Potential health impact of twice-annual influenza vaccination in older adults in Hong Kong

Abstract

BACKGROUND: Each year older adults aged ≥65 years have highest risk for complications, hospitalizations, and deaths from seasonal influenza virus infections. There are numerous reports of lower immunogenicity and effectiveness of influenza vaccines in older adults compared to younger and middle-aged adults. In subtropical and tropical locations where Influenza circulates for prolonged periods of the year, vaccinating older adults every 6 months could increase the likelihood that vaccine components would match circulating influenza viruses as well as enhancing the magnitude, breadth, and duration of immune response and therefore increasing or prolonging protection against infection. METHOD: Using existing surveillance data, we constructed a simulation model to assess the impact of influenza twice-annual vaccination vs annual vaccination where impact is defined as the number of averted health outcomes in Hong Kong. The model parameters were derived from the literature. RESULTS: We estimated that twice-annual vaccination could avert 2201 influenza-associated hospitalizations and 116 influenza-associated deaths each year among older adults, relative to once-annual vaccination, if assuming 50% coverage for both annual vaccination and twice-annual vaccination, and 60% vaccine effectiveness. CONCLUSION: Twice-annual influenza vaccination in Hong Kong has the potential to provide considerable health benefit in terms of averted hospitalizations and deaths in older adults.