Infection with SARS-CoV-2 following Second Dose Pfizer-BioNTech mRNA COVID-19 Vaccine BNT162b2 in Danish Adolescents Aged 12-18 Years: A Real-World Nationwide Danish Cohort Study.

In this real-world cohort study based on Danish nationwide registers, the cumulated proportion, relative risk (RR) of SARS-CoV-2 breakthrough infections, and vaccine effectiveness (VE) were investigated in adolescents aged 12-18 years following vaccination with the BNT162b2 vaccine compared to unvaccinated controls. Adolescents with and without vaccination with the first dose of BNT162b2 between 1 May and 30 September 2021 were included. Effect estimates include proportions with a positive SARS-CoV-2 RT-PCR test among vaccinated and unvaccinated, RR, and VE at three different time points. During Delta-dominance, VE was first 97.6% (95% CI 96.3-98.4), then 96.2% (95% CI 95.4-96.9) in the age group 12-15 and 95.1% (95% CI 94.1-96.0) followed by 95.5% (95% CI 94.8-96.1) in the age group 16-18 years, respectively. During Omicron dominance, VE was 5.8% (95% CI 4.6-7.0) in ages 12-15 years and 9.2% (95% CI 7.7-10.6) in ages 16-18 years. Thus, BNT162b2-vaccine protection was limited during the Omicron era.