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

Abstract

BACKGROUND\nWe 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\u202fyears during 2015-16 and 2016-17 influenza seasons.\n\n\nMETHODS\nIn 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%\u202f×\u202f(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.\n\n\nFINDINGS\nDuring 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%).\n\n\nINTERPRETATION\nDuring a season with well-matched circulating and vaccine strains, IIV3 was moderately effective against laboratory-confirmed influenza among older adults in Thailand.