Weekly Influenza Surveillance

The most recent FluView report shows that flu activity decreased slightly, but remains elevated in United States. While it’s possible that activity might have peaked for the season, some parts of the country are still experiencing high levels of flu activity and ongoing activity is expected to continue for several weeks nationally.

CDC recommends a yearly flu vaccine for everyone 6 months and older. Vaccination can reduce flu illnesses, doctors' visits, and missed work and school due to flu illness, as well as prevent flu-related hospitalizations.

CDC also recommends that patients suspected of having influenza who are athigh risk of flu complications or who are very sick with flu-like illness should receive prompt treatment with influenza antiviral drugs without waiting for confirmatory testing.

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Situation Update: Summary of Weekly FluView Report

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 29 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 March 26, 2016:

For the week ending March 26, the proportion of people seeing their health care provider for influenza-like illness (ILI) decreased for the second week from 3.2% to 2.9%. This is still above the national baseline of 2.1%. Nine of 10 regions (Regions 1, 2, 3, 4, 5, 7, 8, 9, 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 14 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 2 states (New Jersey and New Mexico) experienced high ILI activity. This is a decrease from 7 states with high ILI activity last week. New York City and 7 states (Alabama, Arizona, Georgia, Kentucky, North Carolina, Pennsylvania, and Virginia) experienced moderate ILI activity. 15 states (Alaska, Colorado, Connecticut, Illinois, Kansas, Louisiana, Massachusetts, Mississippi, Missouri, Nevada, New York, Oklahoma, South Carolina, Utah, and Wyoming) experienced low ILI activity. 26 states (Arkansas, California, Delaware, Florida, Hawaii, Idaho, Indiana, Iowa, Maine, Maryland, Michigan, Minnesota, Montana, Nebraska, New Hampshire, North Dakota, Ohio, Oregon, Rhode Island, South Dakota, Tennessee, Texas, Vermont, Washington, West Virginia, and Wisconsin) 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 Guam, Puerto Rico and 29 states (Alaska, Arizona, California, Colorado, Connecticut, Delaware, Idaho, Iowa, Kentucky, Maine, Maryland, Massachusetts, Michigan, Missouri, Montana, Nebraska, Nevada, New Hampshire, New Jersey, New York, North Carolina, North Dakota, Ohio, Pennsylvania, Rhode Island, Vermont, Virginia, Wisconsin, and Wyoming). This is a decrease from 39 states with widespread activity last week. Regional flu activity was reported by 18 states (Arkansas, Florida, Georgia, Hawaii, Illinois, Indiana, Kansas, Louisiana, Minnesota, Mississippi, New Mexico, Oklahoma, Oregon, South Carolina, South Dakota, Texas, Utah, and Washington). Local flu activity was reported by the District of Columbia and two states (Alabama and Tennessee). Sporadic influenza activity was reported by one state (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, 5,915 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 21.4 hospitalizations per 100,000 people in the United States. This is significantly lower than the hospitalization rate at this time last season (60.7 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 (54.5 per 100,000), followed by adults 50-64 years (31.4 per 100,000) and children younger than 5 years (29.3 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.
Three additional influenza-associated pediatric deaths were reported to CDC this week:
- One death occurred during week 11 (the week ending March 19, 2016) and was associated with an influenza A (H1N1)pdm09 virus.
- One death was associated with an influenza A virus for which no subtyping was performed and occurred during week 11.
- One death was associated with an influenza B virus and occurred during week 8 (the week ending February 27, 2016).
This brings the total number of flu-associated pediatric deaths reported this season to 33 children
Nationally, the percentage of respiratory specimens testing positive for influenza viruses in clinical laboratories during the week ending March 26 was 18.3%. For the most recent three weeks, the regional percentage of respiratory specimens testing positive for influenza viruses in clinical laboratories ranged from 10.3% to 29.4%.
- During the week ending March 26, of the 4,022 influenza-positive tests reported to CDC by clinical laboratories, 2,915 (72.5%) were influenza A viruses and 1,107 (27.5%) were influenza B viruses.
The most frequently identified influenza virus type reported by public health laboratories during the week ending March 26 was influenza A viruses, with influenza A (H1N1)pdm09 viruses predominating.
- During the week ending March 26, 536 (75.0%) of the 715 influenza-positive tests reported to CDC by public health laboratories were influenza A viruses and 179 (25.0%) were influenza B viruses. Of the 527 influenza A viruses that were subtyped, 88 (16.7%) were H3 viruses and 439 (83.3%) were (H1N1)pdm09 viruses.
- Cumulatively from October 4, 2015-March 26, 2016, influenza A (H1N1)pdm09 viruses were predominant in all four age groups (0-4 years age group (73.8%), 5-24 years age group (53.5%), 25-64 years age group (73.9%), and in ages 65 years and older (54.5%).
CDC has characterized 1,341 specimens (550 influenza A (H1N1)pdm09, 336 influenza A (H3N2) and 455 influenza B viruses) collected in the U.S. since October 1, 2015.
- All 550 (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 336 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 151 H3N2 viruses also were antigenically characterized; 143 of 151 (94.7%) 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.
- 155 of 159 (97.5%) 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 1,149 influenza A (H1N1)pdm09, 440 influenza A (H3N2), and 644 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, an additional six influenza A (H1N1)pdm09 viruses showing resistance to oseltamivir and peramivir were reported this week. This brings the total number of such viruses reported to 11 (1.0%) this season.