This year's flu season: Low activity level, antibiotic-resistant viral strain
ATLANTA Low levels of influenza activity throughout the majority of the season, coupled with the fact that the predominant viral strain this year showed some resistance to antiviral remedies, characterized the 2008/2009 influenza season, according to a report published in Morbidity and Mortality Weekly Friday.
After a slow start to the season, influenza activity increased in mid-January, peaked in mid-February and remained high until mid-March. Since mid-March, influenza levels have been decreasing nationally.
During this influenza season, a high level of resistance to the antiviral drug Tamiflu (oseltamivir) was detected among circulating influenza A (H1N1) viruses. Since October 1, 99.3% of influenza A (H1N1) viruses tested were resistant to Tamiflu. To date, influenza A has accounted for 67.3% of all influenza viruses identified, and influenza A (H1N1) has accounted for 89.8% of the influenza A viruses that were subtyped.
No oseltamivir resistance has been detected among influenza A (H3N2) or B viruses currently circulating in the United States; however, all the influenza A (H3N2) viruses tested were resistant to adamantanes (Symmetrel and Flumadine, for example). The adamantanes are not effective against influenza B viruses, either. None of the influenza A (H1N1) viruses tested were resistant to both oseltamivir and the adamantanes, and all influenza viruses tested this season have been susceptible to Relenza (zanamivir).
CDC issued interim guidelines for the use of influenza antiviral medications on Dec. 19, suggesting that health-care providers should review their local surveillance data if available to determine which types (A or B) and subtypes of influenza A (H1N1 or H3N2) are most prominent in their community and consider using diagnostic tests to distinguish influenza A from influenza B. When an influenza A (H1N1) virus infection or exposure is suspected, zanamivir is the preferred medication; combination therapy of Tamiflu and Flumadine is an acceptable alternative.
Since early February, the relative proportion of influenza B viruses had been increasing each week, and more than half of influenza viruses identified since the week ending March 14 were influenza B. Approximately 80% of influenza B viruses tested have not been related to the influenza B vaccine strain, however, all influenza B viruses this season have been susceptible to Tamiflu and Relenza. When the type or subtype is unknown, Relenza was the preferred medication, the Centers for Disease Control and Prevention noted.
To date, this season, the cumulative laboratory-confirmed, influenza-associated hospitalization rate reported by EIP among persons greater than 49-years-old has been lower than rates reported for the previous three seasons, but most similar to the 2006/2007 season.