Influenza

Paul G. Auwaerter, M.D.
Influenza is a topic covered in the Johns Hopkins ABX Guide.

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PATHOGENS

  • Influenza A (seasonal, U.S.):
    • 2018-2019, season longest in a decade > 20 weeks with two waves of influenza A (H1N1 pdm09 predominated early with H3N2 seen more in Feb-May).
    • 2019-2020, preliminary CDC estimates in the U.S. included 29-56 million infections, 410,00-740,000 hospitalizations and 24,000-62,000 deaths.
      • The season may have been worse than 2018-2019, with more pediatric deaths (at least 144). Both influenza A and B strains were dominant, with influenza B striking typically children and younger adults more than the elderly.
    • 2020-2021, minimal influenza activity, likely secondary to mitigation efforts for the pandemic coronavirus.
      • Circulating viruses did alter from the prior year: influenza A accounted for 61.4% with influenza B at 38.6%.
        • The majority (52.5%) of influenza A viruses were H3N2, and the majority (60%) of influenza B viruses were of Victoria lineage.
        • In the U.S., 5 cases of Influenza A H1N1v, this variant infection occurred in patients all of whom had swine exposures.
  • Influenza A: recently active strains
    • H1n1 pdm09 active annually since 2009; in 2018-2019 accounted for most of the early season.
    • H3N2: seasonal influenza, predominant influenza A strain in 2017-2020 in U.S.
    • Pandemic H1N1 remains active since pandemic 2009, continues to be identified in seasonal influenza.
    • Other: multiple avian influenza or other strains
  • Influenza B (seasonal): typically becomes more common later in a seasonal influenza season.

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PATHOGENS

  • Influenza A (seasonal, U.S.):
    • 2018-2019, season longest in a decade > 20 weeks with two waves of influenza A (H1N1 pdm09 predominated early with H3N2 seen more in Feb-May).
    • 2019-2020, preliminary CDC estimates in the U.S. included 29-56 million infections, 410,00-740,000 hospitalizations and 24,000-62,000 deaths.
      • The season may have been worse than 2018-2019, with more pediatric deaths (at least 144). Both influenza A and B strains were dominant, with influenza B striking typically children and younger adults more than the elderly.
    • 2020-2021, minimal influenza activity, likely secondary to mitigation efforts for the pandemic coronavirus.
      • Circulating viruses did alter from the prior year: influenza A accounted for 61.4% with influenza B at 38.6%.
        • The majority (52.5%) of influenza A viruses were H3N2, and the majority (60%) of influenza B viruses were of Victoria lineage.
        • In the U.S., 5 cases of Influenza A H1N1v, this variant infection occurred in patients all of whom had swine exposures.
  • Influenza A: recently active strains
    • H1n1 pdm09 active annually since 2009; in 2018-2019 accounted for most of the early season.
    • H3N2: seasonal influenza, predominant influenza A strain in 2017-2020 in U.S.
    • Pandemic H1N1 remains active since pandemic 2009, continues to be identified in seasonal influenza.
    • Other: multiple avian influenza or other strains
  • Influenza B (seasonal): typically becomes more common later in a seasonal influenza season.

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Last updated: October 10, 2021