The two drugs, Relenza and Tamiflu, bottle flu virus inside infected cells and prevent it from migrating to healthy ones, thereby reducing the duration of the attack. Although Relenza, sold by GlaxoSmithKline, and Tamiflu, from Roche, are similar products, only Tamiflu has been approved in this country for both the treatment and prevention of flu.

"It is the healthy for whom we've got the most information," says study author Nicola Cooper, an epidemiologist at the University of Leicester, England. "It's the high-risk groups that need the most attention." These include the very elderly and those with chronic diseases such as diabetes, heart disease and lung problems.

Flu claims 20,000 lives each year in the United States alone, and sends 110,000 people to the hospital each infection season. Health officials urge vaccination in everybody, and especially those at high risk of contracting serious illness. But while the shots are the best defense against influenza they're not guaranteed protection, and roughly 25 percent of people who receive the vaccine wind up with the flu.

Tamiflu, which is a pill, and the inhaled Relenza have been shown to shorten flu bouts in people who contract the virus, and Tamiflu has also been approved in this country as a means of preventing infection.

The latest study appears in the June 7 issue of the British Medical Journal.

Cooper and her colleagues reviewed 17 previous treatment trials and seven prevention studies with the two drugs that had been published before 2002. The studies they included, out of a pool of 80 published reports, looked at the medications in three groups: children 12 and under, healthy people between the ages of 12 and 65, and high-risk people -- the elderly or those with heart, lung or other chronic ailments.

Both drugs were effective at trimming the duration of flu symptoms -- by about a day or so, on average -- and both slashed the risk of developing the infection by between 70 percent and 90 percent.

But most of the studies didn't include elderly subjects or people at high risk of complications from flu.

Barbara Freund, of Eastern Virginia Medical School's Glennan Center for Geriatrics and Gerontology in Norfolk, helped conduct one study of Tamiflu's effectiveness in nursing home residents. Since 2001, researchers have conducted other trials with the drug and Relenza in older people, Freund says, but much of that work remains unpublished.

"We certainly know more than we did and I think the experience in the older population is probably now greater with Relenza," she says. "But certainly we need to do more in those high-risk populations."

Dr. Paul Drinka, medical director of the Wisconsin Veterans Home, in the town of King, says he agrees that relatively little is known about how the two drugs treat flu in the frail elderly. However, he adds, it would be ethically troubling to conduct the most rigorous study possible -- one that included people who got sugar pills instead of real drug -- in sicker subjects.

"Morally, I could not participate in such a study, to withhold a treatment that's proven effective in younger people. I don't know that such a study will ever be done," says Drinka, who also holds a faculty position at the University of Wisconsin Medical School.

Meanwhile, Drinka says, doctors are prescribing Tamiflu and Relenza to high-risk patients despite the dearth of data. "To me it's logical that those who are at the greatest risk of adverse events like pneumonia or death would have the most to gain from an effective treatment." Evidence shows that not only can the drugs shorten flu symptoms but they can also reduce the risk of pneumonia, a serious complication in older flu sufferers, he says.

Daniel Perry, executive director of the Alliance for Aging Research, calls the failure to include the elderly in clinical trials "ageism." Older Americans consume about 40 percent of the prescription drugs in this country each year, Perry says, "but it is the rare clinical trial that includes people over the age of 65 or 70. And it's almost unheard of to include people with multiple chronic conditions. But that is really the profile of the people who will be using the drug."

The elderly metabolize drugs differently and retain their effects longer than do younger patients. The various drugs they take can interact, sometimes in dangerous ways. While it's true that designing robust studies in people with several illnesses on several medications is tricky, Perry says, "we're going to have to do that."

More information

To learn more about flu, visit the Centers for Disease Control and Prevention. And try the U.S. Food and Drug Administration for more on Tamiflu and Relenza.