CDC Flu Update: Influenza A Viruses Continue to Predominate

According to this week’s FluView report from the Centers for Disease Control and Prevention (CDC), seasonal influenza activity increased slightly in the United States. While influenza A (H3N2) viruses have been most common since October 1, in the past three weeks, influenza A (H1N1)pdm09 viruses have predominated. The vast majority of circulating flu viruses analyzed this season remain similar to the vaccine virus components for this season's flu vaccines. CDC recommends an annual flu vaccine for everyone 6 months of age and older.

For the week ending December 26, the proportion of people seeing their healthcare provider for influenza-like illness (ILI) is 2.6%, which is above the national baseline (2.1%). Six of 10 regions (Regions 1, 2, 3, 4, 6 and 7) reported ILI at or above their region-specific baseline levels. This is the second week this season that ILI has been at or above the national baseline. One way that CDC measures the length of the influenza season is the number of consecutive weeks during which ILI is at or above the national baseline.

Puerto Rico and two states (New Jersey and South Carolina) experienced high ILI activity. Three states (Arizona, Georgia and Virginia) experienced moderate ILI activity. New York City and nine states experienced low ILI activity; 36 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.

Widespread activity was reported by Guam and one state (North Carolina). Five states (Maryland, Massachusetts, New Hampshire, Pennsylvania and Virginia) reported regional activity. Twelve states (Arizona, Connecticut, Indiana, Iowa, Kentucky, Minnesota, Nevada, New Jersey, New Mexico, Oregon, Texas, and Utah) reported local influenza activity. Twenty-nine states reported sporadic influenza activity. No influenza activity was reported by the District of Columbia and three states (Mississippi, Rhode Island and Tennessee). Puerto Rico and U.S. Virgin Islands did not report. 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 system-specific epidemic thresholds.

No influenza-associated pediatric deaths were reported. A total of four influenza-associated pediatric deaths have been reported to CDC during the 2015-2016 season to date.

Nationally, the percentage of respiratory specimens testing positive for influenza viruses in clinical laboratories during the week ending December 26 was 2.5%. For the most recent three weeks, the regional percentage of respiratory specimens testing positive for influenza viruses in clinical laboratories ranged from 0.5% to 5.6%. During the week ending December 26, 157 (55.7%) of the influenza-positive tests reported to CDC by clinical laboratories were influenza A viruses and 125 (44.3%) were influenza B viruses.

The most frequently identified influenza virus type reported by public health laboratories during the week ending December 26 was influenza A viruses, with influenza A (H1N1)pdm09 viruses predominating. During the week ending December 26, 32 (84.2%) of the38 influenza-positive tests reported to CDC by public health laboratories were influenza A viruses and 6 (15.8%) were influenza B viruses. Of the 29 influenza A viruses that were subtyped, 6 (20.7%) were H3 viruses and 23 (79.3%) were A (H1N1)pdm09 viruses. Cumulatively from October 4-December 26, 2015, influenza A (H3) viruses were predominant in three of the four age groups ranging from 42.2% (ages 25-64 years) to 75.6% (ages 65 years and older). Influenza A (H1N1)pdm09 viruses were predominant in the 0-4 years age group (38.8%).

The CDC has characterized 170 specimens (34 influenza A (H1N1)pdm09, 112 influenza A (H3N2) and 24 influenza B viruses) collected in the U.S. since October 1, 2015. All 34 (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 112 H3N2 viruses were genetically sequenced and all viruses belonged to genetic groups for which a majority of viruses antigenically characterized were similar to cell-propagated A/Switzerland/9715293/2013, the influenza A (H3N2) component of the 2015-2016 Northern Hemisphere vaccine. A subset of 66 H3N2 viruses also were antigenically characterized; 65 of 66 (98.5%) H3N2 viruses were similar to A/Switzerland/9715293/2013 by HI testing or neutralization testing. All 17 (100%) of the B/Yamagata-lineage viruses were antigenically characterized as similar to B/Phuket/3073/2013, which is included in 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, which is included in the 2015-16 Northern Hemisphere quadrivalent vaccine.

Since October 1, 2015, CDC has tested 40 influenza A (H1N1)pdm09, 138 influenza A (H3N2), and 38 influenza B viruses for resistance to the neuraminidase inhibitors antiviral drugs. While the vast majority of the viruses that have been tested are sensitive to oseltamivir, zanamivir, and peramivir, one influenza A (H1N1)pdm09 virus was reported during the week ending December 12 that showed resistance to oseltamivir and peramivir (but was sensitive to zanamivir).

Source: CDC