Tests showing that nearly all early-season flu cases are resistant to the main antiviral drug will likely complicate treatment of the elderly this year and planning for pandemics in the future, health officials said.

The impact from flu has been slight so far this winter, and it's unclear how the public will be affected. Very little influenza has been confirmed to date.

But the weaknesses of the leading flu drug, Tamiflu, should encourage people to take more conventional measures, such as frequently washing hands and getting vaccines, experts said.

"The take-home point is that people should get the flu shot," said Neil Fishman, director of infection prevention at the Hospital of the University of Pennsylvania.

Unlike in some previous years, the flu vaccine appears effective and supplies are plentiful. The body takes about two weeks to develop immunity against a disease that can be deadly, especially in the elderly.

Influenza comes in many forms, and they differ in susceptibility to various medications, when and where they circulate, severity of illness, and other factors.

Epidemiologists said the influenza type A strain known as H1N1 is often the first to show up, as it was this year.

The U.S. Centers for Disease Control and Prevention reported on Friday that 86 of the 88 of the H1N1 samples tested were resistant to Tamiflu, known generically as oseltamivir. The Pennsylvania Department of Health confirmed resistance in one sample each from Erie and Bucks Counties.

None of the other type A or B flu strains reported this year has shown a similar resistance, but physicians can't tell what strain a patient has - and what drug would work - without a laboratory test, which often is unavailable. The antivirals must be given within 48 hours of first symptoms to be effective.

"It is very tricky," said Esther Chernak, medical director of Philadelphia's acute communicable disease control program. "It will make the control of influenza in high-risk settings like hospitals and nursing homes more complicated."

Though most people take a drug such as Tamiflu to feel better faster - its main benefit has been to reduce the misery by just one or two days - the drug can serve more urgent needs in those with compromised immune systems, even if they have been vaccinated.

"Elderly people don't always produce robust antibodies," said William Kavesh, director of the geriatric clinic at the Philadelphia VA Medical Center.

Institutions rely on Tamiflu, first approved in 1999, as a preventive measure when they believe that a patient is infected or an epidemic is about to start, said Kavesh, an expert in prevention of infectious diseases among the elderly.

"Fortunately, epidemics in nursing homes are rare. When they do occur, people die from them," he said, adding that very sick, disabled people being cared for at home face similar risks.

The CDC estimates that 36,000 Americans a year die from complications of the flu, the vast majority of them elderly.

In such scenarios, no alternatives to Tamiflu are ideal. Doctors said the next best option, Relenza, must be taken by inhalation, which is harder to administer to nursing-home patients. Relenza, known generically as zanamivir, is made by GlaxoSmithKline. Tamiflu is made by Roche.

Two other, older drugs, amantadine and rimantadine, have not been recommended by the CDC in recent years because of increased resistance, although they appear to work on H1N1. In an advisory to doctors three weeks ago, the agency recommended combinations of drugs depending on various conditions.

The latest statistics, released late Friday, show little or no influenza activity in most of the country. Pennsylvania and New Jersey are among 15 states with higher - but still minor - activity.

The reports from both states and the nation as a whole show far less flu, at this early date, than in recent years, although experts said it was impossible to say whether that meant the season would be light or was merely starting later than usual. It usually peaks in February and March.

County statistics, which are not intended to be comprehensive, show four cases of influenza in Philadelphia through Jan. 3.

Regardless of how this season turns out, a major flu strain's resistance to a major antiviral drug could cause headaches for authorities trying to plan for flu pandemics of both the human and bird varieties.

Many countries, including the United States, have stockpiled antivirals for use against a possible flu epidemic.

And drug-resistant H1N1 has been found in Asia and other parts of the world, "so resistant influenza could be circulating at the same time as bird flu," said Susan Coffin, medical director of infection prevention and control at Children's Hospital of Philadelphia. That raises the possibility of a resistant variety of avian flu, which has proved deadly when it infects humans.

For now, virtually every expert said that rather than worry about Tamiflu, people should get a flu shot.

"Vaccine! Vaccine! Vaccine!" said Chernak, the communicable-disease specialist in the city health department.

"And it underscores the importance of non-pharmacological control measures," she said. "Stay home when you're sick. . . . Wash your hands. . . . Cover your mouth when you cough."

Should You Still Get a Shot?

In a word: Yes.

Who: The government has been gradually expanding its guidelines. The flu shot is now approved for people six months and older, including healthy people and those with chronic medical conditions.

Alternative: FluMist nasal-spray vaccine is approved for healthy people ages 2 to 49 who are not pregnant.

When: The sooner the better. Immunity develops over two weeks. (Vaccination campaigns usually begin in the fall because predicting the start of flu season is difficult.)

Where: Many doctors' offices and health centers have vaccine, and scheduled flu-shot clinics are continuing. Local clinics are posted online.

For more information or to find a flu-shot clinic near you, go to http://go.philly.com/flu