H1N1 virus predominates as high influenza activity blankets the nation

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CDC reports that circulation of H1N1 is the highest it's been since the 2009 pandemic.

Flu season has been in full swing throughout the United States, with CDC reporting widespread activity in 35 states and high levels of influenza-like illness (ILI) in 20 states for the week ending January 4, 2014. According to the agency, influenza A (H3N2), 2009 influenza A (H1N1), and influenza B viruses have been circulating, but influenza A (H1N1) has been the predominant virus.

"This is the H1N1 virus that emerged in 2009 to cause a pandemic," reported CDC in its weekly FluView summary. "H1N1 viruses have continued to circulate among people since that time, but this is the first season that the virus has circulated at high levels since the pandemic. During the week ending January 4, 2,421 of the 2,486 influenza-positive tests reported to CDC were influenza A viruses and 65 were influenza B viruses. Of the 1,391 influenza A viruses that were subtyped, 1.4% were H3 viruses and 98.6% were 2009 H1N1 viruses."

Consistent with recommendations of CDC's Advisory Committee on Immunization Practices, the agency emphasized the importance of every individual 6 months or older receiving the influenza vaccine. All currently available influenza vaccine formulations offer protection against H1N1 viruses.

A total of 2,622 laboratory-confirmed hospitalizations related to influenza have been reported since October 1, 2013, which equates to 9.7 hospitalizations per 100,000 people in the United States. Of these 2,622 hospitalizations, 61.6% have been in individuals aged 18 to 64 years. CDC noted that the high rate of hospitalizations among people in this age range also was seen for the 2009 H1N1 pandemic, while flu hospitalizations related to "traditional" virus activity typically occur among people 65 years or older.

Four influenza-associated pediatric deaths were reported during the week ending January 4, three of which were attributable to 2009 H1N1 virus. The other death was associated with an influenza A virus for which subtyping was not performed. A total of 10 pediatric deaths related to flu have been reported thus far for the 2013–14 season. Additional information on pediatric deaths is available at CDC's FluView Interactive page.

A total of 639 influenza viruses have been antigenically characterized by CDC (572 as 2009 influenza A [H1N1], 59 as influenza A [H3N2], and 8 as influenza B) since October 1, 2013. All 572 of the H1N1 viruses tested were characterized as A/California/7/2009-like viruses; this is the H1N1 component of the quadrivalent and trivalent vaccines for the 2013–14 influenza season in the Northern Hemisphere. All 59 of the H3N2 viruses tested were characterized as Texas/50/2012-like viruses, and this is the H3N2 component of the quadrivalent and trivalent vaccines for the 2013–14 season in the Northern Hemisphere. For the eight influenza B viruses tested, three belonged to the B/Yamagata lineage of viruses and were characterized as B/Massachusetts/02/2012-like, while the five other influenza B viruses belonged to the B/Victoria lineage of viruses and were characterized as B/Brisbane/60/2008-like.

CDC reported testing 1,100 2009 H1N1, 76 influenza A (H3N2), and 17 influenza B virus samples for resistance to the neuraminidase inhibitor influenza antiviral drugs since October 1, 2013. Oseltamivir and zanamivir—two neuraminidase inhibitors—are the only recommended influenza antiviral drugs at the present time.

"While the vast majority of the viruses that have been tested are sensitive to oseltamivir and zanamivir, three additional 2009 H1N1 viruses proved resistant to oseltamivir during the week ending January 4," CDC reported. "So far this season 13 (1.2%) 2009 H1N1 viruses have shown resistance to oseltamivir. No viruses have shown resistance to zanamivir."

The agency also noted that similar to recent influenza seasons, high levels of resistance to the adamantanes (amantadine and rimantadine) continue to be seen among 2009 H1N1 and influenza A (H3N2) viruses. In addition, adamantanes are not effective against influenza B viruses. Therefore, CDC does not recommend use of adamantanes to combat influenza during the 2013–14 season.

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