Influenza vaccine effectiveness in older adults compared with younger adults over five seasons

Background: There have been inconsistent reports of decreased vaccine effectiveness (VE) against influenza viruses among older adults (aged >= 65 years) compared with younger adults in the United States. A direct comparison of VE over multiple seasons is needed to assess the consistency of these observations. Methods: We performed a pooled analysis of VE over 5 seasons among adults aged >= 18 years who were systematically enrolled in the U.S. Flu VE Network. Outpatients with medically-attended acute respiratory illness (cough with illness onset <= 7 days prior to enrollment) were tested for influenza by reverse transcription polymerase chain reaction. We compared differences in VE and vaccine failures among older adult age group (65-74, >= 75, and >= 65 years) to adults aged 18-49 years by influenza type and subtype using interaction terms to test for statistical significance and stratified by prior season vaccination status. Results: Analysis included 20,022 adults aged >= 18 years enrolled during the 2011-12 through 2015-16 influenza seasons; 4,785 (24%) tested positive for influenza. VE among patients aged >= 65 years was not significantly lower than VE among patients aged 18-49 years against any subtype with no significant interaction of age and vaccination. VE against A(H3N2) viruses was 14% (95% confidence interval [CI] -14% to 36%) for adults >= 65 years and 21% (CI 9-32%) for adults 18-49 years. VE against A(H1N1) pdm09 was 49% (95% CI 22-66%) for adults >= 65 years and 48% (95% CI 41-54%) for adults 18-49 years and against B viruses was 62% (95% CI 44-74%) for adults >= 65 years and 55% (95% CI 45-63%) for adults 18-49 years. There was no significant interaction of age and vaccination for separate strata of prior vaccination status. Conclusions: Over 5 seasons, influenza vaccination provided similar levels of protection among older and younger adults, with lower levels of protection against influenza A(H3N2) in all ages. Published by Elsevier Ltd.