2018-2019 Influenza Season Week 16 ending April 20, 2019
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    All data are preliminary and may change as more reports are received.

    An overview of the CDC influenza surveillance system, including methodology and detailed descriptions of each data component, is available at http://www.cdc.gov/flu/weekly/overview.htm.

    Influenza activity continues to decrease in the United States. Influenza A(H1N1)pdm09 viruses predominated from October to mid-February, and influenza A(H3N2) viruses have been more commonly identified since late February. Small numbers of influenza B viruses also have been reported. Below is a summary of the key influenza indicators for the week ending April 20, 2019:

    • Viral Surveillance: The percentage of respiratory specimens testing positive for influenza viruses in clinical laboratories decreased. During the most recent three weeks, influenza A(H3) viruses were reported more frequently than influenza A(H1N1)pdm09 viruses nationally, and in all 10 HHS Regions.

    o Virus Characterization: The majority of influenza A(H1N1)pdm09 and influenza B viruses characterized antigenically are similar to the cell-grown reference viruses representing the 2018–2019 Northern Hemisphere influenza vaccine viruses. However, the majority of influenza A(H3N2) 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.

    o Antiviral Resistance: The vast majority of influenza viruses tested (>99%) show susceptibility to oseltamivir and peramivir. All influenza viruses tested showed susceptibility to zanamivir.

    • Influenza-like Illness Surveillance: The proportion of outpatient visits for influenza-like illness (ILI) decreased to 2.1%, which is below the national baseline of 2.2%. This is the first week ILI activity was below the national baseline since mid-November 2018. Four of 10 regions reported ILI at or above their region-specific baseline level.

    o ILI State Activity Indictor Map: Puerto Rico experienced high ILI activity; one state experienced moderate ILI activity; nine states experienced low ILI activity; New York City, the District of Columbia and 40 states experienced minimal ILI activity; and the U.S. Virgin Islands had insufficient data.

    • Geographic Spread of Influenza: The geographic spread of influenza in five states was reported as widespread; Puerto Rico and 17 states reported regional activity; 19 states reported local activity; the District of Columbia, the U.S. Virgin Islands and nine states reported sporadic activity; and Guam did not report.

    • Influenza-associated Hospitalizations A cumulative rate of 64.2 laboratory-confirmed influenza-associated hospitalizations per 100,000 population was reported. The highest hospitalization rate is among adults 65 years and older (214.1 hospitalizations per 100,000 population).

    • Pneumonia and Influenza Mortality: The proportion of deaths attributed to pneumonia and influenza (P&I) was below the system-specific epidemic threshold in the National Center for Health Statistics (NCHS) Mortality Surveillance System.

    • Influenza-associated Pediatric Deaths: Five influenza-associated pediatric deaths were reported to CDC during week 16.

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