Global & Disaster Medicine

Influenza activity in the United States is increasing. Influenza A(H1N1)pdm09, influenza A(H3N2), and influenza B viruses continue to co-circulate.



Influenza activity in the United States is increasing. Influenza A(H1N1)pdm09, influenza A(H3N2), and influenza B viruses continue to co-circulate. Below is a summary of the key influenza indicators for the week ending December 22, 2018:

  • Viral Surveillance: Influenza A viruses have predominated in the United States since the beginning of October. Influenza A(H1N1)pdm09 viruses have predominated in most areas of the country, however influenza A(H3) viruses predominated in the southeastern United States (HHS Region 4). The percentage of respiratory specimens testing positive for influenza viruses in clinical laboratories is increasing.
    • Virus Characterization: The majority of influenza viruses characterized antigenically and genetically are similar to the cell-grown reference viruses representing the 2018–2019 Northern Hemisphere influenza vaccine viruses. A comparison of gene sequences of recent influenza A(H1N1)pdm09 viruses from the U.S. and Mexico/Central America showed them to be similar.
    • Antiviral Resistance: All viruses tested show susceptibility to the neuraminidase inhibitors (oseltamivir, zanamivir, and peramivir).
  • Influenza-like Illness Surveillance:The proportion of outpatient visits for influenza-like illness (ILI) increased to 3.3%, which is above the national baseline of 2.2%. Nine of 10 regions reported ILI at or above their region-specific baseline level.
    • ILI State Activity Indictor Map: New York City and nine states experienced high ILI activity; Puerto Rico and seven states experienced moderate ILI activity; 11 states experienced low ILI activity; the District of Columbia and 22 states experienced minimal ILI activity; and one state had insufficient data. Among the four states along the southern U.S. border, ILI activity increased to moderate in Arizona and high in New Mexico.
  • Geographic Spread of Influenza: The geographic spread of influenza in Guam and 11 states was reported as widespread; Puerto Rico and 19 states reported regional activity; 15 states reported local activity; the District of Columbia, the U.S. Virgin Islands and three states reported sporadic activity; and two states did not report.
  • Influenza-associated Hospitalizations A cumulative rate of 3.6 laboratory-confirmed influenza-associated hospitalizations per 100,000 population was reported.  The highest hospitalization rate is among children younger than 5 years (10.0 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: Four influenza-associated pediatric deaths were reported to CDC during week 51.

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national levels of ILI and ARI

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