Commentary: Benefits of influenza vaccine in US elderly—new studies raise questions

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Abstract
During the period from 1989 to 1997 the vaccination rate for elderly persons ≥65 years of age in the US increased from 30 to 67%. Despite this increase in coverage, mortality and hospitalization rates continued to increase rather than decline as would be expected if the vaccine were optimally efficacious. Currently over 50 000 deaths result from influenza virus infections each year in the US.1 The rate of hospitalizations owing to complications of influenza has increased steadily since 1979 and the average number for the last decade is almost 400 000 per year.2 About 85% of deaths and 63% of hospitalizations attributable to influenza occur in persons ≥65 years of age. About two-thirds of the US elderly have been vaccinated each year since 1997. Despite substantial vaccine coverage in the most vulnerable group, no effect on national all-cause excess mortality or hospitalizations has been evident—even after taking population aging and changing virus dominance patterns into account.3 Furthermore, observational studies of large cohorts have reported that elderly persons who elect to take the influenza vaccine were less likely to die or to be hospitalized during the influenza season than were unvaccinated elderly.4 Protection rates were estimated at 47% for all wintertime deaths and 22–27% for hospitalization. The investigators noted that the elderly who chose to be vaccinated had more co-morbidities, like diabetes or cardiopulmonary diseases, listed on their administrative data files and were, therefore, considered to be at greater risk for complications of influenza than their unvaccinated cohorts. The apparent 50% reduction in mortality is remarkable because the wintertime excess all-cause mortality that might be attributed to influenza was found to be 1,3 The finding that influenza vaccine protection in these observational studies exceeded any possible expectation requires some explanation.