The decrease in vaccine effectiveness in preventing hospital and ICU admission can probably be attributed to waning effectiveness over time. Another partial explanation could be that many people who were not vaccinated against COVID-19 recently had a SARS-CoV-2 infection and built up immunity as a result. The risk of hospital admission for people who received a booster vaccination is more than 5 times lower after a booster vaccination compared to unvaccinated people, and the risk of ICU admission is 11 times lower.
In the period from 1 February 2022 to 22 March 2022, when more than 99% of hospital admissions were caused by the Omicron variant, vaccine effectiveness of the basic series in preventing hospital admission was 45%. After the booster vaccination, that percentage was 82%. Vaccine effectiveness against ICU admission was 57% after the basic series, compared to 91% after booster vaccination.