Title : The impact of expanded adolescent vaccination against Omicron waves depends on the epidemic status: A mathematical modelling study
Abstract:
Background: Deployment of effective vaccination against the various SARS-CoV-2 variants was crucial for controlling the spread of COVID-19. These retrospective modelling analyses evaluated the impact of the expanded adolescent COVID-19 vaccination in England at two different epidemic points: in the autumn (August to November) 2021, in the presence of a large Omicron epidemic wave, and in the autumn 2022, when the subsequent Omicron epidemic was at an endemic stage.
Methods: We used the Covasim SARS-CoV-2 model for England to run two scenario analyses: a) contrasting the strength of the BA.1 Omicron wave when vaccinating 18+ only versus additional 12+ vaccination from the autumn 2021, considering varying adolescent vaccine uptake and vaccine implementation timing; and b) contrasting the strength of the BA.2/BA.4/BA.5/XBB Omicron waves, under the current immunisation strategy at the time versus additional 12+ vaccination from September 2022. We projected the number of new daily SARS-CoV-2 infections, hospitalisations and deaths related to SARS-CoV-2 during the periods 01/08/2021 - 28/02/2022 and 01/08/2022 - 28/02/2023 for the respective analyses.
Findings: In presence of the BA.1 Omicron wave in late 2021, the expanded adolescent vaccination averted around 3,000,000 cases across all-ages. 1,010,000 SARS-CoV-2 infections were averted during the period between 2 months to 6 months after vaccination began in the vaccinated 12-17-year old cohort. The impact declined six months after the onset of the vaccination, while earlier onset of the immunisation campaign had greater overall impact. During the later Omicron waves in 2022, additional adolescents vaccination did not lead to significant reduction in COVID-19 cases, hospitalisations or deaths in the entire population, nor within the vaccinated cohort.
Interpretation: Our findings highlight that adolescent vaccination impact depends on the timing/speed of implementation, other present intervention strategies, and the status of the epidemic at the time. Hence, it should not be considered as a stand-alone immunisation strategy, but alongside information on overall vaccine effectiveness, potential adverse events, the cost of roll out implementation and the operational constraints of its delivery.
