WASHINGTON (Reuters) - 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.