Anti-flu shot does not reduce death risk


Washington : Flu vaccine does confer protection against specific strains of influenza, but its overall benefit seems to have been exaggerated by observational studies, especially in case of the elderly.

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“Over the last two decades in the US, 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,” said Dean T. Eurich, clinical epidemiologist and assistant professor at the School of Public Health, University of Alberta.

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

The Alberta study included more than 700 elderly subjects, half of whom had taken the vaccine. After controlling for variables that were largely not considered or simply not available in previous studies, researchers concluded that any such benefit “. . . was very small and statistically non-significant and may simply be a healthy-user artifact that they were unable to identify.”

“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,” said Eurich.

Eurich and colleagues hypothesized that if the healthy-user effect was responsible for the benefit associated with influenza vaccination seen in observational studies, there should also be a significant mortality benefit present during the “off-season.”

To determine whether the observed mortality benefits were actually an effect of the flu vaccine, therefore, they analysed clinical data from records of all six hospitals in the Capital Health region in Alberta.

They analysed 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.

In examining in-hospital mortality, they found that 12 percent of the patients died overall, with a median length of stay of approximately eight days. Further analyses that included more than 3,400 patients from the same cohort did not significantly alter the relative risk.

“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 . . . 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, associate professor in the Faculty of Medicine & Dentistry at the University of Alberta.

The results will appear in the September issue of the American Journal of Respiratory and Critical Care Medicine.