PATHOGENS

• Influenza A (seasonal, U.S.): though never predictable in any given respiratory season, the pandemic remains an influence as SARS-CoV-2 has joined RSV as the "big 3" viruses.
  
  • 2019-2020, 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 influenza activity increased 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 accounted for most infections, while influenza B displayed minimal activity.
        • Vaccine efficacy was 36% overall but ranged from 10% in those > 50 years to 51% in children 6 mos-8 years.
  • 2022-2023, preliminary estimates by the CDC have figured 27-54 million flu illnesses, 12-26 million medical visits for flu, 300,000-650,000 flu-related hospitalizations and 19,000-58,000 flu deaths.
    • 10% of respiratory samples were positive for influenza in the US.
      • Mostly influenza A (97.8%), primarily H3N2 (72.4%).
    • Vaccine efficacy was 49% overall.