In a notice to clinicians, the Centers for Disease Control and Prevention in Atlanta said for the 2013-14 season, if pH1N1 virus continues to circulate widely, the illness might disproportionately affect young and middle-age adults.
The risk of severe disease and complications from influenza is usually higher among children younger than age 5, adults age 65 and older, pregnant women, and those with underlying medical conditions such as diabetes or cancer. In most influenza seasons, the majority of influenza-associated hospitalizations and deaths are among adults age 65 and older.
However, during the 2009 pandemic, pH1N1 sickened more children and young adults than older adults. The pandemic also was notable for reports of severe illness among pregnant women infected with pH1N1 and adverse neonatal outcomes.
Although influenza activity nationally is currently at low levels, some areas of the U.S. South and Southeast are already experiencing high activity.
From November through December, the CDC has received a number of reports of severe respiratory illness among young and middle-age adults, many of whom were infected with influenza A (H1N1) and pdm09 (pH1N1) virus. Multiple pH1N1-associated hospitalizations, including many requiring intensive care unit admission, and some fatalities have been reported.
Early observations from the 2013-14 influenza season indicate some people infected with pH1N1 virus have had severe illness. Most with severe illness had risk factors for influenza-associated complications, including pregnancy and obesity, but several did not, the CDC said.
The spectrum of illness observed thus far this season ranges from mild to severe and is consistent with that of other years, the CDC said. The CDC said it has not detected any significant changes in pH1N1 viruses that would suggest increased virulence or transmissibility, but the agency is continuing to monitor for antigenic and genetic changes in circulating viruses.
The CDC said clinicians should encourage all patients 6 months and older who have not yet received an influenza vaccine this season to be vaccinated. There are several flu vaccine options for the 2013-14 flu season and all available vaccine formulations contain a pH1N1 component. The CDC does not recommend one flu vaccine formulation over another.