Health officials in Vietnam are reporting what appears to be the largest cluster yet of Tamiflu-resistant H1N1 cases — seven people who traveled together on a long train ride.

Six were from a group of students, and the seventh was in a different train car. They didn't know each other before the train trip in July. They all got sick later, and recovered.

Vietnamese officials reported the cluster in a letter released Wednesday by the New England Journal of Medicine.

Roughly 100 resistant cases have been reported worldwide since the H1N1 virus was identified in April. Tamiflu is one of two flu medicines that help.

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