Despite the arrival of a highly touted new influenza drug, health experts caution that the nation's first and best line of defense against flu remains the annual shot in the arm.

The new drug, Relenza, reached the market last week and comes in the form of a powder that is inhaled. If taken within 24 to 48 hours after symptoms strike, it can bring a "modest" reduction in the severity and duration of the typical flu bout, according to the Food and Drug Administration, which approved the drug in July.

But doctors say the drug, which costs about $50 per treatment, is no substitute for the more fundamental preventive approach of the vaccine.

"The mainstay of any successful flu prevention campaign has to be immunization," said Jehan El-Bayoumi, an internist at George Washington University Medical Center.

Relenza provides one more weapon against flu, she said, though the studies of its effectiveness "are fairly small, and we'll have to see how this translates into real use." The drug, inhaled through a plastic mouthpiece provided with each prescription, has fewer side effects than other available antiviral flu drugs.

Its maker, Glaxo Wellcome Inc., is promoting Relenza in part as a way to reduce the likelihood of a case of flu spreading through a family.

Influenza is a huge double threat to public health. For most of its victims, it causes a week or two of misery, malaise and missed school or work. But many also develop serious and life-threatening complications, such as pneumonia. About 20,000 Americans die of flu-related illness in a typical year, according to the federal Centers for Disease Control and Prevention (CDC), and influenza and pneumonia together are the nation's sixth-leading cause of death.

Flu-related complications can occur at any age, but are most common in older people and those with chronic illnesses. Among Americans over 80 years old, flu and pneumonia kill more than any other illness except heart disease and stroke.

The flu vaccine is not perfect, but it lessens symptoms or prevents them altogether in an estimated 70 to 90 percent of healthy adults. Flu season usually runs from November through March. Because it takes four weeks or more to develop maximum protection from the vaccine, the best time to get a flu shot is this month.

Last winter, flu cases peaked nationally in early February, and deaths from flu peaked in early March. This year's vaccine, based on health experts' best guess as to which versions of the shifty influenza virus will strike, is aimed at two A-type flu strains and one B-type.

The CDC recommends an annual flu shot for people most likely to have complications from flu, including those aged 65 or older; residents of nursing homes and other facilities for the chronically ill; and those with chronic conditions such as asthma, anemia, diabetes or heart, lung or kidney disease. Health care workers and people who share a household with high-risk individuals should also get a flu shot, CDC says.

"I think we have to immunize healthy people as well as high-risk people, because those healthy people are coming into contact with the high-risk ones," El-Bayoumi said.

The American Academy of Family Physicians last month revised its guidelines to recommend that everyone past age 50 get an annual flu shot.

Relenza, taken twice a day for five days, is sold only with a doctor's prescription. It is approved for treatment of influenza in adults and children aged 12 and older who have had flu symptoms for less than 48 hours. Another new anti-flu drug, a tablet with the brand name Tamiflu, is expected to be approved by the FDA for marketing soon, and more may follow in the potentially lucrative flu treatment market.

Relenza and Tamiflu have advantages over the two antiviral flu drugs already on the market: amantidine (brand name: Symmetrel) and rimantidine (Flumadine).

Those older drugs can help prevent or diminish flu caused by A-type influenza viruses, but are not effective against B-type flu, the other main variant. Relenza and Tamiflu work against both types and are less likely to have side effects.

Both Relenza and Tamiflu combat the flu virus in the same way, by blocking a protein crucial to the virus's life cycle. Studies of their effectiveness have been mixed, but some suggest that the drugs shorten a typical flu case by up to a day and a half, on average, and diminish the intensity of aches and other symptoms.

"That makes [Relenza] a useful drug, but its actual impact won't be clear until we go through a season or two," said Norman Edelman, scientific adviser to the American Lung Association.

"It would be unfortunate if people relied on these drugs to handle their flu and stopped getting vaccinated," said Edelman, who is dean of the school of medicine and vice president of the Health Sciences Center at the State University of New York at Stony Brook.

One limitation of the new drugs, Edelman noted, is that they must be taken right away, within 24 to 48 hours after the first hint of symptoms. Yet flu symptoms overlap insidiously with those of the common cold, and Americans typically get three to five colds each winter. That means people on guard against flu symptoms might end up using a drug like Relenza repeatedly--and perhaps needlessly--at $50 or more per treatment.