Risk of infection, hospitalisation, and death up to 9 months after a second dose of COVID-19 vaccine: a retrospective, total population cohort study in Sweden

We found progressively waning vaccine effectiveness against SARS-CoV-2 infection of
any severity across all subgroups, but the rate of waning differed according to vaccine
type. With respect to severe COVID-19, vaccine effectiveness seemed to be better maintained,
although some waning became evident after 4 months. The results strengthen the evidence-based
rationale for administration of a third vaccine dose as a booster.