Four North Carolina patients at a single hospital tested positive for a type of swine flu that is resistant to Tamiflu, health officials said Friday. The cases reported at Duke University Medical Center over six weeks make up the biggest cluster seen so far in the U.S. Tamiflu - made by Switzerland's Roche Group - is one of two flu medicines that help against swine flu, and health officials have been closely watching for signs that the virus is mutating, making the drugs ineffective.

When the nation's swine flu vaccination program began in early October, health officials predicted it was going to be "messy." They were right. The program has been plagued with problems and information gaps.

Health experts say extraordinary measures against swine flu - most notably quarantines imposed by China, where entire planeloads of passengers were isolated if one traveler had symptoms - have failed to contain the disease. Despite initially declaring success, Beijing now acknowledges its swine flu outbreak is much larger than official numbers show.