Modelling of Biological and Social Processes Prague Czechia

Department of Biochemistry Cell and Molecular Biology 3rd Faculty of Medicine Charles University Prague Czechia

Department of Econometrics Institute of Information Theory and Automation Czech Academy of Sciences Prague Czechia

Department of Mathematics and Statistics Faculty of Science Masaryk University Brno Czechia

Department of Probability and Mathematical Statistics Faculty of Mathematics and Physics Charles University Prague Czechia

Institute of Biostatistics and Analyses Faculty of Medicine Masaryk University Brno Czechia

Institute of Biostatistics and Analyses Ltd Brno Czechia

Institute of Health Information and Statistics of the Czech Republic Prague Czechia

School of Psychology University of Nottingham University Park Nottingham United Kingdom

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