Effectiveness of influenza vaccines in preventing severe influenza illness among adults: A systematic review and meta-analysis of test-negative design case-control studies.

ABSTRACT

OBJECTIVES: Summary evidence of influenza vaccine effectiveness (IVE) against hospitalized influenza is lacking. We conducted a meta-analysis of studies reporting IVE against laboratory-confirmed hospitalized influenza among adults. METHODS: We searched Pubmed (January 2009 to November 2016) for studies that used test-negative design (TND) to enrol patients hospitalized with influenza-associated conditions. Two independent authors selected relevant articles. We calculated pooled IVE against any and (sub)type specific influenza among all adults, and stratified by age group (18-64 and 65 years and above) using random-effects models. RESULTS: We identified 3411 publications and 30 met our inclusion criteria. Between 2010-11 and 2014-15, the pooled seasonal IVE was 41% (95%CI34;48) for any influenza (51% (95%CI44;58) among people aged 18-64y and 37% (95%CI30;44) among ≥65 years). IVE was 48% (95%CI37;59),37% (95%CI24;50) and 38% (95%CI23;53) against influenza A(H1N1)pdm09, A(H3N2) and B, respectively. Among persons aged ≥65 year, IVE against A(H3N2) was 43% (95%CI33;53) in seasons when circulating and vaccine strains were antigenically similar and 14% (95%CI-3;30) when A(H3N2) variant viruses predominated. CONCLUSIONS: Influenza vaccines provided moderate protection against influenza-associated hospitalizations among adults. They seemed to provide low protection among elderly in seasons where vaccine and circulating A(H3N2) strains were antigenically variant.