Does influenza vaccination among the elderly protect against death? A prospective cohort study of all elderly persons in a large geographical area

¹ Karolinska Institutet, Dept of Medicine, Infectious Diseases Unit, Karolinska Solna, and Dept of Communicable Diseases Control and Prevention, Stockholm County Sweden
² Karolinska Institutet, Dept of Internal Medicine, Clinical Epidemiology Unit, Karolinska Solna, Sweden
³ Karolinska Institutet, Dept of Internal Medicine, Clinical Epidemiology Unit, , Karolinska Solna, Sweden
⁴ Karolinska Institutet, Dept of Medicine, Infectious Diseases Unit, Karolinska Solna, Sweden

^* To whom correspondence should be addressed. E-mail: ake.ortqvist{at}sll.se.

	Abstract

The around 50% reduced overall mortality previously associated with influenza vaccination among the elderly is based on studies neither fully taking into account systematic differences between individuals who accept or decline vaccination, nor encompassing the entire general population. We performed a population based prospective cohort study in Stockholm County, Sweden, including all persons 65 years, or older, (n260.000) during three influenza seasons 1998/99, 1999/00, and 2000/01. Relative risks for mortality among vaccinated versus unvaccinated were estimated using Cox' proportional hazards regression, adjusted for, and stratified by, demographic factors and co-morbidity during and off, respectively, each of the three seasons.

Influenza vaccination was associated with an un-adjusted reduction in all-cause mortality by 50%, 46%, and 42%, respectively, which decreased to 14% (95% CI 5-23%), 19% (95% CI 11-27%), and 1% (95% CI -10-11%), following adjustment for confounders and differences in mortality between vaccinated and unvaccinated after the influenza season. The NNTs to prevent one death were 297 (212-495), 158 (123-222), and 743 (314-).

Influenza vaccination remains a very effective intervention, since only 150 to 300 elderly needed be vaccinated to prevent one death, although the effectiveness in reducing all-cause mortality in elderly persons is lower than previously thought.

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