Moderate influenza vaccine effectiveness in a B mismatch season: preliminary results from the 2017/2018 season in Portugal

Letters to editorThis study aimed to provide early 2017/2018 seasonal IVE using data from the Portuguese influenza surveillance system. This analysis included patients with influenza-like illness (ILI) from primary care and emergency departments swabbed for the detection of influenza using RT-PCR between weeks 38/2017 to 5/2018. We used the test-negative case–control study design, where influenza laboratory confirmed incident ILI patients (Cases) were compared to laboratory influenza negative ILI patients (Controls). IVE analysis was restricted to ILI patients with symptoms compatible with the European Union ILI definition, i.e., with sudden onset of at least one systemic and one respiratory symptom. Chi-square test was used to compare baseline characteristics between Cases and Controls. Crude IVE, design adjusted for calendar time, was estimated using 1-odds ratio (OR) of being vaccinated in Cases vs. Controls and was further adjusted for confounding by age group and presence of chronic condition.A total of 732 ILI patients were reported to the NISS and approximately 74% adhered to the ILI definition. In a season with B/Yamagata dominance and with vaccine lineage mismatch, the 2017/2018 trivalent seasonal influenza vaccine conferred moderate protection against medically attended influenza. The use of surveillance data constituted a useful tool to have early in the season IVE estimates. These results assist modifications to health interventions, such as using antiviral treatment in high-risk patients, reinforcement of social eviction and individual hygiene measures to reduce risk of influenza transmission, regardless of the vaccination status.