Influenza Vaccine Effectiveness Against Mild and Severe Outcomes: A Comparison Study in Portugal

Abstract

Influenza vaccination is an effective public health intervention to lessen influenza effect on healthcare and has been recommended to prevent severe outcomes. Previous studies present higher influenza vaccine effectiveness (IVE) estimates against primary care medically attended (outpatients) compared to hospitalised cases (inpatients). Inconsistencies in study designs may hinder comparisons of IVE in preventing mild (outpatient consultations) versus severe influenza (inpatient admissions). We aimed to compare IVE against mild and severe influenza within primary care and hospital networks in Portugal from 2015/16 to 2023/24.