Influenza A showing resistance to oseltamivir

A high prevalence of oseltamivir resistance to circulating influenza A (H1N1) virus strains in some states has prompted the Centers for Disease and Control Prevention to issue interim recommendations regarding the use of the antiviral drug during the current influenza season.

To date, this season, influenza activity across the United States has been low. But tests of 50 H1N1 viruses from 12 states found that 98% were resistant to oseltamivir, according to a CDC Health Advisory. All were susceptible to the other available influenza antiviral drugs: zanamivir, amantadine, and rimantadine.

Therefore, “when influenza A (H1N1) virus infection or exposure is suspected, zanamivir or a combination of oseltamivir and rimantadine are more appropriate options than oseltamivir alone,” the CDC advised. “The proportion of influenza A (H1N1) viruses among all influenza A and B viruses that will circulate during the 2008-2009 season cannot be predicted and will likely vary over the course of the season and among communities,” the advisory said.

The neuraminidase inhibitors oseltamivir and zanamivir have been the only influenza antivirals recommended since January 2006 because of widespread resistance to amantadine and rimantadine among influenza A (H3N2) virus strains (Amantadine and rimantadine are active against influenza A viruses only.) During the 2007-2008 flu season, a significant increase in oseltamivir resistance among H1N1 viruses was detected worldwide, and oseltamivir resistance was detected in almost 11% of the H1N1 viruses tested in the United States.

The CDC advisory also noted that the oseltamivir-resistant viruses are anti-genically similar to the influenza A (H1N1) strain that is included in the current flu vaccine and emphasized that vaccination should continue as the “primary method to prevent influenza.”

Chair of the department of preventive medicine, Vanderblt University, Nashville, Tenn., stressed the importance of these early data indicating that the current flu vaccine is a close match to circulating strains. Because influenza peaks in February in the United States, it is not too late for people to get vaccinated, and there is plenty of vaccine still available.

That there is a surprise during every flu season, and that this season, evidence of oseltamivir resistance among the H1N1 viruses is an early surprise.

For health care professionals, this has made it even more important to use rapid tests to distinguish whether a patient has influenza A or B virus, because oseltamivir can be used to treat patients who test positive for influenza B. Because the rapid tests do not distinguish between H1N1 and H3N2 strains, “physicians need to remain very alert to what local and state health departments are reporting about the dominant circulating strains” in their area, which will help them make a better educated guess about empiric treatment of patients who test positive for influenza A.

In addition, it is important that laboratories that perform subtyping of influenza A viruses quickly perform tests on samples received from physicians and promptly communicate the results.