Clinical Scorecard: Assessing the Efficacy of the 2023 Southern Hemisphere Influenza Vaccine Against Outpatient Influenza-Like Illness: Findings from a Multi-Nation Test-Negative Study

At a Glance

Key Highlights

• Pooled vaccine effectiveness (VE) was 68% against medically attended outpatient ILI caused by any influenza virus.
• VE estimates were 62% against influenza A(H1N1)pdm09, 60% against influenza A(H3N2), and 76% against influenza B.
• VE declined from 82% at 14 days post-vaccination to 43% at 150 days, indicating waning protection over time.

Guideline-Based Recommendations

Diagnosis

• Use RT-PCR testing for influenza virus detection in patients presenting with ILI within 10 days of symptom onset.
• Apply standardized ILI case definitions (fever ≥38°C and cough) for surveillance and study eligibility.

Management

• Administer the 2023 Southern Hemisphere seasonal influenza vaccine to eligible populations to reduce outpatient influenza illness.
• Vaccination should be timed to maximize protection during peak influenza circulation periods.

Monitoring & Follow-up

• Conduct routine sentinel surveillance and test-negative design studies to monitor influenza vaccine effectiveness across countries.
• Adjust VE estimates for confounders including age, sex, underlying conditions, and calendar week of symptom onset.

Risks

• Consider potential waning of vaccine-induced protection over time, especially beyond 150 days post-vaccination.
• Exclude patients vaccinated within 14 days prior to illness onset from VE analyses due to insufficient immune response development.

Patient & Prescribing Data

Outpatients aged 12 months and older presenting with ILI in Australia, South Africa, and Thailand during the 2023 influenza season.

Influenza vaccination coverage was low among test-positive cases (8%) compared to test-negative controls (29%), supporting vaccine effectiveness in preventing influenza illness.

Clinical Best Practices

• Implement influenza vaccination programs targeting outpatient populations to reduce influenza burden.
• Use test-negative design studies combining multi-country sentinel surveillance data for robust VE estimation.
• Exclude patients with recent vaccination (<14 days) or incomplete data to ensure accurate VE assessment.
• Monitor VE by influenza subtype and time since vaccination to inform vaccination timing and policy.

References

• Australian Sentinel Practices Research Network (ASPREN)
• Viral Watch sentinel influenza surveillance program (South Africa)
• FluNet Global Influenza Surveillance