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 typically striking children and younger adults more than the elderly.
  • 2020-2021, minimal influenza activity, likely secondary to mitigation efforts for the coronavirus pandemic.
    • Circulating viruses did alter from the prior year: influenza A accounted for 61.4%, with influenza B at 38.6%.
      • Most (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 with swine exposures.
  • 2021-2022, some increase in influenza activity but not nearly to levels seen before the COVID-19 pandemic.
    • CDC estimates that influenza virus infection resulted in 8.0–13.0 million symptomatic illnesses, 3.7–6.1 million medical visits, 82,000–170,000 hospitalizations, and 5,000–14,000 deaths in the U.S.
      • Influenza A: recently active strains
        
        • H1n1 pdm09 has been active annually since 2009; 2018-2019 accounted for most of the early season.
        • H3N2: seasonal influenza, predominant influenza A strain in 2017-2022 in the U.S, and reflected the bulk of isolates in the 2021-2022 season.
        • Pandemic H1N1 remains active since the pandemic in 2009 and 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.
        • Minimal activity during the 2021-2022 season.