H1N1 virus returns at high levels this flu season

The same H1N1 virus to cause a pandemic in 2009 is by far the predominant influenza virus for the 2013-2014 season. This is the first season that the H1N1 virus has circulated at high levels since the pandemic.



However, the Northern Hemisphere quadrivalent and trivalent vaccines for the 2013-2014 season are well-matched to the H1N1 virus in circulation, according to the Centers for Disease Control and Prevention’s FluView report for the week ended Jan. 4.

Even though flu incidence started picking up later into the season, more people had gotten flu shots this season. “Overall, about 40% of the general population had reported getting a flu vaccine by mid-November,” noted Ann Schuchat, CDC’s director of the National Center for Immunization and Respiratory Diseases. “That is about three percentage points higher than last year at the same time, and most of the increase that we see is increases in adults getting vaccinated.”

“We estimate based on a conservative model that during last year’s flu season, … flu vaccination prevented at least 6.6 million people from getting sick with the flu, 3.2 million people from going to see a doctor or other healthcare professional and at least 79,000 hospitalizations,” said Tom Frieden, CDC director. “We’ve looked at the last few flu years going back with a similar model, all the way back to 2005, and this is by far the largest number of hospitalizations and other illnesses we’ve seen prevented. The high numbers prevented from last year were partly attributable to the fact that last year was a relatively severe season.”

As of Jan. 4, 35 states were experiencing widespread activity, and 20 states were reporting high levels of influenza-like illness. However, flu incidence this year is less pervasive than last year, running 5% to 10% below last year.

Nationwide for the week ended Jan. 4, 4.4% of patient visits reported through the U.S. Outpatient Influenza-like Illness Surveillance Network were due to influenza-like illness. This percentage is above the national baseline of 2%. Influenza-like illness is defined as a temperature of 100°F or greater, and cough and/or sore throat.

The neuraminidase inhibitors Tamiflu (oseltamivir) and Relenza (zanamivir) are currently the only recommended influenza antiviral drugs.

While the vast majority of the viruses that have been tested are sensitive to Tamiflu and Relenza, three additional 2009 H1N1 viruses proved resistant to Tamiflu during the week ended Jan. 4. So far this season, 13 (1.2%) 2009 H1N1 viruses have shown resistance to Tamiflu. No viruses have shown resistance to Relenza.

As in recent past seasons, high levels of resistance to the adamantanes — Symmetrel (amantadine) and Flumadine (rimantadine) — continue to persist among 2009 H1N1 and influenza A (H3N2) viruses. Adamantanes are not effective against influenza B viruses. Adamantanes are not recommended for use against influenza this season, the CDC noted.