CDC Flu Update: Influenza A Viruses are Most Frequently Identified Virus Type Reported

According to this week’s FluView report from the Centers for Disease Control and Prevention (CDC), overall seasonal influenza activity increased slightly in the United States but remains low overall.

For the week ending December 5, the proportion of people seeing their health care provider for influenza-like illness (ILI) is 1.8%, and remains below the national baseline (2.1%). Four of 10 regions (Regions 2, 3, 4 and 6) reported ILI at or above their region-specific baseline levels.

One state (South Carolina) experienced high ILI activity. Puerto Rico and four states (Arizona, Mississippi, New Jersey, and Texas) experienced low ILI activity. New York City and 45 states experienced minimal ILI activity. The District of Columbia did not have sufficient data to calculate an activity level. ILI activity data indicate the amount of flu-like illness that is occurring in each state.

Guam reported widespread influenza activity. Puerto Rico reported regional activity. Ten states (Connecticut, Iowa, Kentucky, Maryland, Massachusetts, Nevada, New Hampshire, North Carolina, Oregon, and Texas) reported local influenza activity. The U.S. Virgin Islands and 37 states reported sporadic influenza activity. No influenza activity was reported by the District of Columbia and three states (Alabama, Mississippi, and Rhode Island). Geographic spread data show how many areas within a state or territory are seeing flu activity.

Influenza-associated hospitalization data from the Influenza Hospitalization Surveillance Network (FluSurv-NET) for the 2015-2016 influenza season will be updated weekly starting later this season.

The proportion of deaths attributed to pneumonia and influenza (P&I) based on the NCHS Mortality Surveillance System and the 122 Cities Mortality Reporting System is below their system-specific epidemic threshold.

Two influenza-associated pediatric deaths were reported to CDC during the week ending December 5. One death was associated with an influenza B virus and occurred during week 48 (the week ending December 5, 2015). A total of 3 influenza-associated pediatric deaths have been reported for the 2015-2016 season at this time. One death was associated with an influenza B virus and occurred during the 2014-2015 season and brings the total number of reported pediatric deaths occurring during that season to 148.

Nationally, the percentage of respiratory specimens testing positive for influenza viruses in clinical laboratories during the week ending December 5 was 1.6%. For the most recent three weeks, the regional percentage of respiratory specimens testing positive for influenza viruses in clinical laboratories ranged from 0.2% to 3.5%. During the week ending December 5, 122 (65.9%) of the influenza-positive tests reported to CDC by clinical laboratories were influenza A viruses and 63 (34.1%) were influenza B viruses.

The most frequently identified influenza virus type reported by public health laboratories during the week ending December 5 was influenza A viruses, with influenza A (H3) viruses predominating. During the week ending December 5, 23 (63.9%) of the 36 influenza-positive tests reported to CDC by public health laboratories were influenza A viruses and 13 (36.1%) were influenza B viruses. Of the 20 influenza A viruses that were subtyped, 15 (75.0%) were H3 viruses and 5 (25.0%) were A (H1N1)pdm09 viruses. Influenza A (H3) viruses were predominant in all age groups ranging from 40.5% (ages 0-4 years) to 82.4% (ages 65 years and older).

The CDC has antigenically characterized 91 specimens (18 influenza A (H1N1)pdm09, 57 influenza A (H3N2) and 16 influenza B viruses) collected in the U.S. since October 1, 2015. All 18 (100%) influenza A (H1N1)pdm09 viruses were antigenically characterized as similar to A/California/7/2009, the influenza A (H1N1) component of the 2015-2016 Northern Hemisphere vaccine. All 57 H3N2 viruses were genetically sequenced and all viruses belonged to genetic groups for which a majority of viruses antigenically characterized were similar to A/Switzerland/9715293/2013, the influenza A (H3N2) component of the 2015-2016 Northern Hemisphere vaccine. A subset of 23 H3N2 viruses also were antigenically characterized; 22 of 23 (95.7%) H3N2 viruses were similar to A/Switzerland/9715293/2013 by HI testing or neutralization testing.

All nine (100%) of the B/Yamagata-lineage viruses were antigenically characterized as similar to B/Phuket/3073/2013, the influenza B/Yamagata component of both the 2015–16 Northern Hemisphere trivalent and quadrivalent vaccines. All seven (100%)of the B/Victoria-lineage viruses were antigenically characterized as similar to B/Brisbane/60/2008, the influenza B/Victoria component of the 2015-16 Northern Hemisphere quadrivalent vaccine.

The CDC has characterized 375 U.S. flu viruses collected by U.S. laboratories during May 24–September 30, 2015, including 17 influenza A (H1N1)pdm09 viruses, 277 influenza A (H3N2) viruses, and 81 influenza B viruses. The 277 H3N2 viruses collected from May 24-September 30, 2015 have been genetically sequenced and all viruses belonged to genetic groups for which a majority of viruses antigenically characterized were similar to A/Switzerland/9715293/2013. 219 viruses (17 A (H1N1)pdm09, 121 A (H3N2), 44 B/Yamagata-lineage, 52 B/Yamagata-lineage, and 29 B/Victoria-lineage) collected from May 24-September 30, 2015 have been antigenically characterized. All but one virus (one influenza A (H3N2) virus) were similar to the appropriate vaccine virus.

Since October 1, 2015, CDC has tested 17 influenza A (H1N1)pdm09, 34 influenza A (H3N2), and 18 influenza B viruses for resistance to the neuraminidase inhibitors antiviral drugs. None of the tested viruses were found to be resistant to either oseltamivir, zanamivir, or peramivir.

Source: CDC