WASHINGTON — Vaccines are less effective at protecting people 65 or older from influenza than has been thought, and some of the drugs commonly used to treat the flu are losing their power at a surprisingly fast rate, according to two new studies.

The findings, published online by The Lancet medical journal, come as the world is bracing for a possible flu pandemic from a dangerous strain of virus spreading among birds in Asia.

In the first study, an independent international collaboration of scientists reviewed all studies evaluating the effectiveness of flu vaccination for those 65 or older during the past 40 years.

The analysis, the most comprehensive conducted, found that vaccination was modestly effective, preventing 30 percent to 42 percent of hospitalizations. Those living in nursing homes fared somewhat better than those living on their own.

"The bottom line is the effectiveness is modest, at best," said Tom Jefferson, an epidemiologist in Rome who led the analysis for the Cochrane Collaboration, an independent international effort that evaluates the efficacy of medical care. "It's very disappointing."

The findings are particularly disappointing because people 65 or older are among those at highest risk. About 36,000 Americans die each year from the flu, many of them elderly.

Vaccines may be less effective in older people because their immune systems are less able to mount a vigorous response, Jefferson and others said. "People should ask whether it's worth investing these trillions of dollars and euros in these vaccines," Jefferson said.

Other experts said the findings illustrate the need to develop more effective approaches, but cautioned they should not discourage anyone from getting vaccinated. "We still urge the elderly and other high-risk groups to be vaccinated," said Dick Thompson of the World Health Organization.

Even a 30 percent effectiveness prevents suffering, said William Schaffner of Vanderbilt University.

In the second study, Rick Bright of the U.S. Centers for Disease Control and Prevention and colleagues analyzed 7,000 samples of flu virus collected around the world for genetic mutations that indicate the germs have developed resistance to a class of drugs known as adamantanes, which includes the widely used drugs amantadine and rimantadine.

The researchers found that the rate of resistance increased from 0.4 percent of the samples in 1994 to 12.3 percent in 2004. In the United States, the rate jumped from 1.9 percent in 2004 to 14.5 percent in the first six months of 2005, the researchers found.

"We were alarmed to find such a dramatic increase in drug resistance in circulating human influenza viruses in recent years," Bright said.

Other researchers agreed, especially because this class of drugs is less expensive than newer flu drugs and can be used to treat and prevent the disease.

"This level of resistance is quite alarming," said Robert Belshe of St. Louis University School of Medicine. "This is saying that these resistant viruses are spreading, and that's something we have to be concerned about."

So far, flu viruses, including the bird flu circulating in Asia, remain vulnerable to two newer drugs, Tamiflu and Relenza, which the federal government has been stockpiling.

Also

Indonesia announced plans yesterday for a mass chicken slaughter amid fears of a bird-flu epidemic after two more children suspected of having the disease died in the capital, Jakarta.

The government scrambled to calm fears after the deaths of two girls, ages 2 and 5.