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Situation Update: Summary of Weekly FluView Report

Key Flu Indicators

According to the FluView report for the week ending April 22, 2017 (week 16), flu activity continues to decrease in the United States. Levels of flu-like illness have been below the national baseline for two weeks now.  While the 2016-2017 flu season has peaked and is winding down, 7 states continue to report widespread flu activity and another six flu-related pediatric deaths were reported. This brings the total number of flu deaths in children reported to CDC this season to 83. Sporadic flu activity may continue for a number of weeks. While influenza A (H3N2) viruses have been most common overall this season, influenza B viruses accounted for 69% of the viruses reported by public health laboratories during week 16. Interim vaccine effectiveness (VE) estimates indicate flu vaccines this season reduced a vaccinated person’s risk of getting sick and having to go to the doctor because of flu by about half (48%). Estimated VE against H3N2 viruses was 43% while VE against B viruses was 73%. CDC recommends annual flu vaccination for everyone 6 months of age and older. Vaccination efforts should continue for as long as influenza viruses are circulating. Below is a summary of the key flu indicators for the week ending April 22, 2017:

  • Influenza-like Illness Surveillance: For the week ending April 22, the proportion of people seeing their health care provider for influenza-like illness (ILI) was 1.8%.and is now below the national baseline of 2.2% for the second consecutive week season since early December. Two regions (Regions 1 and 4) reported ILI at or above their region-specific baseline level. For the 2016-2017 season, ILI was at or above baseline for 17 consecutive weeks this season. For the last 15 seasons, the average duration of a flu season by this measure has been 13 weeks, with a range from one week to 20 weeks.
  • Influenza-like Illness State Activity Indicator Map: No states experienced high or moderate flu activity. Four states (Alaska, Arizona, Minnesota, and Tennessee) experienced low ILI activity. New York City, Puerto Rico and 46 states (Alabama, Arkansas, California, Colorado, Connecticut, Delaware, Florida, Georgia, Hawaii, Idaho, Illinois, Indiana, Iowa, Kansas, Kentucky, Louisiana, Maine, Maryland, Massachusetts, Michigan, Mississippi, Missouri, Montana, Nebraska, Nevada, New Hampshire, New Jersey, New Mexico, New York, North Carolina, North Dakota, Ohio, Oklahoma, Oregon, Pennsylvania, Rhode Island, South Carolina, South Dakota, Texas, Utah, Vermont, Virginia, Washington, West Virginia, Wisconsin, 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.
  • Geographic Spread of Influenza Viruses: Widespread influenza activity was reported by 7 states (Alaska, Connecticut, Delaware, Maine, Massachusetts, New Hampshire, and New York).  Regional influenza activity was reported by Guam, Puerto Rico and 11 states (Arizona, Colorado, Iowa, Kentucky, New Jersey, North Carolina, Ohio, Pennsylvania, Rhode Island, South Carolina, and Texas). Local influenza activity was reported by the District of Columbia and 19 states (Arkansas, California, Florida, Kansas, Louisiana, Maryland, Michigan, Minnesota, Missouri, Montana, Nebraska, Nevada, New Mexico, North Dakota, Oklahoma, Tennessee, Washington, West Virginia, and Wisconsin). Sporadic activity was reported by 13 states (Alabama, Georgia, Hawaii, Idaho, Illinois, Indiana, Mississippi, Oregon, South Dakota, Utah, Vermont, Virginia and Wyoming). No influenza activity was reported by the U.S. Virgin Islands. Geographic spread data show how many areas within a state or territory are seeing flu activity.
  • Flu-Associated Hospitalizations: Since October 1, 2016, a total of 17,560 laboratory-confirmed influenza-associated hospitalizations have been reported. This translates to a cumulative overall rate of 62.7 hospitalizations per 100,000 people in the United States. This is higher than the cumulative hospitalization rate for the 2012-2013 flu season (44.0 per 100,000), when influenza A (H3N2) viruses also predominated, and is similar to the cumulative hospitalization rate during 2014-2015 (64.1 per 100,000) which also was an H3N2 predominant season. Vaccine effectiveness during 2012-13 was 49%, similar to interim estimates for the current season, but was 19% during 2014-2015 as a result of a high proportion of drifted influenza viruses during that season.
