In the study involving more than 700 matched elderly subjects, half of whom had taken the vaccine and half of whom had not, the researchers found that any such benefit "if present at all, was very small and statistically non-significant and may simply be a healthy-user artifact that they (previous studies) were unable to identify."
Principal investigator Sumit Majumdar, M.D., M.P.H., associate professor in the Faculty of Medicine and Dentistry at the University of Alberta, suggested that flu shot's overall benefit appears to have been exaggerated.
The researchers analysed the clinical data from 704 patients 65 years of age and older who were admitted to the hospital for community-acquired pneumonia during non-flu season, half of whom had been vaccinated, and half of whom had not.
Each vaccinated patient was matched to a non-vaccinated patient with similar demographics, medical conditions, functional status, smoking status and current prescription medications.
The study showed that that 12 percent of the in patients died overall, with a median length of stay of approximately eight days.
While analysis with a model similar to that employed by past observational studies indeed showed that patients who were vaccinated were about half as likely to die as unvaccinated patients.
"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 Sumit Majumdar, M.D., M.P.H., associate professor in the Faculty of Medicine and Dentistry at the University of Alberta.
"While such a reduction in all-cause mortality would have been impressive, these mortality benefits are likely implausible," said Dean T. Eurich,Ph.D. clinical epidemiologist and assistant professor at the School of Public Health at the University of Alberta.
"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," he added.