From Nov through Dec 2013, CDC receivednumerous reports of severe respiratory illness among young and middle-aged adults.
Many 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 . 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.