Background. In Switzerland, SARS-CoV-2 vaccination campaigns started early 2021. Vaccine coverage reached 65% of the population in December 2021, mostly using mRNA vaccines from Moderna and Pfizer-BioNtech. Simultaneously, the proportion of vaccinated among COVID-19-related hospitalizations and deaths rose, creating some confusion in the general population. We aim to assess vaccine effectiveness against severe forms of SARS-CoV-2 infection using routine surveillance data on the vaccination status of COVID-19-related hospitalizations and deaths and data on vaccination coverage in Switzerland.Methods. We consider all routine surveillance data on COVID-19-related hospitalizations and deaths received at the Swiss Federal Office of Public Health from 1 July 2021 to 1 December 2021. We estimate the relative risk of COVID-19 related hospitalization or death for non-fully vaccinated compared to fully vaccinated individuals, adjusted for the dynamics of vaccination coverage over time, by age and location. We stratify the analysis by age group and by calendar month. We assess variations in the relative risk of hospitalization associated with the time since vaccination.Results. We include a total of 5,948 COVID-19-related hospitalizations of which 1,245 (21%) were fully vaccinated, and a total of 739 deaths of which 259 (35%) were fully vaccinated. We find that the relative risk of COVID-19 related hospitalization is 12.5 (95%CI: 11.7 to 13.4) times higher for non-fully vaccinated than for fully vaccinated individuals. This translates into a vaccine effectiveness against hospitalization of 92.0% (95%CI: 91.4 to 92.5%). Vaccine effectiveness against death is estimated to 90.3% (95%CI: 88.6 to 91.8%). Effectiveness appears comparatively lower in age groups over 70 and during the months of October and November 2021. We also find evidence of a decrease in vaccine effectiveness against hospitalization for individuals vaccinated for 25 weeks or more, but this decrease only appears in age groups below 70.Conclusions. The observed proportions of vaccinated among COVD-19-related hospitalizations and deaths in Switzerland are compatible with a high effectiveness of mRNA vaccines from Moderna and Pfizer-BioNtech against hospitalization and death in all age groups. Effectiveness appears comparatively lower in older age groups, suggesting the importance of booster vaccinations. We find inconclusive evidence that vaccine effectiveness is waning over time. Repeated analyses will be able to better assess waning and the effect of boosters.