THURSDAY, Jan. 8 (HealthDay News) -- The most common strain of flu this season is resistant to the popular antiviral drug Tamiflu, but government health officials said Thursday there is no reason to panic.
The fact that the flu season so far has been slow, and that other drugs work well against this particular flu virus, has health officials adopting a watchful attitude for now.
While the cause of the mutation that made the virus resistant to Tamiflu (oseltamivir) isn't known, experts suspect it was caused by the wide use of Tamiflu in other countries to treat upper respiratory infections.
There were reports last year from Europe and other countries that a certain type of flu -- H1N1 -- was resistant to oseltamivir, according to Dr. Joseph Bresee, chief of flu prevention at the U.S. Centers for Disease Control and Prevention.
This year, the CDC was on the lookout for flu resistance to Tamiflu in the United States and, sure enough, it showed up. Moreover, the proportion of the strain that is resistant has gone up from around 10 percent last year to all of the H1N1 strains this year, Bresee said.
Two weeks ago, the CDC issued new guidelines urging doctors to test suspected flu cases to see if they are influenza A or influenza B, and if they are A, whether they are H1 or H3 viruses, Bresee said.
"We didn't want to make a big deal out of this," Bresee said. "We think it's interesting. We think it's worth monitoring closely, but we are still watching. When we see more, then we will get a better sense of what the public health impact of it is. We don't have a good feel of that yet."
The resistant strain is the most common type of flu circulating in the United States right now, but the season is early and the number of cases is low, Bresee said.
As more flu cases are reported, other strains of the virus may become more common, Bresee added.
"What happens frequently is that one type of flu will circulate at one time in the season and be replaced by another type later in the season, so in a week's time, a month's time, we may have mostly H3s or mostly Bs, which oseltamivir works wonderfully for," he explained.
To be most effective, Tamiflu, made by Roche, needs to taken within 48 hours after flu symptoms appear, Bresee said. "Generally, antivirals for influenza are underused in the U.S.," he added.
Most people with flu are not treated, Bresee noted. "Clearly, it's a self-limited disease and you will get over it," he said.
Bresee noted there are alternatives to Tamiflu, including Relenza (zanamivir) made by GlaxoSmithKline. "The viruses that we are seeing that are resistant to Tamiflu are sensitive to zanamivir," he said.
Zanamivir has some limitations, Bresee said. It can't be given to young children and patients with certain lung diseases.
Other treatment options include the antiviral rimantadine (brand name Flumadine).
Bresee added that all the circulating flu stains are prevented by the flu vaccine, and it's not too late to get vaccinated. "Folks who haven't gotten the vaccine so far can still go get it," he said.
Dr. Pascal James Imperato, dean and Distinguished Service Professor of the Graduate Program in Public Health at SUNY Downstate Medical Center in New York City, thinks resistance to Tamiflu developed because of its use in other countries to treat upper respiratory infections.
"The emerging resistance of H1N1 influenza strains to oseltamivir was first noted a few years ago," Imperato said. "At present, based on analyses of H1N1 isolates from 16 countries in 2008, some 31 percent demonstrated resistance to Tamiflu."
Tamiflu resistance has not been documented for the two other common current human influenza strains, H3N2 and influenza B, or for avian influenza (H5N1), Imperato said.
"On the face of it, the emergence of resistance in the presence of limited drug use would seem unusual," Imperato said. "However, in many areas of the world, there is significant Tamiflu use for upper respiratory infections. It is believed that minor mutations in the H1N1 virus can in effect block the action of Tamiflu," he said.
It is important to note that no H1N1 influenza resistance to Relenza has developed, Imperato said. "This is the other drug in the neuraminidase group used to prevent and treat influenza," he said.
Flu usually causes about 36,000 deaths in the United States each year. Most deaths are among the elderly, very young or those with medical conditions such as asthma or heart disease. Pneumonia resulting from flu is usually the fatal complication.
For more on the flu, visit the U.S. Centers for Disease Control and Prevention.
SOURCES: Joseph Bresee, M.D., chief, Epidemiology and Prevention Branch Influenza Division, U.S. Centers for Disease Control and Prevention, Atlanta; Pascal James Imperato, M.D., M.P.H., Dean and Distinguished Service Professor, Graduate Program in Public Health, SUNY Downstate Medical Center, New York City