The flu shot does not lower the overall risk of death, according to Canadian researchers.

A study compared more than 700 elderly people, half of whom had been vaccinated. A team from the University of Alberta said that it could not find any statistically significant reduction in risk of death from any cause.

Author Dean T. Eurich said that previous studies that found a strong benefit were probably flawed.

"Over the last two decades in the United Sates, even while 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. 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."

The study looked at patients who had checked in to hospitals with pneumonia outside of flu season. It found that 12 percent of the patients died overall. Unvaccinated patients were more likely to die, but not more likely than a vaccinated patient with a similar age, background, medical history and sex.

Therefore, it could be that earlier studies found a higher mortality rate in the unvaccinated because people who were more likely to die anyhow or were in poorer health were also less likely to get the vaccine.

"The healthy-user effect 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," said principal investigator Dr. Sumit Majumdar.