BACKGROUND: For adolescents, data on the long-term effectiveness of the BNT162b2 and mRNA-1273 vaccines against severe COVID-19 outcomes are scarce. Additionally, only a few studies have evaluated vaccine effectiveness (VE) for mRNA-1273 or heterologous mRNA vaccine schedules (ie, mixing BNT162b2 and mRNA-1273). METHODS: Nationwide register-based 1-to-1 matched cohort analyses were conducted in Denmark, Finland, Norway, and Sweden between May 28, 2021, and April 30, 2023, to estimate VE for primary COVID-19 vaccine (2-dose) schedules among adolescents aged 12 to 17 years. Cumulative incidences of COVID-19-related hospitalization (primary outcome) and laboratory-confirmed SARS-CoV-2 infection (secondary outcome) were compared for vaccinated and unvaccinated at 6 months of follow-up using the Kaplan-Meier estimator. Country-specific VE (1-risk ratio) and risk differences (RD) were combined by random-effects meta-analyses. RESULTS: The study included 526 966 primary schedule vaccinated adolescents. VE against COVID-19-related hospitalization was 72.6% (95% confidence interval [CI], 62.5-82.7) and RD was -2.8 (95% CI, -4.5 to -1.0) per 10 000 vaccinated for BNT162b2 at 6 months of follow-up compared with unvaccinated. The corresponding VE and RD were 86.0% (95% CI, 56.8-100.0) and -2.1 (95% CI, -4.0 to -0.2) per 10 000 vaccinated for mRNA-1273 and 80.7% (95% CI, 58.0-100.0) and -5.5 (95% CI, -15.5 to 4.6) per 10 000 vaccinated for heterologous mRNA vaccine schedules. Estimates were comparable when restricting to a period of omicron predominance and extending follow-up to 12 months. CONCLUSIONS: Across 4 Nordic countries, severe COVID-19 in adolescents was a rare event. Compared with unvaccinated, BNT162b2, mRNA-1273, and heterologous mRNA vaccination schedules provided high protection against COVID-19-related hospitalization, including hospitalizations during the omicron period.