CDC Flu Update: Influenza-Like Illness Remains Below National Baseline

According to this week’s FluView report from the Centers for Disease Control and Prevention (CDC), overall seasonal influenza activity is low across the United States. For the week ending November 7, the proportion of people seeing their healthcare provider for influenza-like illness (ILI) is 1.4% and remains below the national baseline (2.1%). All 10 U.S. regions reported ILI activity below region-specific baseline levels.

Puerto Rico experienced moderate ILI activity. New York City and all 50 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.  Four states (Hawaii, Iowa, Massachusetts, and Oregon) reported local influenza activity. The District of Columbia and 39 states reported sporadic influenza activity.  The U.S. Virgin Islands and 7 states reported no influenza activity. 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.

At this time, no influenza-associated pediatric deaths have been reported for the 2015-2016 flu season.

Nationally, the percentage of respiratory specimens testing positive for influenza viruses in clinical laboratories during the week ending November 7 was 1.2%. For the most recent three weeks, the regional percentage of respiratory specimens testing positive for influenza viruses in clinical laboratories ranged from 0.3% to 2.7%. During the week ending November 7, 75 (58.6%) of the influenza-positive tests reported to CDC by clinical laboratories were influenza A viruses and 53 (41.4%) were influenza B viruses.

The most frequently identified influenza virus type reported by public health laboratories during the week ending November 7 was influenza A viruses, with influenza A (H3) viruses predominating. During the week ending November 7, 10 (55.6%) of the 18 influenza-positive tests reported to CDC by public health laboratories were influenza A viruses and 8 (44.4%) were influenza B viruses. Of the 8 influenza A viruses that were subtyped, 7 (87.5%) were H3 viruses and one (12.5%) was an A (H1N1)pdm09 virus.

The CDC has characterized 335 U.S. flu viruses collected by U.S. laboratories during May 24–September 30, 2015, including 14 influenza A (H1N1)pdm09 viruses, 250 influenza A (H3N2) viruses, and 71 influenza B viruses. All 14 influenza A (H1N1)pdm09 viruses were antigenically characterized as A/California/7/2009-like, the influenza A (H1N1) component of the 2015-2016 Northern Hemisphere. All 250 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 104 H3N2 viruses also were antigenically characterized; 103 of 104 (99%) H3N2 viruses were A/Switzerland/9715293/2013-like by HI testing or neutralization testing. 44 (62%) of the 71 influenza B viruses collected and analyzed during this period belonged to the B/Yamagata lineage, and all were antigenically similar to the B/Phuket/3073/2013 virus, the influenza B component for both the 2015–16 Northern Hemisphere trivalent and quadrivalent vaccines. The remaining 27 (38%) influenza B viruses were antigenically characterized as B/Brisbane/60/2008-like, the recommended influenza B component of the 2015-16 Northern Hemisphere quadrivalent flu vaccine.

In addition, CDC has antigenically characterized ten specimens (1 influenza A (H1N1)pdm09, 8 influenza A (H3) and 1 influenza B/Yamagata-lineage) collected in the U.S. since October 1, 2015. The eight H3N2 viruses collected since October 1, 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, the influenza A (H3N2) component of the 2015-2016 Northern Hemisphere vaccine.  Six viruses (one A (H1N1)pdm09, four A (H3N2), and one B/Yamagata-lineage) collected since October 1, 2015 have been antigenically characterized. All six were similar to the 2015-2016 Northern Hemisphere influenza vaccine components.

Since Oct. 1, 2015, CDC has tested 13 influenza A (H3N2) 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