CDC Diagnosis: Last Flu Season Was Extremely Mild

The 2011-2012 flu season saw relatively low rates of hospitalization for influenza and of deaths attributed to pneumonia and influenza.

The 2011-2012 influenza season was among the mildest and latest on record, according to an update from the Centers for Disease Control and Prevention (CDC) published in the September 5, 2012, edition of the Journal of the American Medical Association. The CDC reports that, compared with recent seasons, 2011-2012 had a relatively low percentage of outpatient visits for suspected influenza, low rates of hospitalization for influenza, and fewer deaths attributed to pneumonia and influenza.

The 2011-2012 influenza season ran from October 2, 2011, to May 19, 2012. The weekly portion of outpatient visits for suspected influenza in a nationwide surveillance network peaked at 2.4% in the week ending March 17, 2012, the latest peak in more than 2 decades. By comparison, the peak portion was 3.6% in the 2008-2009 season, 7.7% in the 2009-2010 season (which included the H1N1 pandemic), and 4.5% in 2010-2011.

The cumulative hospitalization rate (per 100,000 people) in the 2011-2012 season for confirmed influenza was 8.6 for all age groups, 14.2 among those 4 years of age and younger, 4.2 among those aged 5 to 17 years, 4.1 among those aged 18 to 49 years, 8.5 among those aged 50 to 64 years, and 30.4 among those aged 65 years and older. By comparison, over the last 3 seasons, the hospitalization rate for confirmed influenza has ranged from 35.5 to 72.8 for those 4 years of age and younger, 6.4 to 27.3 for those aged 5 to 17 years, 3.6 to 23.1 for those aged 18 to 49 years, 5.1 to 30.8 for those aged 50 to 64 years, and 13.5 to 65.9 for those aged 65 years and older.

In 2011-2012, the portion of deaths attributed to pneumonia and influenza exceeded the epidemic threshold for just 1 week, peaking at 7.9% during the week ending January 21, 2012. In the last 3 seasons, the number of consecutive weeks above the epidemic threshold has ranged from 3 to 13. There were also 26 pediatric deaths confirmed to be associated with influenza. Of these, 6 were associated with influenza B, 5 with influenza A (H3N2), 7 with influenza A (pH1N1), 7 with influenza A of undetermined strain, and 1 with an undetermined virus type. By comparison, there were 122 influenza-related pediatric deaths during the 2010-2011 season, 348 during the 2009 H1N1 pandemic (from April 15, 2009, through October 2, 2010), and 67 during the 2008-2009 season.

Of 22,417 positive influenza samples tested by labs during the 2011-2012 season, 86% were influenza A viruses and 14% were influenza B viruses. Of the influenza A viruses that were subtyped, 74% were influenza A (H3N2) viruses, and 26% were influenza A (pH1N1) viruses. There have also been 13 cases of novel swine-origin influenza A (H3N2) variant virus reported since August 2011, 1 in an adult and 11 in children, resulting in 3 hospitalizations but no deaths.

Of 2576 influenza virus specimens tested for resistance to the antiviral medications oseltamivir and zanamivir, none of the influenza B or influenza A (H3N2) specimens had resistance to either medication. Among 1614 influenza A (pH1N1) viruses tested for resistance to oseltamivir, however, 16 (1.4%) were resistant, although none of the 518 influenza A (pH1N1) viruses tested for resistance to zanamivir were resistant. In addition, the authors note that high levels of resistance to amantadine and rimantadine continue among influenza A (pH1N1) and influenza A (H3N2) viruses around the world.