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"What happens frequently is that one type of flu will circulate at one time in the season and be replaced by another type later in the season, so in a week's time, a month's time, we may have mostly H3s or mostly Bs, which oseltamivir works wonderfully for," he explained.

To be most effective, Tamiflu, made by Roche, needs to taken within 48 hours after flu symptoms appear, Bresee said. "Generally, antivirals for influenza are underused in the U.S.," he added.

Most people with flu are not treated, Bresee noted. "Clearly, it's a self-limited disease and you will get over it," he said.

Bresee noted there are alternatives to Tamiflu, including Relenza (zanamivir) made by GlaxoSmithKline. "The viruses that we are seeing that are resistant to Tamiflu are sensitive to zanamivir," he said.

Zanamivir has some limitations, Bresee said. It can't be given to young children and patients with certain lung diseases.

Other treatment options include the antiviral rimantadine (brand name Flumadine).

Bresee added that all the circulating flu stains are prevented by the flu vaccine, and it's not too late to get vaccinated. "Folks who haven't gotten the vaccine so far can still go get it," he said.

Dr. Pascal James Imperato, dean and Distinguished Service Professor of the Graduate Program in Public Health at SUNY Downstate Medical Center in New York City, thinks resistance to Tamiflu developed because of its use in other countries to treat upper respiratory infections.

"The emerging resistance of H1N1 influenza strains to oseltamivir was first noted a few years ago," Imperato said. "At present, based on analyses of H1N1 isolates from 16 countries in 2008, some 31 percent demonstrated resistance to Tamiflu."

Tamiflu resistance has not been documented for the two other common current human influenza strains, H3N2 and influenza B, or for avian influenza (
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