Top 5 Takeaways

  1. Low severity and unusual pattern: The 2021–22 influenza season was characterized by low severity, with activity spanning from October 2021 through mid-June 2022, showcasing two distinct waves of influenza A, primarily A(H3N2) viruses.
  2. Continuous surveillance and vaccination: Due to atypical influenza activity timing and an ongoing outbreak of avian A(H5N1), continuous surveillance and annual vaccination are emphasized as key protective measures.
  3. Virus surveillance and characterization: Among tested specimens, 4.5% were positive for influenza, predominantly A(H3N2). Genetic and antigenic analysis supported vaccine effectiveness, with all collected viruses since October 3 showing susceptibility to major antivirals.
  4. 2022–23 Influenza vaccine composition: The vaccine composition was updated based on WHO recommendations, highlighting the importance of adaptation to circulating strains for optimal protection.
  5. Implications of COVID-19 on influenza activity: Compared to pre-pandemic seasons, the 2021–22 season saw reduced influenza activity, potentially influenced by COVID-19 mitigation measures, underscoring the dynamic interaction between respiratory viruses and public health interventions.

Original Article Author and Citation

Corresponding Author

Angiezel Merced-Morales,;
Peter Daly,

Suggested Citation

Merced-Morales A, Daly P, Abd Elal AI, et al. Influenza Activity and Composition of the 2022–23 Influenza Vaccine — United States, 2021–22 Season. MMWR Morb Mortal Wkly Rep 2022;71:913–919. DOI:


The 2021–22 influenza season in the United States was marked by low severity, featuring two waves of influenza A activity. A(H3N2) viruses predominated, with influenza B viruses playing a minor role. The season’s extended duration and atypical timing highlight the need for year-round vigilance and influenza vaccination.


Surveillance data was collected from a network of clinical and public health laboratories across the United States. This included testing over 2.8 million respiratory specimens for influenza viruses, with genetic and antigenic characterization of selected specimens to monitor circulating strains and inform vaccine composition.


The season’s unusual pattern, with extended activity and low severity, underscores the impacts of COVID-19 mitigation measures on influenza circulation. The update to the 2022–23 vaccine composition aims to enhance protection against expected strains.


Continued influenza testing, surveillance for novel viruses, and vaccination are crucial for public health, especially with the presence of ongoing threats like avian influenza A(H5N1). Annual vaccination remains the key strategy for preventing influenza and its complications.

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