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

Key Flu Indicators

According to this week’s FluView report, flu activity decreased slightly, but remains elevated in the United States. It’s possible that the percentage of patient visits for influenza-like-illness (ILI) may have peaked nationally for this season and is winding down, but there are still 39 states reporting widespread activity at this time. Flu activity will likely continue to be elevated 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 19, 2016:

For the week ending March 19, the proportion of people seeing their health care provider for influenza-like illness (ILI) decreased from 3.7% to 3.2%. This is still above the national baseline of 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 10 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 7 states (Alabama, Kentucky, New Jersey, New Mexico, New York, North Carolina, and Virginia) experienced high ILI activity. This is a decrease from 14 states with high ILI activity last week. New York City and 8 states (Alaska, Arizona, Hawaii, Illinois, Michigan, Mississippi, Nevada, and Utah) experienced moderate ILI activity. 20 states (Arkansas, California, Colorado, Florida, Georgia, Idaho, Indiana, Kansas, Louisiana, Massachusetts, Missouri, Montana, Oklahoma, Pennsylvania, Rhode Island, South Carolina, Tennessee, Texas, Wisconsin, and Wyoming) experienced low ILI activity. 15 states (Connecticut, Delaware, Iowa, Maine, Maryland, Minnesota, Nebraska, New Hampshire, North Dakota, Ohio, Oregon, South Dakota, Vermont, Washington, and West Virginia) 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 39 states. This is one fewer state than reported last week. Regional flu activity was reported by Guam and 10 states (Alabama, Georgia, Illinois, Louisiana, Mississippi, Tennessee, Texas, Utah, Washington, and West Virginia). Local flu activity was reported by the District of Columbia and one state (Hawaii). 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, 5,023 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 18.2 hospitalizations per 100,000 people in the United States. This is significantly lower than the hospitalization rate at this time last season (59.6 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 andhttp://gis.cdc.gov/grasp/fluview/FluHospChars.html.
- The highest hospitalization rates are among people 65 years and older (46.1 per 100,000), followed by adults 50-64 years (26.9 per 100,000) and children younger than 5 years (25.6 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.
Two influenza-associated pediatric deaths were reported to CDC during the week ending March 19.
- One death was associated with an influenza A (H3) virus and occurred during week 11 (the week ending March 19, 2016) and one death was associated with an influenza A (H1N1)pdm09 virus and occurred during week 9 (the week ending March 5, 2016).
- A total of 30 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 19 was 20.1%. For the most recent three weeks, the regional percentage of respiratory specimens testing positive for influenza viruses in clinical laboratories ranged from 11.0% to 29.6%.
- During the week ending March 19, of the 4,816 influenza-positive tests reported to CDC by clinical laboratories, 3,557 (73.9%) were influenza A viruses and 1,259 (26.1%) were influenza B viruses.
The most frequently identified influenza virus type reported by public health laboratories during the week ending March 19 was influenza A viruses, with influenza A (H1N1)pdm09 viruses predominating.
- During the week ending March 19, 839 (77.3%) of the 1,086 influenza-positive tests reported to CDC by public health laboratories were influenza A viruses and 247 (22.7%) were influenza B viruses. Of the 824 influenza A viruses that were subtyped, 85 (10.3%) were H3 viruses and 739 (89.7%) were (H1N1)pdm09 viruses.
- Cumulatively from October 4, 2015-March 19, 2016, influenza A (H1N1)pdm09 viruses were predominant in all four age groups (0-4 years age group (74.2%), 5-24 years age group (53.4%), 25-64 years age group (73.7%), and in ages 65 years and older (53.6%).
CDC has characterized 1,229 specimens (507 influenza A (H1N1)pdm09, 324 influenza A (H3N2) and 398 influenza B viruses) collected in the U.S. since October 1, 2015.
- All 507 (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 324 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 139 H3N2 viruses also were antigenically characterized; 131 of 139 (94.2%) 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.
- 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 922 influenza A (H1N1)pdm09, 387 influenza A (H3N2), and 463 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 influenza A (H1N1)pdm09 viruses showing resistance to oseltamivir and peramivir was reported this week. This brings the total of such viruses reported to 5 (0.5%) this season.

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.