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Situation Update: Summary of Weekly FluView Report

FluView Activity Update (Key Flu Indicators)

According to this week’s FluView report, overall influenza activity continued to decrease, but remains elevated and relatively high for this time of year. Twenty states continue to report widespread flu activity. Additionally, four flu-related pediatric deaths occurring during the 2018-2019 season were reported to CDC, bringing the total to 86 flu-related pediatric deaths this season. Most activity at this time is being caused by influenza A(H3) viruses.  At this time, influenza-like-illness levels have been at or above baseline for 20 weeks this season. By this measure, the last five seasons have averaged 16 weeks, with a range of 11 to 20 weeks. The last season that was at or above baseline for 20 weeks occurred during 2014-2015.

CDC expects flu activity to remain elevated for one or two more weeks, but sporadic flu activity will continue to occur after that. While CDC continues to recommend influenza vaccination as long as influenza viruses are circulating, influenza antiviral drugs are an important second line of defense that can be used to treat flu illness. A(H3) viruses are typically associated with more severe illness in older adults, and flu vaccine may protect less well against A(H3) illness in older adults than other virus components, making prompt treatment with flu antivirals in this age group especially important during the current period of A(H3) predominance.

Below is a summary of the key flu indicators for the week ending April 6, 2019:

  • Influenza-like Illness Surveillance: For the week ending April 6 (week 14), the proportion of people seeing their health care provider for influenza-like illness (ILI) decreased from 3.1% to 2.8% and is above the national baseline of 2.2%. The most recent data indicate that ILI activity for the current season peaked the week ending February 16 (week 7) at 5.1%.
    • While ILI activity decreased in all 10 regions, nine of 10 regions continue to experience a proportion of outpatient visits for ILI at or above their region-specific baseline levels.
    • For the past five flu seasons, the peak percent of visits due to ILI has ranged between 3.6% (2015-2016) and 7.5% (2017-2018).
    • Additional ILINet data, including national, regional, and select state-level data for the current and previous seasons, can be found at http://gis.cdc.gov/grasp/fluview/fluportaldashboard.html.
  • Influenza-like Illness (ILI) State Activity Indicator Map: The number of states experiencing high ILI activity decreased from six states last week to four states this week. Additional data, including data for previous seasons, can be found at https://gis.cdc.gov/grasp/fluview/main.html. https://gis.cdc.gov/grasp/fluview/main.html.
  • Geographic Spread of Influenza Viruses: The number of jurisdictions reporting widespread influenza activity decreased from Puerto Rico and 33 states last week to 20 states this week. Geographic spread data reflect how many areas within a state or territory are seeing flu activity. Additional data are available at: https://gis.cdc.gov/grasp/fluview/FluView8.html.
  • Flu-Associated Hospitalizations: Since October 1, 2018, 17,295 laboratory-confirmed influenza-associated hospitalizations have been reported through the Influenza Hospitalization Network (FluSurv-NET), a population-based surveillance network for laboratory-confirmed influenza-associated hospitalizations covering approximately 9% of the U.S. This translates to a cumulative overall rate of 59.9 hospitalizations per 100,000 people in the United States.
    • The highest hospitalization rate is among adults aged 65 years and older (195.9 per 100,000) followed by adults aged 50-64 years (75.5 per 100,000), and children younger than 5 years (69.1 per 100,000). During most seasons, adults 65 years and older have the highest hospitalization rates.
    • For comparison purposes:
      • The final, cumulative overall hospitalization rate for week 14 last season was 99.5 per 100,000.
      • Over the past 5 seasons, cumulative end-of-season hospitalization rates have ranged from 31.4 per 100,000 (2015-2016) to 102.9 per 100,000 (2017-2018).
    • Additional data, including hospitalization rates during previous influenza seasons, can be found at http://gis.cdc.gov/GRASP/Fluview/FluHospRates.html and http://gis.cdc.gov/grasp/fluview/FluHospChars.html.
