Durham, N.C. — Extensive testing has found no more cases of drug-resistant H1N1 flu at Duke University Hospital, officials said Tuesday.

The hospital reported on Nov. 20 that two men and two women had contracted a strain of the virus that was resistant to the drug Tamiflu, one of two medicines used to fight H1N1. Both men and one woman died.

The patients had been treated in an isolated unit of the hospital in the six weeks prior to the Nov. 20 announcement, and all had underlying compromised immune systems and other complex medical conditions at the time, officials said.

The U.S. Centers for Disease Control and Prevention and the state Division of Public Health worked with Duke Hospital to test all other patients in the isolated unit several times in recent days. Other people being treated for H1N1 at the hospital also were tested.

"Much work is still being done to better understand the nature of the four cases that were reported previously," Dr. Daniel Sexton, an infectious disease specialist and director of the Duke Infection Control Outreach Network, said in a statement.

About 52 Tamiflu-resistant cases have been reported in the world since April, including 15 in the U.S. Last summer, health officials said two people in western North Carolina also had a drug-resistant form of the virus.

Tamiflu is still the most effective treatment for the H1N1 virus, and a vaccination is the best prevention for contracting the virus, health officials said. No resistance has been found to Relenza, the other drug approved to treat H1N1, they said.

Since the H1N1 pandemic began, 58 people in North Carolina have died as a result of the virus, health officials said.