The determinants of influenza vaccination effectiveness in adults aged 65 years and older: a systematic review

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Purpose: Influenza vaccination effectiveness is generally lower in adults aged 65 years and above compared with healthy adults below the age of 65. This is generally attributed to weaker immune response in the elder population as a result of immunosenescence. However, the reasons for this phenomenon are not well characterised. The aim of this systematic review is to identify and summarise documented determinants of seasonal vaccine effectiveness in older adults. Methods & Materials: A literature search was conducted on bibliographic databases PubMed and EMBASE to identify relevant articles that investigate the determinants of influenza vaccine effectiveness in older adults up to July 4, 2015. Studies were eligible if they investigated potential determinants of seasonal influenza vaccination effectiveness in reducing the risk of laboratory-confirmed influenza, influenza-related hospitalisation or complications, and death or excess mortality in adults aged above 65. Results: Out of 37 relevant studies that were identified through the search process, eight studies that assess potential determinants of influenza vaccine effectiveness in older adults were identified. Six of the selected studies investigated vaccine factors, one assessed host factors, and one described factors involving vaccination policies and strategies as potential determinants of vaccine effectiveness in adults above 65 years old. The determinants that were significantly associated with influenza include vaccine type and route of administration, time elapsed after vaccination, frailty, and pneumococcal vaccination. However, vaccine effectiveness was evaluated using varying endpoints in these studies as investigators are interested in different primary outcomes, including laboratory-confirmed or diagnosed influenza, influenza-related hospitalisation or complications, and death or excess mortality. Conclusion: Documented determinants of influenza vaccine effectiveness in older adults can be broadly categorized into vaccine factors, host factors, and vaccination policies and strategies. Nevertheless, published studies that assess the impact of these determinants are relatively scarce and have lower internal validity as they are observational in nature. There is a need for more well-designed observational studies to identify and investigate the determinants of vaccine effectiveness in elder individuals, as they are at a higher risk of developing serious complications of influenza. The identification of these determinants will facilitate efforts to improve influenza vaccination in elder individuals.