A retrospective cohort study of incidence and risk factors for severe SARS-CoV-2 breakthrough infection among fully vaccinated people

SARS-CoV-2 vaccination is currently the mainstay in combating the COVID-19 pandemic. However, there are still people among vaccinated individuals suffering from severe forms of the disease. We conducted a retrospective cohort study based on data from nationwide e-health databases. The study included 184,132 individuals who were SARS-CoV-2 infection-naive and had received at least a primary series of COVID- 19 vaccination. The incidence of BTI (breakthrough infection) was 8.03 (95% CI [confidence interval] 7.95.8.13/10,000 person-days), and for severe COVID-19 it was 0.093 (95% CI 0.084 - 0.104/10,000 person-days). The protective effect of vaccination against severe COVID-19 remained constant for up to six months, and the booster dose offered an additional pronounced benefit (hospitalization aHR 0.32, 95% CI 0.19 - 0.54). The risk of severe COVID-19 was higher among those >= 50 years of age (aHR [adjusted hazard ratio] 2.06, 95% CI 1.25 - 3.42) and increased constantly with every decade of life. Male sex (aHR 1.32, 95% CI 1.16 - 1.45), CCI (The Charlson Comorbidity Index) score >= 1 (aHR 2.09, 95% CI 1.54 - 2.83), and a range of comorbidities were associated with an increased risk of COVID-19 hospitalization. There are identifiable subgroups of COVID-19-vaccinated individuals at high risk of hospitalization due to SARS-CoV-2 infection. This information is crucial to driving vaccination programs and planning treatment strategies.