    • FluSurv-Net data is used to generate national estimates of the total numbers of flu cases, medical visits, and hospitalizations. This season, CDC is reporting preliminary cumulative in-season estimates, which are available at https://cdc.gov/flu/about/burden/preliminary-in-season-estimates.htm.
  • Mortality Surveillance: The proportion of deaths attributed to pneumonia and influenza (P&I) was 7.0% during the week ending March 30, 2019 (week 13). This percentage is below the epidemic threshold of 7.1% for week 13 in the National Center for Health Statistics (NCHS) Mortality Surveillance System. P&I has been at or above threshold for a total of 8 weeks this season (weeks 1-3, weeks 7-9, and weeks 11-12).
    • For comparison purposes, over the last five seasons, P&I has been at or above epidemic threshold for a range of four weeks (2015-2016) to 16 weeks (2017-2018).
  • Additional P&I mortality data for current and past seasons and by geography (national, HHS region, or state) are available at https://gis.cdc.gov/grasp/fluview/mortality.html
  • Pediatric Deaths: Four influenza-associated pediatric deaths were reported to CDC during week 14 (the week ending April 6, 2019).
    • One death was associated with an influenza A(H1N1)pdm09 virus and occurred during week 12 (the week ending March 23, 2019). One death was associated with an influenza A(H3) virus and occurred during week 6 (the week ending February 9, 2019). Two deaths were associated with an influenza A virus for which no subtyping was performed and occurred during weeks 13 and 14 (the weeks ending March 30 and April 6, 2019, respectively).
    • A total of 86 influenza-associated pediatric deaths have been reported for the 2018-2019 season.
    • An additional death that occurred during the 2017-2018 season was reported to CDC. This death was associated with an influenza A virus for which no subtyping was performed and brings the total number of reported influenza-associated deaths occurring during that season to 186.
    • Additional information on influenza-associated pediatric deaths reported during past seasons, including basic demographics, underlying conditions, bacterial co-infections, and place of death is available on FluView Interactive at: https://gis.cdc.gov/GRASP/Fluview/PedFluDeath.html. More detailed information about pediatric deaths reported during the current season will be available later in the season.
  • Laboratory Data:
    • Nationally, the percentage of respiratory specimens testing positive for influenza viruses in clinical laboratories during the week ending April 6 was 15.1%, a decrease from 19.0% the prior week.
    • For comparison purposes, since laboratory data from clinical and public health laboratories was disaggregated three seasons ago, the peak percent of respiratory specimens testing positive for flu at clinical laboratories has ranged from 23.6% to 27.4%.
    • This season, the percent of respiratory specimens testing positive for influenza viruses in clinical laboratories peaked during week 8 (the week ending February 23, 2019) at 26.2%.
    • Nationally, during the week ending April 6, influenza A(H3) viruses were reported more frequently than influenza A(H1N1)pdm09 viruses and were predominant in all 10 regions.
      • However, overall for the 2018-2019 flu season, influenza A(H1N1)pdm09 viruses remain predominant nationally.
      • The majority of A(H1N1)pdm09, B/Victoria and B/Yamagata influenza viruses collected in the United States during September 30, 2018 through April 6, 2019, are still characterized antigenically and genetically as being similar to their cell-grown reference viruses representing the 2018–2019 Northern Hemisphere influenza vaccine viruses; However, an increasing proportion of influenza A(H3) viruses are antigenically distinguishable from A/Singapore/INFIMH-16-0019/2016 (3C.2a1), a cell-propagated reference virus representing the A(H3N2) component of 2018-19 Northern Hemisphere influenza vaccines.
    • The vast majority (>99%) of influenza viruses tested showed susceptibility to oseltamivir, zanamivir and peramivir. So far this season, three (0.2%) influenza A(H1N1)pdm09 viruses displayed highly reduced inhibition by oseltamivir and peramivir. An additional two (0.1%) influenza A(H1N1)pdm09 viruses showed reduced inhibition by oseltamivir. All influenza viruses tested showed susceptibility to zanamivir.
  •  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 2018-2019 season can be found on the current FluView and FluView Interactive.
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