Mutant Flu Virus Is Found That Resists Popular Drug

A small but significant percentage of the main influenza virus causing illness this winter in Europe, Canada and the United States has a mutation that makes it resistant to the anti-influenza drug Tamiflu, the World Health Organization said Wednesday.

Scientists said they were surprised by the finding because they had believed that mutations of this type generally made the virus less potent and less easily spread among people. The predominant influenza virus circulating this winter is influenza A/H1N1. The Tamiflu-resistant form of the virus, known as influenza A(H1N1 H274Y), has been found with varying frequency in various areas of four European countries, Canada and the United States.

There are no immediate plans to recommend changes in the use of Tamiflu, which is also known as oseltamivir, officials from the W.H.O. and the United States said in interviews, because the incidence of the mutant virus is still small. Tamiflu is one of the antiviral drugs used to treat influenza in its early stages.

Nevertheless, officials from the W.H.O., a United Nations agency in Geneva, said they were troubled by the discovery.

“Clearly, this is of global concern, but it is not a global problem now,” Dr. Frederick G. Hayden, an influenza expert at the organization, said in a telephone interview.

The standard influenza vaccine still protects against the mutant virus, said Dr. Hayden and Dr. Alicia Fry, an influenza epidemiologist at the Centers for Disease Control and Prevention in Atlanta.

Norwegian epidemiologists first called attention to the problem last week when they reported that the mutation was present in 12 of 16, or 75 percent, of viruses isolated in that country from patients in the earliest part of the influenza season, in November and December.

The Norwegian rate was the highest among the four European countries — Britain, Denmark, France and Norway — that reported the mutant virus. There was no evidence that the 12 cases in Norway were linked to one another.

Over all, the mutant form was found in 19 of the viruses isolated from 148 patients or 13 percent, in a monitoring system that the European Center for Disease Prevention and Control runs in 10 European countries.

The overall percentage fell to 5 if Norway was excluded.

In the United States, the Tamiflu resistant strain was found in 9 of 237, or 3.8 percent of patients from whom influenza type A and B viruses were isolated this winter, and all 9 were in the A(H1N1) category, making them 6.7 percent of those 135 cases, Dr. Fry, said in a telephone interview.

The W.H.O. conducted a teleconference lasting two hours on Tuesday to collect information from experts in a number of countries. The participants agreed that continued close monitoring was needed to collect information on a larger number of patients to determine the frequency, transmission and distribution of the mutant strain as well as its virulence, Dr. Hayden said.

Scientists also want to learn how the resistance developed. It is unlikely to have been from use of Tamiflu, Dr. Hayden said, in part because no cases have been detected in Japan, where the drug is often used in treatment.

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