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, according to an article in The Archives of Internal Medicine.
Although influenza vaccination of the elderly in the U.S. has increased from 15 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, Ph.D., 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 scientist 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.
Based on that information, the authors see a vast disconnect between conclusions from different studies on the impact of the shortage of influenza vaccine in the 2004-2005 season, ranging from tremendous to hardly noticeable consequences.
MEDICA.de; Source: American Medical Association (AMA)