    • The hospitalization rate among people 65 years and older is 281.0 per 100,000. This is the highest rate of any age group. The hospitalization rate for people 65 and older for the same week during the 2012-2013 flu season was 183.4 per 100,000. For week 16 during 2014-2015, it was 307.8 per 100,000.
    • The hospitalization rate among adults 50-64 years is 62.1 per 100,000. During the 2012-2013 and 2014-2015 flu seasons, the hospitalization rate for that age group for the same week was 40.7 per 100,000 and 53.2 respectively.
    • The hospitalization rate among children younger than 5 years is 43.2 per 100,000. During the 2012-2013 and 2014-2015 flu seasons, the hospitalization rate for that age group for the same week was 66.3 per 100,000 and 56.8 per 100,000 respectively.
    • During most seasons, children younger than 5 years and adults 65 years and older have the highest hospitalization rates.
    • Hospitalization data are collected from 13 states and represent approximately 9% 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. Additional data, including hospitalization rates during other influenza seasons, can be found at http://gis.cdc.gov/GRASP/Fluview/FluHospRates.html and http://gis.cdc.gov/grasp/fluview/FluHospChars.html.
  • Mortality Surveillance:
    • The proportion of deaths attributed to pneumonia and influenza (P&I) was 6.9% for the week ending April 8, 2017 (week 14). This percentage is below the epidemic threshold of 7.2% for week 14 in the National Center for Health Statistics (NCHS) Mortality Surveillance System. The weekly percentage of deaths attributed to P&I was at or exceeded the epidemic threshold for 12 consecutive weeks this season.
  • Pediatric Deaths:
    • Six influenza-associated pediatric deaths were reported to CDC for the week ending April 15, 2017.
    • One death was associated with influenza A (H3) virus and occurred during week 6 (the week ending February 11, 2017).
    • One death was associated with an influenza A virus for which no subtyping was performed and occurred during week 15 (the week ending April 15, 2017).
    • Four deaths were associated with an influenza B virus and occurred during week 15.
    • A total of 83 influenza-associated pediatric deaths have been reported for the 2016-2017 season.
    • Additional information on pediatric deaths for the 2016-2017 season is available on FluView Interactive at: https://gis.cdc.gov/GRASP/Fluview/PedFluDeath.html.
  • Laboratory Data:
    • Nationally, the percentage of respiratory specimens testing positive for influenza viruses in clinical laboratories during the week ending April 22 was 9.6%.
    • Regionally, the three week average percent of specimens testing positive for influenza in clinical laboratories ranged from 6.7% to 20.9%.
    • During the week ending April 22, of the 1,426 (9.6%) influenza-positive tests reported to CDC by clinical laboratories, 366 (25.7%) were influenza A viruses and 1,060 (74.3%) were influenza B viruses.
    • While influenza A (H3N2) viruses have predominated this season, the most frequently identified influenza virus type reported by public health laboratories since mid-March  was influenza B viruses.
    • During the week ending April 22, 66 (30.7%) of the 215 influenza-positive tests reported to CDC by public health laboratories were influenza A viruses and 149 (69.3%) were influenza B viruses. Of the 60 influenza A viruses that were subtyped, 58 (96.7%) were H3N2 viruses and 2 (3.3%) was (H1N1)pdm09 virus.
    • Since October 1, 2016, antigenic and/or genetic characterization shows that the majority of the tested viruses remain similar to the recommended components of the 2016-2017 Northern Hemisphere vaccines.
    • Since October 1, 2016, CDC tested 3,056  specimens (291 influenza A (H1N1)pdm09, 2,071 influenza A (H3N2), and 694 influenza B viruses) for resistance to the neuraminidase inhibitors antiviral drugs. None of the tested viruses were found to be resistant to oseltamivir, zanamivir, or peramivir.

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 2016-2017 season can be found on the current FluView.

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