Health officials say four people have tested positive for a type of H1N1 flu that's resistant to the drug Tamiflu.

The four cases were reported at Duke University Hospital in Durham, North Carolina over the past six weeks.

Tamiflu is one of two medicines that help fight against H1N1.

Health officials have been closely watching For signs the virus is mutating, making the drugs ineffective.

About 52 resistant cases have been reported in the world since April, including 15 in the U.S. so far.