Seasonal flu viruses are developing the ability to evade influenza drugs globally, but how and why this is happening is not clear, experts told a conference on Monday.
Europe is the worst-affected by strains of influenza that resist the effects of antiviral drugs, but the resistance is growing globally, they told a meeting of the Infectious Diseases Society of America.
"A significant proportion of resistant viruses were observed in Europe this winter," Dr. Bruno Lina of Claude Bernard University in Lyons, France, told the meeting.
The resistance also varies by strain, with a quarter of H1N1 flu viruses resistant in Europe and about 11 percent of H1N1 in the United States, but far fewer cases of H3N2 and influenza B viruses.
Their findings show that flu viruses — already known to mutate speedily — may be even more unpredictable than anyone thought.
Experts fear drugs may become quickly useless to fight an unusually severe flu season or the emergence of a new strain of flu that may cause a pandemic. They have been stressing the need to develop new flu drugs and also quicker and better ways to make vaccines.
The World Health Organization and the U.S. Centers for Disease Control and Prevention have been collecting samples of the annual flu viruses to check them against the four available flu drugs: amantadine and rimantadine, and the newer drugs Tamiflu and Relenza.
The viruses changed rapidly over the past 2007-2008 flu season, Lina said. "We started with something like 10 percent in Europe," Lina said. By April of this year, 25 percent of the viruses tested were resistant to Tamiflu.
U.S. flu viruses developed a sudden ability to evade the effects of the older drugs amantadine and rimantadine during the 2005-2006 flu season, said Dr. Larisa Gubareva of the CDC. In 2006 the CDC said no one should use those drugs any more.
Doctors had hopes for two newer drugs — Roche AG's Tamiflu, known generically as oseltamivir and licensed from Gilead Sciences, and GlaxoSmithKline's Relenza, known generically as zanamivir.
But already resistance is being seen to Tamiflu, a pill that can be taken to treat symptoms and also to prevent infection.
Lina's team tested more than 2,600 samples of flu viruses from patients in Europe and found baffling patterns of this resistance that appeared to have nothing to do with actual use of Tamiflu.
For instance in France, 54 percent of those tested in Paris carried the mutation that would give resistance to Tamiflu, compared to 29 percent in southeastern France. "Which makes absolutely no sense," Lina said.
Patients showed no difference in their symptoms if they were infected with resistant virus, he noted.
"It's difficult to understand. I have no idea why these viruses emerged," he said.
And in Europe, the H1N1 viruses were the most resistant.
Gubareva said tests across the United States, Canada and Mexico showed very quick development of drug resistance among H1N1 viruses. As of May 15 resistant viruses had been detected in 18 U.S. states out of 43 where virus samples from patients were tested, she said.
In Canada, resistant virus was found in nine of 13 provinces.
But only 6 percent of H3N2 and influenza B samples tested in North America had genetic mutations giving resistance to Tamiflu, she said.