Influenza vaccination of older adults in the current and previous seasons was associated with a reduced risk for severe influenza compared with vaccination in the current season alone or no vaccination, according to the results of a study published in the Canadian Medical Association Journal.

In this case-control Spanish study, adults older than 65 who were admitted to the hospital with laboratory-confirmed influenza were matched with inpatient controls based on sex, age, hospital, and admission date. Severe influenza was considered a case requiring intensive care unit (ICU) admission or death within 30 days of hospital admission. Vaccination status for the current influenza season as well as the 3 previous seasons was evaluated and compared in patients with severe influenza, and nonsevere influenza, and controls.

Based on comparison with controls, the efficacy of the influenza vaccination for hospitalization prevention in the current or any previous season was 31%. Efficacy for prevention of ICU admission due to influenza and prevention of death was higher at 74% and 70%, respectively. Vaccination in the current season, however, did not show efficacy for prevention of severe influenza (–1%).

An increased number of influenza vaccine doses in the current and previous seasons was negatively correlated with the risk for admission to the hospital due to severe influenza (P <.001 for linear trend). Furthermore, vaccine efficacy for severe influenza in the current season was improved in patients who were vaccinated in previous seasons. No significant association was reported for the number of vaccinations and hospitalization due to nonsevere influenza.

Vaccination in the current season as well as any of the previous 3 seasons was associated with reduced influenza severity (odds ratio [OR] 0.45), decreased admission to the ICU (OR 0.35), and reduced mortality (OR 0.44). In contrast, vaccination in only the current season was not associated with reduced odds of severity and was associated with increased odds of death (OR 3.35).

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The investigators noted that “repeated vaccination for influenza was highly effective in preventing severe and fatal infection caused by influenza in older adults.” “Because severe cases of influenza may be prevented by 2 mechanisms, the effectiveness of vaccination against severe influenza may be greater than that for mild cases, and the benefit of influenza vaccination may be greater than that estimated in previous studies,” concluded the researchers.


Casado I, Domínguez Á, Toledo D, et al.; Project PI12/02079 Working Group. Repeated influenza vaccination for preventing severe and fatal influenza infection in older adults: a multicentre case–control study. CMAJ. 2018;190:E3-E12. doi:10.1503/cmaj.170910