Influenza vaccine failure in the tropics: A retrospective cohort study of waning effectiveness

Abstract

Influenza vaccine effectiveness (VE) wanes over the course of a temperate climate winter season but little data is available from tropical countries with year-round influenza virus activity. A retrospective cohort study of adults vaccinated from 2013-2017 in Singapore was conducted. Influenza vaccine failure was defined as hospital admission with polymerase chain reaction (PCR)-confirmed influenza infection 2-49 weeks after vaccination. Relative VE was calculated by splitting the follow up period into 8-week episodes (Lexis expansion) and the odds of influenza infection in the first 8-week period after vaccination (weeks 2-9) compared with subsequent 8-week periods using multivariable logistic regression adjusting for patient factors and influenza virus activity. Records of 19,298 influenza vaccinations were analysed with 617 (3.2%) influenza infections. Relative VE was stable for the first 26 weeks post-vaccination, but then declined for all three influenza types/subtypes to 69% at weeks 42-49 (95% confidence interval [CI] 52-92%, p=0.011). VE declined fastest in older adults, in individuals with chronic pulmonary disease and in those who had been previously vaccinated within the last two years. Vaccine failure was significantly associated with a change in recommended vaccine strains between vaccination and observation period (adjusted odds ratio 1.26, 95% CI 1.06-1.50, p=0.010). © 2020 Cambridge University Press. All rights reserved.