Effectiveness of the 2023 Southern Hemisphere Influenza Vaccine Against Outpatient Influenza-Like Illness: A Multi-Country Test-Negative Design Study - Summary - MDSpire

Effectiveness of the 2023 Southern Hemisphere Influenza Vaccine Against Outpatient Influenza-Like Illness: A Multi-Country Test-Negative Design Study

  • By

  • Annette K Regan

  • Radhika Gharpure

  • Monique Chilver

  • Nigel Stocks

  • Siobhan St George

  • Sibongile Walaza

  • Anne von Gottberg

  • Nicole Wolter

  • Cheryl Cohen

  • Aaron M Samuels

  • Kriengkrai Prasert

  • Prabda Praphasiri

  • William W Davis

  • Chakrarat Pittayawonganon

  • Sheena G Sullivan

  • Eduardo Azziz-Baumgartner

  • September 17, 2025

  • 0 min

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Objective:

To estimate the effectiveness of the 2023 Southern Hemisphere formulation of the seasonal influenza vaccine in preventing outpatient medically attended influenza-like illness (ILI) in Australia, South Africa, and Thailand.

Key Findings:
  • Pooled VE was 68% (95% CI: 57%, 76%) against ILI with any influenza virus.
  • VE was 62% (95% CI: 39%, 76%) against influenza A(H1N1)pdm09.
  • VE was 60% (95% CI: 38%, 75%) against influenza A(H3N2).
  • VE was 76% (95% CI: 59%, 86%) against influenza B.
  • VE declined from 82% to 43% as the time from vaccination increased from 14 days to 150 days.
Interpretation:

The 2023 Southern Hemisphere influenza vaccine demonstrated significant effectiveness against ILI across three countries, highlighting the importance of seasonal vaccination programs.

Limitations:
  • The study may not fully capture all outpatient cases due to reliance on sentinel surveillance.
  • Potential biases in self-reported vaccination status could affect VE estimates.
  • Variability in influenza virus circulation and vaccine strains across countries may influence results.
Conclusion:

The findings support the public health benefits of the 2023 Southern Hemisphere influenza vaccination program in reducing outpatient ILI.

Original Source(s)

Effectiveness of the 2023 Southern Hemisphere Influenza Vaccine Against Outpatient Influenza-Like Illness: A Multi-Country Test-Negative Design Study

  • by Annette K Regan, Radhika Gharpure, Monique Chilver, Nigel Stocks, Siobhan St George, Sibongile Walaza, Anne von Gottberg, Nicole Wolter, Cheryl Cohen, Aaron M Samuels, Kriengkrai Prasert, Prabda Praphasiri, William W Davis, Chakrarat Pittayawonganon, Sheena G Sullivan, Eduardo Azziz-Baumgartner

  • September 17, 2025

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