Situation Update: Summary of Weekly FluView Report

Key Flu Indicators

According to this week’s FluView report, flu activity continues to decrease, but is still elevated overall nationally in the United States. Influenza-like-illness (ILI) may have peaked nationally for this season and be winding down, but there are still 25 states reporting widespread activity at this time. Flu activity will likely continue for several more weeks. CDC continues to recommend influenza vaccination as long as influenza viruses are circulating. In late February, CDC reported flu vaccine effectiveness of nearly 60% this season. CDC also recommends that patients suspected of having influenza who are at high-risk of flu complications or who are very sick with flu-like symptoms should receive prompt treatment with influenza antiviral drugs without waiting for confirmatory testing. Below is a summary of the key flu indicators for the week ending April 2, 2016:

For the week ending April 2, the proportion of people seeing their health care provider for influenza-like illness (ILI) decreased for the third week from 2.9% to 2.4%. This is still above the national baseline of 2.1%. Eight of 10 regions (Regions 1, 2, 3, 4, 5, 7, 8, and 10) reported ILI at or above their region-specific baseline levels. One way that CDC measures the duration 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 15 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.
2 states (New Jersey and New Mexico) experienced high ILI activity. The same number of states reported high activity during the previous week. 7 states (Alabama, Alaska, Arkansas, Georgia, Missouri, North Carolina, and Virginia) experienced moderate ILI activity. New York City and 13 states (Arizona, Colorado, Connecticut, Hawaii, Illinois, Kentucky, Massachusetts, Mississippi, Pennsylvania, South Carolina, Texas, West Virginia, and Wyoming) experienced low ILI activity. Puerto Rico and 27 states (California, Delaware, Florida, Idaho, Indiana, Iowa, Kansas, Louisiana, Maine, Maryland, Michigan, Minnesota, Montana, Nebraska, Nevada, New Hampshire, New York, North Dakota, Ohio, Oklahoma, Oregon, Rhode Island, South Dakota, Tennessee, Vermont, Washington, and Wisconsin) experienced minimal ILI activity. The District of Columbia and one state (Utah) 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 25 states (Alaska, Arizona, California, Colorado, Connecticut, Delaware, Iowa, Kentucky, Maine, Maryland, Massachusetts, Michigan, Missouri, Nebraska, New Hampshire, New Jersey, New York, North Carolina, North Dakota, Ohio, Pennsylvania, Vermont, Virginia, Wisconsin, and Wyoming). This is a decrease from 29 states with widespread activity last week. Regional flu activity was reported by Guam and 18 states (Arkansas, Florida, Georgia, Idaho, Kansas, Louisiana, Minnesota, Montana, Nevada, New Mexico, Oklahoma, Rhode Island, South Carolina, South Dakota, Tennessee, Texas, Utah, and Washington). Local flu activity was reported by the District of Columbia and four states (Hawaii, Illinois, Indiana, and Oregon). Sporadic influenza activity was reported by three states (Alabama, Mississippi, and West Virginia).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, 6,756 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 24.4 hospitalizations per 100,000 people in the United States. This is significantly lower than the hospitalization rate at this time last season (61.6 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 (62.8 per 100,000), followed by adults 50-64 years (35.6 per 100,000) and children younger than 5 years (33.7 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 the NCHS Mortality Surveillance System and above the system-specific epidemic threshold in the 122 Cities Mortality Reporting System.
Seven additional influenza-associated pediatric deaths were reported to CDC this week:
- Two deaths were associated with an influenza A (H1N1)pdm09 virus and occurred during weeks 11 and 12 (the weeks ending March 19 and March 26, 2016).
- One death was associated with an influenza A virus for which no subtyping was performed and occurred during week 11.
- Four deaths were associated with an influenza B virus and occurred during weeks 5, 11, and 12 (the weeks ending February 6, March 19, and March 26, 2016).
This brings the total number of flu-associated pediatric deaths reported this season to 40 children.
Nationally, the percentage of respiratory specimens testing positive for influenza viruses in clinical laboratories during the week ending April 2 was 16.2%. For the most recent three weeks, the regional percentage of respiratory specimens testing positive for influenza viruses in clinical laboratories ranged from 8.9% to 24.7%.
- During the week ending April 2, of the 3,383 influenza-positive tests reported to CDC by clinical laboratories, 2,215 (65.5%) were influenza A viruses and 1,168 (34.5%) were influenza B viruses.
The most frequently identified influenza virus type reported by public health laboratories during the week ending April 2 was influenza A viruses, with influenza A (H1N1)pdm09 viruses predominating.
- During the week ending April 2, 434 (67.6%) of the 642 influenza-positive tests reported to CDC by public health laboratories were influenza A viruses and 208 (32.4%) were influenza B viruses. Of the 421 influenza A viruses that were subtyped, 76 (18.1%) were H3 viruses and 345 (81.9%) were (H1N1)pdm09 viruses.
  - Cumulatively from October 4, 2015-April 2, 2016, influenza A (H1N1)pdm09 viruses were predominant in all four age groups (0-4 years age group (72.9%), 5-24 years age group (52.7%), 25-64 years age group (73.4%), and in ages 65 years and older (54.8%).
CDC has characterized 1,494 specimens (606 influenza A (H1N1)pdm09, 375 influenza A (H3N2) and 513 influenza B viruses) collected in the U.S. since October 1, 2015.
- All 606 (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 375 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 162 H3N2 viruses also were antigenically characterized; 154 of 162 (95.1%) H3N2 viruses were similar to A/Switzerland/9715293/2013 by HI testing or neutralization testing.
- All 296 (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.
- 211 of 217 (97.2%) 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 1,241 influenza A (H1N1)pdm09, 461 influenza A (H3N2), and 706 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, 11 (0.9%) influenza A (H1N1)pdm09 viruses have showed resistance to oseltamivir and peramivir.
The Food and Drug Administration’s Vaccines and Related Biological Products Advisory Committee (VRBPAC) endorsed the WHO-recommended vaccine viruses for use in all U.S. seasonal flu vaccines for the 2016-2017 flu season. These recommendations were as follows:
- It was recommended that trivalent vaccines for use in the 2016-2017 influenza season (Northern Hemisphere winter) contain the following:
  - an A/California/7/2009 (H1N1)pdm09-like virus;
  - an A/Hong Kong/4801/2014 (H3N2)-like virus;
  - a B/Brisbane/60/2008-like virus (B/Victoria lineage).
- It was recommended that quadrivalent vaccines containing two influenza B viruses contain the above three viruses and a B/Phuket/3073/2013-like virus (B/Yamagata lineage).
- This represents a change in the influenza A (H3) component and a change in the influenza B lineage included in the trivalent vaccine compared with the composition of the 2015-2016 influenza vaccine.

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.