Observational studies which report that influenza vaccination reduces winter mortality risk among the elderly by 50 percent may substantially overestimate the vaccination benefit.

Accurate determination of the impact of influenza on mortality is difficult because the infection is often cleared before the onset of the secondary complications that actually cause a person’s death.

Although influenza vaccination of the elderly in the U.S. has increased from 15 to 20 percent before 1980 to 65 percent in 2001, the authors could find no correlation between this increasing vaccination coverage after 1980 and declining deaths rates in any age group. Observational studies may introduce a systematic bias that leads to a substantial over-estimate of the impact of influenza vaccination on mortality, the authors suggest.

Lone Simonsen, of the National Institute of Allergy and Infectious Diseases, and colleagues, used statistical models that estimate the winter-seasonal all-cause mortality above an estimated baseline to determine influenza-related mortality indirectly.

Their model incorporated information on deaths among the elderly from pneumonia and influenza and all other causes from 33 winter seasons from 1968-2001.

“ Our results, based on national vital statistics, are simply not consistent with the very large mortality benefits reported in observational studies,” the authors write. The authors suggest that this disconnect may be explained by a disparity in who is likely to be vaccinated. “ Very ill elderly people, whose fragile health would make them highly likely to die over the coming winter months, are less likely to be vaccinated during the autumn vaccination period,” they stated.

“ Our results have obvious implications for influenza vaccination policy. … The present findings, and those of at least one other study, indicate that the shortage [ of influenza vaccine in the 2004-2005 season ] will have little impact [ on mortality ]…,” the authors conclude.
“ Other cohort studies suggest that the shortage will have a tremendous impact on mortality among the elderly. Either way, this vast disconnect between conclusions from different studies must be sorted out.”

Source: The Archives of Internal Medicine, 2005


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