|source : http://www.medpagetoday.com/
|ATLANTA, March 2 — More than 12% of H1N1 influenza A infections in the U.S. last season were resistant to oseltamivir (Tamiflu) and the prevalence appears to be rising dramatically, according to a team of CDC-led researchers.
Moreover, the resistant strains appear no less virulent, debunking earlier suggestions that oseltamivir resistance would make the virus less dangerous.
Those findings emerged from a study of 1,155 H1N1 isolates from the 2007-2008 season by Nila J. Dharan, M.D., of the CDC, and colleagues from that agency and from several state public health laboratories, and reported online in the Journal of the American Medical Associati
Mid-season data reported in February 2008 showed oseltamivir resistance on the rise generally among all flu virus serotypes. (See: Resistance to Oseltamivir (Tamiflu) Grows Higher)
Dr. Dharan examined records of 274 individual infections with H1N1 viruses, including 99 showing oseltamivir resistance.
“We found no significant difference in our comparison of the clinical symptoms and outcomes of untreated patients with oseltamivir-resistant and oseltamivir-susceptible influenza A(H1N1) infections,” the researchers wrote.
One exception was that fewer hospitalizations were seen among those with resistant strains (2% versus 8%, P=0.005).
But virtually identical percentages of untreated patients took medications for fever and missed work or school, the researchers found.
Among patients who received antiviral agents, illness severity and outcomes were also similar.
Of 47 patients with resistant strains who received oseltamivir, five were hospitalized and four died.
Patient factors including demographics and flu vaccination history did not affect the likelihood of contracting oseltamivir-resistant H1N1 strains.
Dr. Dharan and colleagues also determined that rates of oseltamivir usage within a state did not correlate with the prevalence of resistant strains.
Of 22 states analyzed, only one had records of high oseltamivir usage (as measured by filled prescriptions) and high proportions of resistant strains, whereas four had high drug usage and low rates of resistant infection. Six states had relatively low levels of oseltamivir use and high rates of resistant infection.
The researchers also noted that early surveillance data for the current season have suggested that more than 90% of H1N1 viruses — the most common now in circulation — are resistant to oseltamivir.
In an accompanying editorial, David M. Weinstock, M.D., and Gianna Zuccotti, M.D., both from Harvard, said the study helps “dispel the notion that oseltamivir resistance compromises virulence.”
They also pointed to a small Dutch study, also reported online this week in JAMA, suggesting that one particular oseltamivir resistant strain may be even deadlier than normal for H1N1 viruses.
“The widespread belief that oseltamivir would retain activity against epidemic influenza strains has crumbled,” Drs. Weinstock and Zuccotti wrote.
They added that this should not be surprising, given that a variety of studies have shown rising resistance to the drug beginning in the 2006-2007 season.
“For now, the best tools to mitigate influenza infection are tried-and-true — vaccination, social distancing, hand washing, and common sense,” they concluded.
Primary source: Journal of the American Medical Association
Dharan N, et al “Infections with oseltamivir-resistant influenza A(H1N1) virus in the United States” JAMA 2009; DOI: 10.1001/jama.2009.294.
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