March 2 (HealthDay News) -- A troubling increase in resistance to the antiviral drug Tamiflu is linked mostly to influenza A strains that are circulating this season, a new government report shows.
Almost 100 percent of the influenza A (H1N1) strain is now resistant to the drug, up from 12 percent during last year's flu season, said report author Dr. Alicia Fry, a medical epidemiologist with the influenza division at the U.S. Centers for Disease Control and Prevention.
Last year was the first time Tamiflu resistance was seen anywhere in the world; H1N1 is the most common type of flu circulating in the United States.
"This makes using antiviral drugs very tricky for flu," said Dr. John Treanor, director of the division of infectious diseases at the University of Rochester Medical Center. "It is more complicated."
But, always, Fry emphasized, "Our message is to get vaccinated with the influenza vaccine. The current influenza vaccine has three different virus types in it, and the H1 strain that is in the vaccine is the same strain that is resistant to oseltamivir [Tamiflu], so the best prevention is still getting the vaccine."
"We're fortunate in that this is a very mild flu season, and we're not seeing either the numbers of cases or the complications that we normally see in a bad flu season," said Dr. Neil Schachter, medical director of respiratory care at Mount Sinai Medical Center in New York City and author of The Good Doctor's Guide to Colds and Flu. "While it is of concern that there are viruses that are developing resistance, it's not all the strains and it's not all the drugs. We're not painted into a corner."
Also, Schachter pointed out, many people with the flu are never treated at all.
"While there's still a flu season out there, everyone should get vaccinated," he said.
Another expert said some of the increasing resistance has less to do with Tamiflu and more to do with the natural evolution of viruses.
"It's not the same as the analogy to bacteria that have developed resistance when they get more virulent," explained Dr. Len Horovitz, a pulmonary specialist with Lenox Hill Hospital in New York City. "These are natural mutational shifts and drifts, not a result of the overuse of [Tamiflu]. Just because it's becoming more resistant does not mean that it's a more deadly virus."
A second study has found that the Tamiflu-resistant virus still poses a significant health threat to certain groups of patients, in particular those with compromised immune systems who are hospitalized.
Both studies were expected to be published in the March 11 issue of the Journal of the American Medical Association, but were released Monday.
In light of the increased resistance to Tamiflu, the CDC in December recommended that doctors test suspected flu cases to determine the exact strain of the virus and what medications should be prescribed based on results of these tests.
In 2006, the CDC issued a recommendation against using two other flu drugs, amantadine and rimantadine, owing to resistant strains of influenza B and H3. Those drugs work by a different mechanism than Tamiflu and the other drug in its class, Relenza (zanamivir).
The second study, from researchers in the Netherlands, recommended that patients be tested for specific strains of influenza and, if needed, be isolated so as to avoid hospital outbreaks.
But, the authors also cautioned, the conclusions and recommendations were based on only a small number of patients.
A third study, this one published in the March 4 issue of JAMA, found that the flu shot was more effective in reducing influenza- and pneumonia-related health-care visits than the nasal spray form of the vaccine. Pneumonia is a potentially fatal complication of the flu.
The authors, from the Armed Forces Health Surveillance Center in Silver Spring, Md., based their conclusions on an analysis of more than 1 million military personnel over three flu seasons.
The flu shot appeared to confer greater immunity than the nasal spray over all three seasons (2004-2005, 2005-2006 and 2006-2007), particularly among soldiers who had been vaccinated in previous seasons.
"Vaccination with either version of the vaccine was protective. Vaccine effectiveness was higher among members receiving inactivated vaccine [shot] compared to those receiving intranasal vaccine," study author Zhong Wang said during a news conference Friday. "When we look at individuals not immunized during the prior season, vaccine effectiveness became relatively similar between these two groups."
SOURCES: Alicia M. Fry, M.D., medical epidemiologist, influenza division, U.S. Centers for Disease Control and Prevention, Atlanta; John Treanor, M.D., professor, medicine, and director, infectious diseases division, University of Rochester Medical Center; Neil Schachter, M.D., medical director, respiratory care, Mount Sinai Medical Center, New York City; Len Horovitz, M.D., pulmonary specialist, Lenox Hill Hospital, New York City; Feb. 27, 2009, teleconference with Zhong Wang, Ph.D., Armed Forces Health Surveillance Center, Silver Spring, Md.; March 4, 2009, Journal of the American Medical Association; March 11, 2009, Journal of the American Medical Association