For people over age 65, flu vaccines may not work as well as predicted.

The finding, from two careful University of Washington studies, is a big surprise. Yes, elderly people don't get the same vigorous boost of immunity from flu vaccine as younger people. But public health experts almost universally agree that flu vaccination is a priority for elderly people.

According to the World Health Organization, elderly people not living in nursing homes are 39 percent to 75 percent less likely to die if they get their annual flu shot. The shot, the WHO says, cuts a person's risk of death from any cause by 68 percent.

Not so, find Lisa A. Jackson, MD, MPH, and colleagues. They report that healthy elderly people are much more likely to get flu shots. When this bias is removed -- that is, when one compares vaccinated elderly people with unvaccinated elderly people who have the same health status -- those who get flu shots don't have a significantly lower risk of death.

"The magnitude of this bias … was sufficient to account entirely for the [reduced risk of death in vaccinated people] observed during influenza season," Jackson and colleagues report in the Dec. 20 advance access issue of the International Journal of Epidemiology.

How Effective Is Flu Vaccine for the Elderly?

Jackson's team arrived at their findings by comparing death rates among the vaccinated and unvaccinated elderly people before, during, and after flu season. Those who got flu vaccine had a lower death risk in all time periods -- even when there was no flu going around.

In a second study, the researchers looked at 252 people aged 65 and older who died during a flu season. They matched them with 572 people who died at other times.

Elderly people who got flu vaccines were 41 percent less likely to die during flu season. But those who didn't get flu shots were much more likely to have "functional limitations." When Jackson and colleagues compared people with the same health status, they found that vaccinated elderly people were only 29 percent less likely to die during flu season.

Vaccinate Youngsters, Protect Oldsters?

An editorial accompanying the Jackson team's studies argues that we urgently need better flu vaccines to protect elderly people. And the editorialists, W. Paul Glezen, MD, of Baylor College of Medicine and Lone Simonsen, PhD, of the National Institute of Allergy and Infectious Diseases, offer another idea.

"Many studies have shown that school children have the highest rates of infection with influenza each year, and that they are the major spreaders of influenza in the community and introducers into the household," they write. "Immunization of school children, therefore, will reduce exposure of vulnerable patients to influenza."

Glezen and Simonsen suggest that school-based vaccine clinics vaccinate all kids every year. If the nasal spray vaccine was used, they note, there would still be enough flu shots for elderly people. And with the "major transmitters" of the flu virus already vaccinated, these flu shots would offer even more protection.

By Daniel J. DeNoon, reviewed by Louise Chang, MD

SOURCES: Jackson, L.A. International Journal of Epidemiology, Dec. 20, 2005; advance access. Glezen, W.P. and Simonsen, L. International Journal of Epidemiology,Dec. 20, 2005; advance access.