From Sept. 28 to Dec. 6, almost 125,000 respiratory specimens in the United States were tested for influenza and 10.9% were positive, according to a CDC report published in MMWR.
The majority (89.3%) were influenza A viruses, and among those, 42.1% were subtyped. Almost all of the subtyped specimens (99.1%) were influenza A (H3) viruses. The remainder were influenza A (H1N1)pdm09 viruses. All 50 states, plus the District of Columbia, Guam and Puerto Rico, have reported positive influenza tests.
All of the 10 pH1N1 viruses analyzed genetically or antigenically by the CDC were antigenically similar to the 2014-2015 Northern Hemisphere H1N1 vaccine component. However, of the 197 influenza A(H3N2) viruses analyzed, only 64 were characterized as the H3N2 vaccine component. The remainder showed reduced titers against A/Texas/50/2012 or belonged to a genetic group that showed reduced titers against it. Most were antigenically similar to A/Switzerland/9715293/2013, which is the H3N2 component of the 2015 Southern Hemisphere vaccine. This strain was first detected in the United States in March.
Since Sept. 28, the weekly percentage of outpatient visits for influenza-like illness ranged from 1.2% to 2.6%. It was first reported to be at or above 2%, the national baseline, during week 47. Since Oct. 1, 1,028 laboratory-confirmed, influenza-associated hospitalizations were reported, for a rate of 3.8 per 100,000 population. The highest rates were 13.4 per 100,000 among adults aged 65 years and older and 6.2 per 100,000 among children aged 0 to 4 years. The majority of hospitalizations (92.6%) were attributable to influenza A.
“Even during seasons when the match between the vaccine viruses and circulating viruses is less than optimal and protection against illness might be reduced, vaccination remains the most effective method to prevent influenza and its complications,” the investigators wrote. “Health care providers should recommend vaccination to all unvaccinated persons aged 6 months and older now and throughout the influenza season.”
Disclosure: The researchers report no relevant financial disclosures.