Influenza Vaccine Efficacy: Key Findings from 2023-2024 Season

Strategic Insights -

During the 2023-2024 influenza season, sustained elevated activity was observed from December 2023 to February 2024, with continued activity through May 2024. Multiple influenza viruses, including A(H1N1), A(H3N2), and B, circulated during the season. Researchers from the U.S. Centers for Disease Control and Prevention conducted a study to evaluate the effectiveness of the influenza vaccine against hospitalizations due to influenza A(H1N1), A(H3N2), and B viruses. The study, published in The Journal of Infectious Diseases, found that the vaccine was effective against influenza-associated hospitalizations, particularly in patients vaccinated 14-60 days prior to illness onset.

Key Takeaways:

  • The 2023-2024 influenza season included sustained elevated activity from December 2023 to February 2024 and continued activity through May 2024.
  • Influenza A(H1N1), A(H3N2), and B viruses circulated during the season.
  • Vaccine effectiveness (VE) against influenza-associated hospitalization was calculated to be 40% (95% confidence interval [CI], 31-48%).
  • The VE was statistically significant against influenza B (67%; 95% CI, 35-84%) and A(H1N1) (36%; 95% CI, 21-48%).
  • The VE was higher for patients 14-60 days from vaccination (54%; 95% CI, 40-65%) than 120 days (18%; 95% CI, -1% to 33%).
  • The study found that vaccination reduced the risk of influenza A(H1N1)- and influenza B-associated hospitalizations among adults.

Statistics:

  • 40% vaccine effectiveness against influenza-associated hospitalization (95% CI, 31-48%).
  • 53% vaccine effectiveness against influenza-associated hospitalization among patients 18-49 years old (95% CI, 34-67%).
  • 47% vaccine effectiveness against influenza-associated hospitalization among patients 50-64 years old (95% CI, 31-60%).
  • 31% vaccine effectiveness against influenza-associated hospitalization among patients ≥ 65 years old (95% CI, 16-43%).
  • 67% vaccine effectiveness against influenza B hospitalization (95% CI, 35-84%).
  • 36% vaccine effectiveness against influenza A(H1N1) hospitalization (95% CI, 21-48%).

Sources:

  • "Vaccine Effectiveness Against Influenza A(H1n1), A(H3n2), and B-associated Hospitalizations, United States, 1 September 2023 To 31 May 2024." The Journal of Infectious Diseases. 2025.
  • NewsRx. New Findings from U.S. Centers for Disease Control and Prevention in Vaccine Efficacy Provides New Insights [Vaccine Effectiveness Against Influenza A(H1n1), A(H3n2), and B-associated Hospitalizations, United States, 1 September 2023 To 31 May ...]. Medical Letter on the CDC & FDA. June 22, 2025; p 360.