This study used a rigorous actuarial approach to estimate excess mortality across German federal states during the first three years of the COVID-19 pandemic. Regional trends were analysed alongside associations with state-level indicators: reported COVID-19 deaths and infections, policy stringency, vaccination rates, demographic and socioeconomic factors. Average excess mortality was moderate in the first year, with substantial regional variation. It increased slightly in the second year, with stable regional patterns. In the third year, excess mortality rose sharply, regional differences diminished, and the most affected states shifted, indicating the emergence of a new excess mortality driver. In the first two years, excess mortality strongly correlated with COVID-19 deaths, although reported COVID-19 deaths substantially exceeded excess deaths. Despite rising excess mortality, COVID-19 deaths declined over time. In the third year, only vaccination rate and trust in institutions showed notable associations, with the latter fully mediated by vaccination rate. Higher vaccination rates correlated with larger increases in excess mortality and with smaller declines in COVID-19 deaths and case fatality rates, even after adjusting for prior mortality levels and time-invariant confounders. This robust finding underscores the need for urgent investigation into potential unintended effects of vaccination or other previously neglected mortality drivers.