Population effects of influenza vaccination in children and adolescents: Systematic review

To investigate indirect vaccine effectiveness (indirVE) of vaccination of children and adolescents with seasonal influenza vaccines against influenza-related outcomes occurring in other population groups.

We performed a systematic review of studies (randomized and non-randomized) on indirVE of vaccination of participants aged 6 months–17 years with tri- or quadrivalent seasonal influenza vaccines against influenza (lab-confirmed; non-lab-confirmed) occurring in contacts of vaccinated persons or members of the wider community (last search: 17th March 2024). GRADE certainty of evidence (CoE) was evaluated (PROSPERO: CRD42024546400).

We identified 28 studies (5 randomized; 23 non-randomized). In community-based studies, indirect protection against laboratory-confirmed influenza (LCI) ranged from −38 % [95 % CI: −574 to 72] to 61 % [95 % CI: 8–83] (very low CoE). In household-based settings, indirVE against LCI varied between −151.2 % [95 % CI: −1194.6 to 51.3] and 39.4 % [95 % CI: 7.4 to 60.3] (very low CoE). In school-based settings, highly variable indirect effects were observed on LCI, hospitalization, emergency department visits and school/work absenteeism (very low CoE).

There is no clear evidence of indirect effects from influenza vaccination in children. While plausible, effect size is uncertain and varies by study design, population, and vaccine type. Stronger indirect effects appeared only when direct VE was high.