NEW YORK (Reuters Health) - While influenza vaccination does provide protection against catching the flu, it does not have a major impact on death in the elderly, contrary to what some studies have suggested, a new study suggests.

In prior studies, an impressive 50 percent reduction in death from any cause had been noted in elderly people who got a flu shot, but some researchers were skeptical of this degree of benefit, suggesting that it may have been the result of the "healthy user effect." The new study supports this line of thinking.

The study included more than 700 elderly people, half of whom had gotten a flu shot and half of whom had not. After controlling for a variety of factors that were largely not considered or simply not available in previous studies, the researchers concluded that any death benefit "if present at all, was very small and statistically non-significant and may simply be a healthy-user artifact that they were unable to identify."

"The healthy-user effect," study chief Dr. Sumit Majumdar of the University of Alberta in Edmonton, Canada explained in a statement, "is seen in what doctors often refer to as their 'good' patients patients who are well-informed about their health, who exercise regularly, do not smoke or have quit, drink only in moderation, watch what they eat, come in regularly for health maintenance visits and disease screenings, take their medications exactly as prescribed and quite religiously get vaccinated each year so as to stay healthy. Such attributes are almost impossible to capture in large scale studies using administrative databases."

"Over the last two decades in the United Sates, even while (flu) vaccination rates among the elderly have increased from 15 to 65 percent, there has been no commensurate decrease in hospital admissions or all-cause mortality," added co-investigator Dr. Dean T. Eurich, who is also with the University of Alberta.

"Further, only about 10 percent of winter-time deaths in the United States are attributable to influenza, thus to suggest that the vaccine can reduce 50 percent of deaths from all causes is implausible in our opinion," he added.

The study involved 352 patients given the vaccine and 352 matched control subjects. Overall, 85 percent of patients were over 64 years of age. Severe pneumonia was seen in 29 percent of patients and 12 percent of the patients died.

Flu vaccination was, in fact, associated with reduced mortality of about 50 percent (8 percent vs. 15 percent mortality in the vaccinated and unvaccinated groups, respectively), and this finding did not change after accounting for age, gender, or co-existing illnesses.

However, after adjusting for other potential confounders, including functional and socioeconomic status, the mortality reduction was weakened and no longer statistically significant.
"Previous studies were likely measuring a benefit not directly attributable to the vaccine itself, but something specific to the individuals who were vaccinated a healthy-user benefit or frailty bias," Eurich concluded in a statement.

SOURCE: American Journal of Respiratory and Critical Care Medicine, September 2008.