According to this week’s FluView report from the Centers for Disease Control and Prevention (CDC), flu activity increased in the United States. The number of states reporting widespread flu activity increased from 37 to 40 and an additional eight influenza-associated pediatric deaths were reported this week. CDC recommends an annual flu vaccine for everyone 6 months of age and older. In late February, CDC reported flu vaccine effectiveness of nearly 60% this season. Vaccination is recommended as long as influenza viruses are still circulating.
For the week ending March 12, the proportion of people seeing their health care provider for influenza-like illness (ILI) increased from 3.5% to 3.7%, which is above the national baseline (2.1%). All 10 regions 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. ILI has been at or above the national baseline for 9 consecutive weeks so far this season. For the last 13 seasons, the average duration of a flu season by this measure has been 13 weeks, with a range from 1 week to 20 weeks.
New York City, Puerto Rico and 14 states (Alabama, Arizona, Arkansas, Georgia, Hawaii, Illinois, Indiana, Kentucky, Mississippi, New Jersey, New Mexico, North Carolina, Oregon, and Virginia) experienced high ILI activity. 13 states (California, Florida, Kansas, Louisiana, Maryland, Massachusetts, Minnesota, Missouri, New York, Oklahoma, Pennsylvania, Tennessee, and Utah) experienced moderate ILI activity. 11 states (Colorado, Idaho, Michigan, Nevada, Rhode Island, South Carolina, South Dakota, Texas, West Virginia, Wisconsin, and Wyoming) experienced low ILI activity. 12 states (Alaska, Connecticut, Delaware, Iowa, Maine, Montana, Nebraska, New Hampshire, North Dakota, Ohio, Vermont, and Washington) 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 40 states. Regional flu activity was reported by Guam and 10 states (Alabama, Hawaii, Illinois, Louisiana, Mississippi, Tennessee, Texas, Utah, Washington, and West Virginia). Local flu activity was reported by the District of Columbia. The U.S. Virgin Islands did not report. Geographic spread data show how many areas within a state or territory are seeing flu activity.
Since October 1, 2015, 4,006 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 14.5 hospitalizations per 100,000 people in the United States. This is significantly lower than the hospitalization rate at this time last season (58.4 per 100,000). 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 (37.2 per 100,000), followed by adults 50-64 years (21.3 per 100,000) and children younger than 5 years (20.9 per 100,000). During most seasons, adults 65 years and older and children younger than 5 years 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.
Eight influenza-associated pediatric deaths were reported to CDC during the week ending March 12. Five deaths were associated with an influenza A (H1N1)pdm09 virus and occurred during weeks 8 and 9 (the weeks ending February 27 and March 5, 2016) and three deaths were associated with an influenza A virus for which no subtyping was performed and occurred during weeks 8, 9, and 10 (the weeks ending February 27, March 5, and March 12, 2016). A total of 28 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 March 12 was 23.1%. For the most recent three weeks, the regional percentage of respiratory specimens testing positive for influenza viruses in clinical laboratories ranged from 10.9% to 26.7%. During the week ending March 12, of the 5,968 influenza-positive tests reported to CDC by clinical laboratories, 4,557 (76.4%) were influenza A viruses and 1,411 (23.6%) were influenza B viruses.
The most frequently identified influenza virus type reported by public health laboratories during the week ending March 12 was influenza A viruses, with influenza A (H1N1)pdm09 viruses predominating. During the week ending March 12, 1,055 (81.5%) of the 1,294 influenza-positive tests reported to CDC by public health laboratories were influenza A viruses and 239 (18.5%) were influenza B viruses. Of the 957 influenza A viruses that were subtyped, 114 (11.9%) were H3 viruses and 843 (88.1%) were (H1N1)pdm09 viruses. Cumulatively from October 4, 2015-March 12, 2016, influenza A (H1N1)pdm09 viruses were predominant in all four age groups (0-4 years age group (73.9%), 5-24 years age group (52.7%), 25-64 years age group (73.0%), and in ages 65 years and older (51.4%).
CDC has characterized 1,098 specimens (446 influenza A (H1N1)pdm09, 295 influenza A (H3N2) and 357 influenza B viruses) collected in the U.S. since October 1, 2015. All 446 (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 295 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 119 H3N2 viruses also were antigenically characterized; 112 of 119 (94.1%) H3N2 viruses were similar to A/Switzerland/9715293/2013 by HI testing or neutralization testing. All 239 (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. 117 of 118 (99.2%) 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 844 influenza A (H1N1)pdm09, 361 influenza A (H3N2), and 409 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, so far this season, 4 (0.5%) influenza A (H1N1)pdm09 viruses have showed resistance to oseltamivir and peramivir.