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.
Very few people aged 60 and over who received a repeat vaccination against COVID-19 have been admitted to hospital. In people aged 70 years and older, the repeat vaccination seems to restore vaccine effectiveness against hospital admission to the level just after the booster vaccination. Due to the small numbers involved, exact figures on vaccine effectiveness of the repeat vaccination against COVID-19 have not yet been established.