Primary Series and Booster Vaccine Effectiveness against COVID-19 Infection and Hospitalisation among 12 to 17-year-old Adolescents in Singapore

Abstract

Background: Singapore offered the Pfizer-BioNTech/Comirnaty vaccine to adolescents aged 12 to 17 years old in May 2021, and extended booster vaccines to this group in January 2022. Literature on primary series and booster vaccine effectiveness among adolescents is limited outside of Europe and North America.

Methods: We studied the incidence of confirmed COVID-19 infection and hospitalisation among adolescents aged 12 to 17 years old vaccinated with Pfizer-BioNTech/Comirnaty in Singapore from 1 September to 15 December 2021 during the Delta-variant wave, and from 21 January to 28 April 2022 during the Omicron-variant wave.

Findings: Two doses of vaccination achieved vaccine effectiveness of 66% (95% CI 63% – 69%) against Delta-variant infection and 25% (95% CI 21% – 29%) against Omicron-variant infection, and 83% (95% CI 74% – 89%) against Delta-variant hospitalisation and 75% (95% CI 56% – 86%) against Omicron-variant hospitalisation. Booster vaccination with three doses achieved vaccine effectiveness of 56% (95% CI 53% – 58%) against Omicron-variant infection and 94% (95% CI 86% – 97%) against Omicron-variant hospitalisation, compared with unvaccinated adolescents. Vaccine effectiveness against infection after two doses waned over time, while vaccine effectiveness against hospitalisation remained stable; both were increased after three doses.

Interpretation: Among adolescents aged 12 to 17 years old, vaccine effectiveness against confirmed COVID-19 infection after two doses of Pfizer-BioNTech/Comirnaty decreased over time, and increased after a third dose. Boosted adolescents were also most protected from hospitalisation, compared with fully vaccinated, partially vaccinated and unvaccinated adolescents.

Note:
Funding: Funding: This study was not funded.

Declaration of Interests: The authors declare that they have no conflict of interest.

Ethics Approval Statement: This study was conducted as part of national COVID-19 public health response under the Infectious Disease Act, Ministry of Health, Singapore, with exemption from ethics review, as provided for under the Human Biomedical Research Act 2015, Singapore.

Keywords: COVID-19, SARS-CoV-2, vaccines, mRNA vaccine, adolescent, booster