For a study, researchers sought to assess the benefits of mRNA boosters fully and comprehend how booster immunization can prevent mild and severe sickness without hospitalization.

Throughout the Omicron BA.1/2 wave, they monitored 85,801 people (aged 31 to 81) in 2 large population-based cohorts. From biweekly surveys covering the period from 12 January 2022 to 7 April 2022, information on home testing, PCR testing, and COVID-19 symptoms were taken. National registries were used to determine vaccination status and information on prior SARS-CoV-2 infections. The efficacy of the booster immunization compared to receiving the 2-dose primary series >130 days before was estimated using Cox regression.

The efficacy of the booster immunization reduced over time after it was given and increased with the severity of COVID-19. In the period >90 days following booster immunization, the efficacy against severe COVID-19 decreased from 80.9% to 63.4%. Against weak COVID-19, there was practically any impact. Patients ≥60 tended to have lesser efficacy than those under 50.

The efficacy of boosters against self-reported mild, moderate, and severe COVID-19 was assessed for the first time in population-based research. In addition, the research provided important data on the protection’s duration and the best time for further booster shots.