New data published in a recent MMWR revealed that influenza A(H3N2) is the predominate strain circulating in the United States, and that most circulating viruses are similar to the reference viruses used in vaccines recommended for the 2016-2017 season.
“Influenza A (H3N2) virus–predominant seasons are typically more severe overall than influenza A (H1N1)pdm09 virus–predominant seasons, and are especially severe among the elderly and the very young,” Mei Shang, MBBS, MPH, of CDC’s Epidemic Intelligence Service and Influenza Division, National Center for Immunization and Respiratory Diseases, and colleagues wrote. “Annual influenza vaccination is the most effective method of preventing influenza and its complications.”
The report included data on influenza activity in the United States from Oct. 2 to Dec. 17. According to the data, 5,157 of the 177,867 respiratory specimens tested in clinical laboratories and 2,103 of 12,496 respiratory specimens tested in public health laboratories were positive for influenza. Among them, 73.4% in clinical laboratories and 91.8% in public health laboratories were influenza A viruses.
Additional information was available for 1,824 influenza A specimens and 97 influenza B specimens from public health laboratories. Most influenza A viruses were H3N2, representing 94.8% of viruses. Meanwhile, 5.2% of strains were influenza A (H1N1)pdm09. One patient from Iowa was infected with a novel influenza A (H1N2) variant [(H1N2)v] virus. The patient reported exposure to swine the week before illness, and no evidence of ongoing human-to-human transmission was identified. Among the influenza B specimens, 40.2% were of the B/Yamagata lineage and 59.8% were of the B/Victoria lineage.
Researchers antigenically characterized 42 influenza A (H3N2) viruses, 26 influenza A (H1N1)pdm09 viruses and 21 influenza B viruses collected since Oct. 1. They found that 92.9% of influenza A (H3N2) viruses, 85.7% of influenza B/Victoria-lineage viruses, and all the influenza A(H1N1)pdm09 and influenza B/Yamagata-lineage viruses were similar to components of this season’s Northern Hemisphere vaccine recommendations.
Researchers tested 205 influenza A and B specimens for resistance against antiviral medications, including Tamiflu (oseltamivir, Roche), Relenza (zanamivir, GlaxoSmithKline) and Rapivab (peramivir, BioCryst Pharmaceuticals). All viruses were sensitive to all three antivirals.
A surveillance network of approximately 2,000 centers reported that the weekly percentage of outpatient visits for influenza-like illnesses (ILI) ranged from 1.2% to 2.3%. The week ending on Dec. 17 was the first of the season to have the percentage of outpatient visits for ILIs (2.3%) above the national baseline of 2.2%, indicating that influenza activity is slowly increasing, Shang and colleagues wrote. Regions experiencing the most ILI activity as of Dec. 17 were Oklahoma and Puerto Rico.
The CDC identified 676 laboratory-confirmed hospitalizations linked to influenza. Patients aged 65 years and older accounted for more than half (53.1%) of hospitalizations. The overall hospitalization rate was 2.4 per 100,000 population.
According to data from the National Center for Health Statistics (NCHS) Mortality Reporting System, 5.9% (1,763 of 29,760) of all U.S. deaths at week 48 (ending on Dec. 3) were attributed to pneumonia and influenza, which is below the epidemic threshold for week 48 (6.9%). So far this season, no influenza-associated deaths among children were reported. However, the researchers noted that deaths will likely increase as more information becomes available.
Although influenza vaccination is the best way to prevent the infection, the researchers reported that approximately 60% of the population did not receive a vaccine for the 2016-2017 season.
“Because the peak month for influenza activity typically ranges from December to March, and influenza activity for the current season is just beginning to increase, receiving influenza vaccine at this time still offers substantial public health benefits,” they wrote. “Health care providers should recommend influenza vaccine now and throughout the influenza season to all unvaccinated persons aged ≥6 months who do not have contraindications.” – by Stephanie Viguers
Shang M, et al. MMWR Morb Mortal Wkly Rep. 2016:doi:10.15585/mmwr.mm655051a5.
Disclosure: The researchers report no relevant financial disclosures.