Weekly Influenza Surveillance

The most recent FluView report shows that flu activity continues to decrease in the United States. While flu activity has peaked nationally for this season, some parts of the country are still experiencing widespread flu. Ongoing activity is expected to continue for a number of weeks. Also, flu viruses circulate at low levels during the summer.

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 continues to decrease in the United States. Flu activity has peaked nationally for this season and is winding down. Only two states continue to report widespread flu activity. Influenza B viruses have been most common in recent weeks, however H1N1 viruses have been predominant overall this season. Second waves of influenza B activity occur during many flu seasons. CDC continues to recommend influenza vaccination as long as influenza viruses are circulating. 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 May 14, 2016:

For the week ending May 14, the proportion of people seeing their health care provider for influenza-like illness (ILI) decreased to 1.4%. This is below the national baseline of 2.1%. One of 10 regions (Region 4) 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 was at or above the national baseline for 17 consecutive weeks 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 experienced moderate ILI activity. Three states (Georgia, North Carolina, and New Jersey) experienced low ILI activity. New York City and 46 states experienced minimal ILI activity. The District of Columbia and one state (Idaho) 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 2 states (Delaware and New York). This is a decrease from 3 states with widespread activity last week. Regional flu activity was reported by 7 states (Connecticut, Kentucky, Maine, Massachusetts, New Jersey, Ohio, and Pennsylvania). Local flu activity was reported by the District of Columbia, Guam, and 19 states (Alabama, Arizona, California, Colorado, Hawaii, Iowa, Michigan, New Hampshire, New Mexico, North Carolina, Oklahoma, South Carolina, Tennessee, Texas, Vermont, Virginia, Washington, West Virginia, and Wisconsin). Sporadic flu activity was reported by the U.S. Virgin Islands and 22 states (Alaska, Arkansas, Florida, Georgia, Idaho, Illinois, Indiana, Kansas, Louisiana, Maryland, Minnesota, Mississippi, Missouri, Montana, Nebraska, Nevada, North Dakota, Oregon, Rhode Island, South Dakota, Utah, and Wyoming). Geographic spread data show how many areas within a state or territory are seeing flu activity.
Between October 1, 2015 and April 30, 2016, 8,666 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 31.3 hospitalizations per 100,000 people in the United States. This is significantly lower than the hospitalization rate at this time last season (64.1 per 100,000), which was an H3N2 predominant season. During the last H1N1 predominant season (2013-2014), the cumulative hospitalization rate was 35.1. More data on hospitalization rates, including hospitalization rates during other influenza seasons, are available athttp://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 (84.2 per 100,000), followed by adults 50-64 years (45.2 per 100,000) and children younger than 5 years (42.1 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 at the system-specific epidemic threshold in the 122 Cities Mortality Reporting System.
One additional influenza-associated pediatric death was reported to CDC this week:
- This death was associated with an influenza B virus and occurred during week 14 (the week ending April 9, 2016).
- This brings the total number of flu-associated pediatric deaths reported this season to 68 children.
Nationally, the percentage of respiratory specimens testing positive for influenza viruses in clinical laboratories during the week ending May 14 was 7.1%. For the most recent three weeks, the regional percentage of respiratory specimens testing positive for influenza viruses in clinical laboratories ranged from 4.7% to 12.8%.
- During the week ending May 14, of the 741 influenza-positive tests reported to CDC by clinical laboratories, 205 (27.7%) were influenza A viruses and 536 (72.3%) were influenza B viruses.
The most frequently identified influenza virus type reported by public health laboratories during the week ending May 14 was influenza B viruses.
- During the week ending May 14, 38 (42.7%) of the 89 influenza-positive tests reported to CDC by public health laboratories were influenza A viruses and 51 (57.3%) were influenza B viruses. Of the 31 influenza A viruses that were subtyped, 11 (35.5%) were H3 viruses and 20 (64.5%) were (H1N1)pdm09 viruses.
Cumulatively from October 4, 2015-May 14, 2016, influenza A (H1N1)pdm09 viruses were predominant in all four age groups: 0-4 years age group (70.4%), 5-24 years age group (48.9%), 25-64 years age group (68.9%), and in ages 65 years and older (50.7%).
CDC has characterized 2,292 specimens (901 influenza A (H1N1)pdm09, 562 influenza A (H3N2) and 829 influenza B viruses) collected in the U.S. since October 1, 2015.
- 900 of 901 (99.9%) 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 562 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 265 H3N2 viruses also were antigenically characterized; 257 of 265 (97.0%) H3N2 viruses were similar to A/Switzerland/9715293/2013 by HI testing or neutralization testing.
- All 467 (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.
- 355 of 362 (98.1%) 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 2,015 influenza A (H1N1)pdm09, 671 influenza A (H3N2), and 1063 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, 15 (0.7%) 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.