2018-2019 Influenza Season Week 13 ending March 30, 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. Synopsis: Influenza activity decreased but remains elevated 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 have also been reported. Below is a summary of the key influenza indicators for the week ending March 30, 2019: • Viral Surveillance: The percentage of respiratory specimens testing positive for influenza viruses in clinical laboratories decreased. Nationally, during the most recent three weeks, influenza A(H3) viruses were reported more frequently than influenza A(H1N1)pdm09 viruses and in all 10 HHS Regions. o Virus Characterization: The majority of influenza viruses characterized antigenically are similar to the cell-grown reference viruses representing the 2018–2019 Northern Hemisphere influenza vaccine viruses. However, an increasing proportion 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 3.2%, and remains above the national baseline of 2.2%. All 10 regions reported ILI at or above their region-specific baseline level. o ILI State Activity Indictor Map: Six states experienced high ILI activity; 19 states experienced moderate ILI activity; New York City, the District of Columbia, Puerto Rico and 13 states experienced low ILI activity; 12 states experienced minimal ILI activity; and the U.S. Virgin Islands had insufficient data. • Geographic Spread of Influenza: The geographic spread of influenza in Puerto Rico and 33 states was reported as widespread; 15 states reported regional activity; the District of Columbia and one state reported local activity; the U.S. Virgin Islands and Guam did not report. • Influenza-associated Hospitalizations A cumulative rate of 56.4 laboratory-confirmed influenza-associated hospitalizations per 100,000 population was reported. The highest hospitalization rate is among adults 65 years and older (181.8 hospitalizations per 100,000 population). • Pneumonia and Influenza Mortality: The proportion of deaths attributed to pneumonia and influenza (P&I) was at the system-specific epidemic threshold in the National Center for Health Statistics (NCHS) Mortality Surveillance System. • Influenza-associated Pediatric Deaths: Six influenza-associated pediatric deaths were reported to CDC during week 13. Five deaths occurred during the 2018-2019 season and one death occurred during the 2017-2018 season.
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