Influenza vaccine effectiveness among high-risk groups: a systematic literature review and metaanalysis of case-control and cohort studies

Vaccination represents the most effective intervention to prevent infection, hospitalization and mortality due to influenza. This meta-analysis quantifies data reporting influenza vaccine effectiveness (VE) on influenza visits and hospitalizations of case-control and cohort studies amonghigh-risk groups.A systematic literature review including original articles published between 2007 and 2016, using aprotocol registered on Prospero with No. 42017054854, and a meta-analysis were conducted.For three high-risk groups (subjects with underlying health conditions, pregnant women and healthcare workers) only a qualitative evaluation was carried out. The VE quantitative analysisdemonstrated a clear significant overall effect of 39% (95%CI: 32%-46%) for visits and 57%(95%CI: 30%-74%) for hospitalization among children. Considering the elderly influenza VE had a clear effect of 25% (95%CI: 6%-40%) for visits and 14% (95%CI: 7%-21%; p<0.001) forhospitalization. This study showed the high VE of influenza vaccination among high-risk groups, representing a tool for public health decision-makers to develop evidence-based preventive interventions to avoid influenza outcomes