Situation Update: Summary of Weekly FluView Report

According to this week’s FluView report, flu activity increased in the United States. Influenza-like-illness levels climbed this week over last, and the number of states reporting widespread flu activity increased from 33 to 37. In late February, CDC reported flu vaccine effectiveness of 60% this season. CDC recommends an annual flu vaccine for everyone 6 months of age and older. Vaccination is recommended as long as influenza viruses are still circulating. Below is a summary of the key flu indicators for the week ending March 5, 2016:

For the week ending March 5, the proportion of people seeing their health care provider for influenza-like illness (ILI) increased from 3.2% to 3.5%, 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 8 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.
Puerto Rico and 10 states (Alabama, Arizona, Arkansas, Illinois, Kentucky, Mississippi, Nevada, New Jersey, New Mexico, and North Carolina) experienced high ILI activity. New York City and 13 states (Georgia, Hawaii, Indiana, Kansas, Louisiana, Minnesota, New York, Pennsylvania, South Carolina, Tennessee, Texas, Utah, and Virginia) experienced moderate ILI activity. 12 states (California, Colorado, Connecticut, Florida, Maryland, Massachusetts, Michigan, Missouri, Oklahoma, Vermont, West Virginia, and Wisconsin) experienced low ILI activity. 15 states (Alaska, Delaware, Idaho, Iowa, Maine, Montana, Nebraska, New Hampshire, North Dakota, Ohio, Oregon, Rhode Island, South Dakota, Washington, and Wyoming) 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 37 states. Regional flu activity was reported by Guam and 13 states (Alabama, Colorado, Georgia, Hawaii, Illinois, Louisiana, Maine, Mississippi, Missouri, South Carolina, Tennessee, Washington, and West Virginia). The District of Columbia reported local influenza activity. The U.S. Virgin Islands reported sporadic influenza activity. Geographic spread data show how many areas within a state or territory are seeing flu activity.
Since October 1, 2015, 2,870 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 10.4 hospitalizations per 100,000 people in the United States. This is significantly lower than the hospitalization rate at this time last season (57.2 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 andhttp://gis.cdc.gov/grasp/fluview/FluHospChars.html.
- The highest hospitalization rates are among people 65 years and older (27.6 per 100,000), followed by children younger than 5 years (15.8 per 100,000), and adults 50-64 years (14.5 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.
Two influenza-associated pediatric deaths were reported to CDC during the week ending March 5.
- One death was associated with an influenza A (H1N1)pdm09 virus and occurred during week 6 (the week ending February 13, 2016) and one death was associated with an influenza B virus and occurred during week 8 (the week ending February 27, 2016).
- A total of 20 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 5 was 20.6%. For the most recent three weeks, the regional percentage of respiratory specimens testing positive for influenza viruses in clinical laboratories ranged from 8.5% to 22.5%.
- During the week ending March 5, of the 4,918 influenza-positive tests reported to CDC by clinical laboratories, 3,816 (77.6%) were influenza A viruses and 1,102 (22.4%) were influenza B viruses.
The most frequently identified influenza virus type reported by public health laboratories during the week ending March 5 was influenza A viruses, with influenza A (H1N1)pdm09 viruses predominating.
- During the week ending March 5, 810 (75.4%) of the 1,074 influenza-positive tests reported to CDC by public health laboratories were influenza A viruses and 264 (24.6%) were influenza B viruses. Of the 742 influenza A viruses that were subtyped, 66 (8.9%) were H3 viruses and 676 (91.1%) were (H1N1)pdm09 viruses.
- Cumulatively from October 4, 2015-March 5, 2016, influenza A (H1N1)pdm09 viruses were predominant in all four age groups (0-4 years age group (71.8%), 5-24 years age group (50.3%), 25-64 years age group (72.4%), and in ages 65 years and older (47.7%).
CDC has characterized 970 specimens (385 influenza A (H1N1)pdm09, 275 influenza A (H3N2) and 310 influenza B viruses) collected in the U.S. since October 1, 2015.
- All 385 (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 275 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 113 H3N2 viruses also were antigenically characterized; 106 of 113 (93.8%) H3N2 viruses were similar to A/Switzerland/9715293/2013 by HI testing or neutralization testing.
- All 205 (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.
- 104 of 105 (99.1%) 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 596 influenza A (H1N1)pdm09, 350 influenza A (H3N2), and 376 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, 2 (0.3%) influenza A (H1N1)pdm09 viruses have showed resistance to oseltamivir and peramivir (but both were sensitive to zanamivir).

FluView is available – and past issues are archived – on the CDC website.

Note: Delays in reporting may mean that data changes over time. The most up to date data for all weeks during the 2015-2016 season can be found on the current FluView.