Government health officials announced Friday that a leading flu medicine, Tamiflu, might not work against all cases of the flu this year. The most common flu bug right now is overwhelmingly resistant to Tamiflu, they said.

CDC results of samples show 98 percent of the cases were resistant to Tamiflu. That drug has been the most common medicine used to control the virus for the past decade. The Tamiflu-resistant virus is the flu bug most commonly seen so far this year. It's been detected in 12 states so far, mostly in Hawaii and Texas.

Three different flu bugs are in circulation among humans. The resistant bug is the type A H1N1 strain. There's also the type A H3N2 strain, and one type B strain. However health officials say they are not too worried, for several reasons. First, it is early in the flu season, and it is not clear whether this strain will dominate through the next several months. Second, not many people take antiviral medications for the flu.

The alert is an early heads up for doctors who may need to find other ways to treat the flu. Tamiflu is given within two days of the onset of symptoms. The CDC said patients suspected of having Influenza A should instead be treated with the antiviral drug Relenza or a combination Tamiflu and rimantadine. The agency based in Atlanta issued the health alert advisory Friday after studying preliminary flu data from a limited number of states.

The flu causes 200,000 hospitalizations and 36,000 deaths annually, according to official estimates. The elderly, young children and people with chronic illnesses are considered at greatest risk.