WASHINGTON 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
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
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
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
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
But only 6 percent of H3N2 and influenza B samples tested
in North America had genetic mutations giving resistance to
Tamiflu, she said.
(Editing by Michael Kahn and Jackie Frank)