Influenza Vaccine Effectiveness in Patients Hospitalized with Severe Acute Respiratory Infection in Lithuania During the 2019/2020 Influenza Season

Abstract

Background Influenza is a contagious viral airborne disease that adds to the clinical and economic burden on the healthcare system that could be prevented substantially by seasonal influenza vaccination. Seasonal influenza vaccine effectiveness (SIVE) varies a lot and should therefore be monitored. This report aims to update age-stratified SIVE estimates among patients hospitalized due to severe acute respiratory infection (SARI) during the 2019-2020 influenza season. Method We performed a test-negative case-control study between December, 2019 and April, 2020 influenza season. We estimated SIVE and its 95% confidence intervals (95% CI) with logistic regression as (1-odds ratio)*100%. The models were adjusted for covariates that changed the unadjusted SIVE by ≥10%. Result Among 84 participants, 32 (38.1%) were influenza positive, mostly with A(H1N1)pdm09. SIVE against any influenza adjusted for age and heart disease was 60.8% (95% CI: 13.2%, 86.3%). Age-stratified point estimates adjusted for heart diseases indicated different SIVE, and was 36.0% (95% CI: 3.2%, 409.2%) and 78.4% (95% CI: 17.4%, 352.2%) for ≥ 65 and 18-64 year-old participants, respectively. Conclusion The point estimates suggested high SIVE against any influenza among hospitalized 18-64-year-old SARI participants, while lower estimates were found in the ≥65-year-old group. Although SIVE estimates confidence intervals are broad and results can serve only as indicatory due to lacking sample size and therefore statistical power, they are in line with the estimates reported by other studies during the 2019-2020 season.