WASHINGTON Virtually all cases of the most common strain of flu circulating in the United States now resist the main drug used to treat it, the U.S. Centers for Disease Control and Prevention reported on Monday.
CDC researchers said 98 percent of all flu samples from the H1N1 strain were resistant to Roche AG's Tamiflu, a pill that can both treat flu and prevent infection. Four patients infected with the resistant strain have died, including two children.
This year, H1N1 is the most common strain of flu in the United States, although the flu season is a mild one so far, and still below the levels considered an epidemic.
Few doctors even test patients for flu, and Tamiflu is not widely prescribed. But the news is sobering because the pill, known generically as oseltamivir, is one of the few weapons against influenza, which kills an estimated 36,000 people in the United States in an average year.
It is also considered a key weapon against a potential pandemic of a new type of influenza, and this study suggests the virus can rapidly evade its effects.
This season, nine children have died from influenza, most apparently healthy before they got infected, the CDC reports.
Last flu season, only 19 percent of H1N1 viruses tested were Tamiflu-resistant, Dr. Nila Dharan and colleagues at the CDC reported.
"As of February 19, 2009, resistance to oseltamivir had been identified among 264 of 268 (98.5 percent) U.S. influenza A(H1N1) viruses tested," the researchers wrote in the Journal of the American Medical Association.
They interviewed 99 patients and found 30 percent of them had been vaccinated against flu but became infected anyway. The vaccine is known not to fully protect against infection.
"Two patients died on the way to the hospital or in the emergency department. One patient was 4 years old and previously healthy, and one patient was 4 years old with neurological problems," Dharan's team wrote.
Dr. David Weinstock of the Dana-Farber Cancer Institute and Dr. Gianna Zuccotti of Brigham and Women's Hospital, both in Boston, said the quick spread of Tamiflu-resistant flu had surprised doctors.
"Undoubtedly, new surprises await in the perpetual struggle with influenza as one thing is certain -- the organism will continue to evolve," they wrote.
"For now, the best tools to mitigate influenza infection are tried-and-true -- vaccination, social distancing, hand washing, and common sense."
GlaxoSmithKline, which makes the rival flu drug Relenza, said there was no indication influenza viruses were resistant to its drug. Relenza, known generically as zanamivir, is inhaled and is used even less commonly than Tamiflu.
Flu already resists two older drugs, rimantadine and amantadine. There is no indication the two other types of season flu now circulating, H3N2 and influenza B, resist the effects of Tamiflu and the CDC recommends using a cocktail of flu drugs in patients now.
"Hundreds of thousands of flu patients continue to be treated with Tamiflu this season," Roche said in a statement.
"Treatment with antivirals such as Tamiflu can lessen the duration and severity of illness and can help curb the spread of flu, especially in crowded settings like households, schools and nursing homes."