Semiannual versus annual influenza vaccination in older adults in the tropics: An observer-blind, active-comparator-controlled, randomized superiority trial

Abstract

Background. Antibody titres and vaccine effectiveness decline within 6 months after influenza vaccination in older adults. Biannual vaccination may be necessary to provide year-round protection in the tropics, where influenza circulates throughout the year. Methods. Tropical Influenza Control Strategies (TROPICS1) was a single-center, 1:1 randomized, observer-blinded, active-comparator- controlled, superiority study in 200 community-resident adults aged ≥65 years. Participants received a standard-dose trivalent inactivated influenza vaccination (IIV3) at enrollment, and either tetanus-diphtheria-pertussis vaccination or IIV3 6 months later. The primary outcome was the proportion of participa..