Providing a new weapon against one of mankind's most threatening ailments, the Food and Drug Administration yesterday approved the first of a new class of drugs designed to disrupt the influenza virus.

The drug, which must be delivered by inhaler, will be sold under the brand name Relenza by Glaxo Wellcome PLC, a pharmaceutical concern based in London. If given early enough in the course of a flu infection, Relenza makes people feel better and shaves a day to a day and a half off the typical six- to 10-day course of illness.

It's the first of a new group of drugs that block the action of neuraminidase, an essential protein in the life cycle of influenza viruses. A similar drug that can be given as a pill is under development by F. Hoffmann-La Roche Ltd., the Swiss pharmaceutical giant, and could be licensed by October. This means doctors are likely to have two new choices in treating influenza this winter.

Vaccination is the primary means of controlling the flu. But the vaccine must be administered every year, and many people don't bother to get it. In addition, flu strains sometimes pop up too quickly to be controlled by vaccination. The new drugs show effects against all known strains of flu and in people who have already been infected with the virus, assuming they get treated within the first couple of days of illness.

"We're very excited about this," said Michael Elliott, project director for influenza research at Glaxo Wellcome. "It's great to have a new, breakthrough drug."

The effects of the neuraminidase inhibitors as treatments are relatively modest. But their most important eventual use could be as preventives in case of a worldwide flu pandemic. Scientists are constantly worried about a repeat of the flu pandemic of 1918, when a virulent strain wiped out 20 million people worldwide.

Indeed, a virulent flu strain popped up in Hong Kong in 1997, killing six of the 18 people who got it. But health authorities stopped that outbreak in its tracks by destroying all of Hong Kong's poultry, which were passing the virus to people.

Research on using the new drugs for prevention is still underway, but the studies so far suggest that neuraminidase inhibitors may be nearly as effective as vaccine, even though they can be deployed much later in the flu season. (Flu vaccine, whose formula is tweaked yearly, takes months to manufacture and, once administered as a shot, it takes several more weeks to induce immunity.) Many experts are betting the new drugs will become the world's primary control strategy if a virulent flu strain crops up suddenly and begins to spread.

Two older drugs are available to treat influenza, but they only work against a type called Influenza A, and many viruses have developed resistance to the drugs. So far, the newer drugs don't appear to have these limitations.