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Press Release

October 03, 2010

Oakland, CA – Influenza vaccine has a modest – albeit still significant – effect on prevention of hospitalizations for pneumonia and influenza in those over 50, according to researchers at the Kaiser Permanente Vaccine Study Center.

In the study, which appears in the current issue of Vaccine, researchers estimated that over the 11-year period studied, influenza vaccination prevented 8.5 percent of hospitalizations for pneumonia and influenza in people 65 and older and 12.4 percent in those 50-64 years of age.

The findings suggest that the 8.5 percent vaccine effectiveness estimate in the elderly amounts to a 28 percent reduction in influenza-attributable hospitalization.  The corresponding reduction for people aged 50-64 years of age was 48 percent. Researchers found no effect of influenza vaccination on hospitalization for ischemic heart disease, congestive heart failure, cerebrovascular disease, or trauma.

The study builds on prior research that has found that people who get flu shots experience unexpectedly low mortality even when influenza is not circulation – indicating that prior observational studies may have overstated vaccine effectiveness due to selection bias or other factors.

In this study, researchers examined the association between prior influenza vaccination and hospitalizations when influenza was not circulating and compared it to the association found when influenza was circulating. Vaccine effectiveness was estimated from the difference in the association between hospitalization and prior vaccination, inside vs. outside influenza seasons.

“The bottom line here is that people should be vaccinated,” said Roger Baxter, MD, co-director of the Kaiser Permanente Vaccine Study Center. “But we should be looking for more effective vaccines for the elderly.”

The retrospective population-based study analyzed data from health plan members age 50 and older during September1997 and August 2008, when there were about 68,000 pneumonia hospitalizations in 10 million person years. Researchers did not have access to data on influenza vaccinations that might have been given outside of Kaiser Permanente. Vaccine effectiveness may vary for the severity of the influenza season and the match of the vaccines to circulating strains. Determination of  year-specific vaccine effectiveness would require a larger population and more years.

Additional authors on the study included G. Thomas Ray and Bruce Fireman, both with the Kaiser Permanente Division of Research. The study was funded by a Kaiser Permanente Community Benefits grant.