From November through December 2013, CDC has received a number of reports of severe respiratory illness among young and middle-aged adults, many of whom were infected with influenza A (H1N1) pdm09 (pH1N1) virus. Multiple pH1N1-associated hospitalizations, including many requiring intensive care unit (ICU) admission, and some fatalities have been reported. The pH1N1 virus that emerged in 2009 caused more illness in children and young adults, compared to older adults, although severe illness was seen in all age groups. While it is not possible to predict which influenza viruses will predominate during the entire 2013-14 influenza season, pH1N1 has been the predominant circulating virus so far. For the 2013-14 season, if pH1N1 virus continues to circulate widely, illness that disproportionately affects young and middle-aged adults may occur.

Seasonal influenza contributes to substantial morbidity and mortality each year in the United States. In the 2012-13 influenza season, CDC estimates that there were approximately 380,000 influenza-associated hospitalizations [1]. Although influenza activity nationally is currently at low levels, some areas of the United States are already experiencing high activity, and influenza activity is expected to increase during the next few weeks.

The spectrum of illness observed thus far in the 2013-14 season has ranged from mild to severe and is consistent with that of other influenza seasons. While CDC has not detected any significant changes in pH1N1 viruses that would suggest increased virulence or transmissibility, the agency is continuing to monitor for antigenic and genetic changes in circulating viruses, as well as watching morbidity and mortality surveillance systems that might indicate increased severity from pH1N1 virus infection. In addition, CDC is actively collaborating with state and local health departments in investigation and control efforts.

CDC recommends annual influenza vaccination for everyone 6 months and older. Anyone who has not yet been vaccinated this season should get an influenza vaccine now. While annual vaccination is the best tool for prevention of influenza and its complications, treatment with antiviral drugs (oral oseltamivir and inhaled zanamivir) is an important second line of defense for those who become ill to reduce morbidity and mortality. Antiviral treatment is recommended as early as possible for any patient with confirmed or suspected influenza who is hospitalized; has severe, complicated, or progressive illness; or is at higher risk for influenza complications.