Using data from Swedish nationwide registers, researchers conducted this retrospective, total population cohort study investigating the effectiveness of COVID-19 vaccination against the risk of infection, hospitalization, and death during the first 9 months after vaccination for the total population of Sweden.

• Researchers retrieved data of 842,974 individuals who were fully vaccinated (two doses), and matched them (1:1) to an equal number of unvaccinated individuals (total study cohort n = 1,685,948).

• Vaccine effectiveness against SARS-CoV-2 infection of any severity progressively waned across all subgroups, but there were differences in the rate of waning according to vaccine type.

• Vaccine effectiveness appeared to be better maintained with respect to severe COVID-19, although there was some waning after 4 months.

• These results yield support for administration of a third vaccine dose as a booster.