CDC Reports US Influenza Activity, New Vaccine Strains

Laurie Barclay, MD

June 17, 2013

Influenza A (H3N2), influenza A (H1N1)pdm09, and influenza B viruses cocirculated during the 2012 to 2013 US influenza season, according to surveillance data published online in the June 14 issue of the Centers for Disease Control and Prevention's (CDC's) Morbidity and Mortality Weekly Report. Two cases of variant influenza A infection, both H3N2v, were also reported.

"During the 2012–13 influenza season in the United States, influenza activity increased through November and December before peaking in late December," write Lynnette Brammer, MPH, from the World Health Organization Collaborating Center for Surveillance, Epidemiology, and Control of Influenza, and colleagues. "This influenza season was moderately severe, with a higher percentage of outpatient visits for influenza-like illness..., higher rates of hospitalization, and more reported deaths attributed to pneumonia and influenza compared with recent years."

From September 30, 2012, through May 18, 2013, the World Health Organization and US National Respiratory and Enteric Virus Surveillance System laboratories tested 311,333 specimens for influenza viruses. Of 73,130 (23%) positive specimens, 51,675 (71%) were influenza A viruses and 21,455 (29%) were influenza B viruses. Subtyping of 68% of samples for seasonal influenza A viruses showed that 96% were influenza A (H3N2) and 4% were pH1N1 viruses.

Among 3626 influenza virus specimens tested for antiviral resistance since October 1, 2012, all 961 influenza B viruses tested were sensitive to both oseltamivir and zanamivir. Of 2123 influenza A (H3N2) viruses tested, 1 (0.05%) was resistant to oseltamivir alone and 1 (0.05%) to both oseltamivir and zanamivir. Two (0.4%) of 542 pH1N1 viruses tested were resistant to oseltamivir, but all 258 viruses tested for resistance to zanamivir were sensitive.

"High levels of resistance to the adamantanes (amantadine and rimantadine) persist among influenza A viruses currently circulating globally (the adamantanes are not effective against influenza B viruses)," the report authors write.

Recommendations for 2013 to 2014 Influenza Vaccine Composition

The US Food and Drug Administration recommends that the 2013 to 2014 influenza trivalent vaccines used in the United States contain an A/California/7/2009(H1N1)pdm09-like virus, an A(H3N2) virus antigenically similar to the cell-propagated A/Victoria/361/2011 virus (A/Texas/50/2012), and a B/Massachusetts/2/2012-like (B/Yamagata lineage) virus. In addition, they recommend that quadrivalent vaccines contain a B/Brisbane/60/2008-like (B/Victoria lineage) virus.

"These recommendations were based on global influenza virus surveillance data related to epidemiology, antigenic and genetic characteristics, and serological responses to 2012–13 seasonal vaccines, and the availability of candidate strains and reagents," the report authors write.

Implications for Public Health

The CDC recommends that throughout the influenza season, clinicians offer influenza vaccine to all unvaccinated persons at least 6 months of age, and throughout the summer months, influenza surveillance should continue, including for novel influenza viruses.

Patients with severe, complicated, or progressive influenza illness; those at higher risk for influenza complications; or those who can start treatment within 48 hours of symptom onset should receive timely empiric antiviral treatment, and clinicians should consider influenza as a cause of respiratory illness outside of the typical season.

"Recommended antiviral medications include oseltamivir and zanamivir," the authors conclude. "Recent viral surveillance and resistance data indicate that the majority of currently circulating influenza viruses are sensitive to these medications. Amantadine and rimantadine should not be used because of sustained high levels of resistance to these drugs among circulating influenza A viruses."

The authors have disclosed no relevant financial relationships.

Morb Mortal Wkly Rep. 2013;62:473-479. Full text


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