Young B, etc.,al. Six-monthly versus annual influenza vaccination in older adults in the tropics: an observer-blind, active-comparator controlled, randomised superiority trial. Clin Infect Dis. 2018 Oct 1.
Antibody titres and vaccine effectiveness decline within six 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.
Tropical Influenza Control Strategies (TROPICS1) was a single-centre, 1:1 randomised, observer-blinded, active-comparator controlled, superiority study in 200 community-resident adults aged ≥65 years. Participants received standard-dose trivalent inactivated influenza vaccination (IIV3) at enrolment, and either tetanus-diphtheria-pertussis vaccination or IIV3 six months later. Primary outcome was the proportion of participants with haemagglutination-inhibition (HI) geometric mean titre (GMT) ≥1:40 one month after the second vaccination (Month 7). Secondary outcomes included GMTs to Month 12, the incidence of influenza-like illness (ILI), and adverse reactions after vaccination.
At Month 7, the proportion of participants with a HI tire ≥1:40 against A/H1N1 increased by 21.4% (95% CI 8.6-33.4) in the six-monthly vaccination group. This proportion was not significantly higher for A/H3N2 (4.3, 95% CI -1.1-10.8) or B (2.1, 95% CI -2.0-7.3). Six-monthly vaccination significantly increased GMTs against A/H1N1 and A/H3N2 at Month 7, but not B. Participants receiving repeat IIV3 reported a significantly lower incidence of ILI in the six months after the second vaccination (relative vaccine effectiveness 57.1%, 95% CI 0.6-81.5). The frequency of adverse events was similar after first or second influenza vaccination.
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