Effectiveness of trivalent inactivated influenza vaccine among community-dwelling older adults in Thailand: A two-year prospective cohort study

Abstract Background We conducted a two-year prospective cohort study to measure the effectiveness of trivalent inactivated influenza vaccine (IIV3) to prevent laboratory-confirmed influenza among community-dwelling Thai adults aged ≥65 years during 2015–16 and 2016–17 influenza seasons. Methods In 2015, we enrolled a cohort of 3220 participants. Trained health volunteers collected baseline data and followed participants for two years with weekly surveillance for new or worsened cough with self-collection of nasal swabs. Vaccine effectiveness (VE) was estimated as 100% × (1- rate ratio of rRT-PCR -confirmed influenza) among vaccinated versus unvaccinated participants. Propensity score stratification was used to reduce differences between vaccinated and unvaccinated participants associated with access to and receipt of IIV3. Findings During 2015–16 and 2016–17, 1666 (52%) and 1498 (48%) participants received IIV3, respectively. The overall incidence of influenza during the two seasons was 14.3/1000 person-years among vaccinated participants and 20.2/1000 person-years among unvaccinated participants. VE was −4% (95% confidence interval [CI], −83%–40%) during 2015–16 when there was poor antigenic match between the dominant circulating A/H3N2 viruses and the vaccine strain, and 50% (95% CI, 12–71%) during 2016–17 when circulating and vaccine strains were well-matched. Of all three influenza subtypes in both years, significant protection was observed only against Influenza A/H3N2 during 2016–17 (VE, 49%; 95% CI, 3–73%). Interpretation During a season with well-matched circulating and vaccine strains, IIV3 was moderately effective against laboratory-confirmed influenza among older adults in Thailand.