CDC Flu Update: Percentage of Respiratory Specimens Testing Positive for Flu Increases

According to this week’s Fluview report from the Centers for Disease Control and Prevention (CDC), flu activity continues to increase slowly; however there are localized pockets of high activity in parts of the country and the percentage of respiratory specimens testing positive for flu in clinical laboratories increased. H1N1 viruses are most common at this time. This H1N1 virus emerged in 2009 to cause a pandemic. Seasonal flu vaccines have included the H1N1 pandemic virus since 2010. CDC recommends an annual flu vaccine for everyone 6 months of age and older.

For the week ending January 30, the proportion of people seeing their healthcare provider for influenza-like illness (ILI) remained at 2.2%, which is above the national baseline (2.1%). Six of 10 regions (Regions 1, 2, 3, 4, 6 and 10) reported ILI at or above their region-specific baseline levels. 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 experienced high ILI activity. Two states (Connecticut and Arkansas) experienced moderate ILI activity. New York City and 11 states (Arizona, Florida, Hawaii, Illinois, Maryland, Nevada, New Jersey, Oklahoma, Pennsylvania, Texas, and Utah) experienced low ILI activity. 37 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 flu activity was reported by Puerto Rico and three states (California, Iowa, and Massachusetts). Regional flu activity was reported by Guam and 18 states (Arizona, Connecticut, Indiana, Kentucky, Maine, Maryland, Nevada, New Hampshire, New Jersey, New Mexico, New York, North Carolina, Rhode Island, Texas, Utah, Vermont, Virginia, and Washington). The District of Columbia and 16 states (Alabama, Alaska, Arkansas, Colorado, Florida, Idaho, Illinois, Michigan, Minnesota, North Dakota, Ohio, Oklahoma, Oregon, Pennsylvania, South Carolina, and Wyoming) reported local influenza activity. The U.S. Virgin Islands and 12 states reported sporadic influenza activity. No flu activity was reported by one state (Mississippi). Geographic spread data show how many areas within a state or territory are seeing flu activity.

Since October 1, 2015, 723 laboratory-confirmed influenza-associated hospitalizations have been reported through FluSurv-NET, a population-based surveillance network for laboratory-confirmed influenza-associated hospitalizations. This translates to a cumulative overall rate of 2.6 hospitalizations per 100,000 people in the United States. More data on hospitalization rates, including hospitalization rates during other influenza seasons, are available at http://gis.cdc.gov/GRASP/Fluview/FluHospRates.html and http://gis.cdc.gov/grasp/fluview/FluHospChars.html. The highest hospitalization rates are among people 65 years and older (8.5 per 100,000), followed by children younger than 5 years (3.8 per 100,000). During most seasons, children younger than 5 years and adults 65 years and older have the highest hospitalization rates. FluSurv-NET hospitalization data are collected from 13 states and represent approximately 8.5% of the total U.S. population. The number of hospitalizations reported does not reflect the actual total number of influenza-associated hospitalizations in the United States.

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

Two influenza-associated pediatric deaths were reported to CDC during the week ending January 30. A total of nine influenza-associated pediatric deaths have been reported during the 2015-2016 season.

Nationally, the percentage of respiratory specimens testing positive for influenza viruses in clinical laboratories during the week ending January 30 was 6.8%. For the most recent three weeks, the regional percentage of respiratory specimens testing positive for influenza viruses in clinical laboratories ranged from 1.0% to 10.9%. During the week ending January 30, of the 1,085 influenza-positive tests reported to CDC by clinical laboratories, 739 (68.1%) were influenza A viruses and 346 (31.9%) were influenza B viruses.

The most frequently identified influenza virus type reported by public health laboratories during the week ending January 30 was influenza A viruses, with influenza A (H1N1)pdm09 viruses predominating. During the week ending January 30, 273 (74.2%) of the 368 influenza-positive tests reported to CDC by public health laboratories were influenza A viruses and 95 (25.8%) were influenza B viruses. Of the 222 influenza A viruses that were subtyped, 55 (24.8%) were H3 viruses and 167 (75.2%) were  (H1N1)pdm09 viruses. Cumulatively from October 4, 2015-January 30, 2016, influenza A (H3) viruses were predominant in two of the four age groups ranging from 35.7% (ages 5-24 years) to 51.0% (ages 65 years and older). Influenza A (H1N1)pdm09 viruses were predominant in the 0-4 years age group (60.4%) and in the 25-64 years age group (58.8%).

CDC has characterized 407 specimens (130 influenza A (H1N1)pdm09, 190 influenza A (H3N2) and 87 influenza B viruses) collected in the U.S. since October 1, 2015. All 130 (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 190 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 93 H3N2 viruses also were antigenically characterized; 92 of 93 (98.9%) H3N2 viruses were similar to A/Switzerland/9715293/2013 by HI testing or neutralization testing. All 52 (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 35 (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 229 influenza A (H1N1)pdm09, 225 influenza A (H3N2), and 130 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 additional influenza A (H1N1)pdm09 virus that was reported during the week ending January 30, 2016, showed resistance to oseltamivir and peramivir.  So far this season, 2 (0.9%) influenza A (H1N1)pdm09 viruses have showed resistance to oseltamivir and peramivir (but both were sensitive to zanamivir).

Source: CDC