Pamploma, Spain—For pharmacies providing flu shots, older adults who are repeat customers appear to get the most benefit.

Getting a shot over multiple years was twice as effective in preventing severe influenza in patients hospitalized for influenza, according to a report in the Canadian Medical Association Journal, which also noted that the effect was consistent regardless of flu season, virus subtypes, or age of patient.

For the study, Spanish researchers looked at the effect of repeated influenza vaccinations in the current and three previous seasons in those aged 65 years and older. The patients were admitted to 20 Spanish hospitals in 2013/14 and 2014/15. 

“Repeated vaccination for influenza was highly effective in preventing severe and fatal infection caused by influenza in older adults,” write the authors, led by researchers from the Instituto de Salud Pública de Navarra in Pamplona, Spain.

The study enrolled 130 inpatients with severe and 598 with nonsevere influenza, matching them to 333 and 1,493 controls, respectively. Results indicate that, compared with patients who were unvaccinated in the current and three previous seasons, adjusted effectiveness of influenza vaccination in the current and any previous season was 31% (95% confidence interval [CI] 13%–46%) in preventing admission to hospital for nonsevere influenza, 74% (95% CI 42%–88%) in preventing admissions to ICU and 70% (95% CI 34%–87%) in preventing death.

On the other hand, the research found that vaccination only in the current season had no significant effect on cases of severe influenza. Yet, among inpatients with influenza, vaccination in the current and any previous season did reduce the risk of severe outcomes (adjusted odds ratio 0.45, 95% CI 0.26–0.76).

“Because severe cases of influenza may be prevented by two 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,” study authors concluded. “The prevention of severe and fatal infection caused by influenza was observed mainly in patients who were vaccinated in both the current and previous seasons, which reinforces the recommendation of annual vaccination for influenza in older adults.”
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