Effectiveness of the 2023 Southern Hemisphere Influenza Vaccine Against Outpatient Influenza-Like Illness: A Multi-Country Test-Negative Design Study - Report - MDSpire
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Effectiveness of the 2023 Southern Hemisphere Influenza Vaccine Against Outpatient Influenza-Like Illness: A Multi-Country Test-Negative Design Study
Efficacy of 2023 Southern Hemisphere Influenza Vaccine Against Outpatient ILI
Overview
A multi-nation test-negative study across Australia, South Africa, and Thailand demonstrated that the 2023 Southern Hemisphere influenza vaccine provided substantial protection against outpatient medically attended influenza-like illness (ILI). Pooled vaccine effectiveness (VE) was 68% against any influenza virus, with notable protection against influenza A(H1N1)pdm09, A(H3N2), and influenza B.
Background
Seasonal influenza vaccination programs aim to reduce the burden of influenza illness in the community. Monitoring vaccine effectiveness (VE) in real-world outpatient settings is critical to optimize vaccination strategies and inform public health policies. Test-negative design studies using sentinel surveillance systems provide valuable data on VE across different populations and influenza virus subtypes. This study evaluated the 2023 Southern Hemisphere influenza vaccine's performance in three countries with diverse influenza epidemiology and vaccine formulations.
Data Highlights
Parameter
Value
Total ILI outpatients enrolled
2469
Test-positive influenza cases
966 (39%)
Test-negative non-cases
1503 (61%)
Influenza A viruses detected
78% of cases (750/966)
Vaccinated among cases
8% (72/966)
Vaccinated among non-cases
29% (439/1503)
Pooled VE against any influenza
68% (95% CI: 57%, 76%)
VE against influenza A(H1N1)pdm09
62% (95% CI: 39%, 76%)
VE against influenza A(H3N2)
60% (95% CI: 38%, 75%)
VE against influenza B
76% (95% CI: 59%, 86%)
VE decline over time
From 82% at 14 days post-vaccination to 43% at 150 days
Key Findings
The 2023 Southern Hemisphere influenza vaccine demonstrated 68% effectiveness against outpatient medically attended ILI caused by any influenza virus.
Vaccine effectiveness was subtype-specific: 62% against A(H1N1)pdm09, 60% against A(H3N2), and 76% against influenza B.
Influenza A viruses accounted for the majority (78%) of detected influenza cases among outpatients.
Vaccination coverage was low among influenza-positive cases (8%) compared to test-negative controls (29%).
Vaccine protection waned over time, with VE decreasing from 82% at 14 days post-vaccination to 43% at 150 days.
Data were pooled from three countries with different vaccine formulations and surveillance systems, enhancing generalizability.
Clinical Implications
These findings support the continued use of the 2023 Southern Hemisphere influenza vaccine to reduce outpatient influenza illness. Clinicians should encourage timely vaccination early in the influenza season to maximize protection, considering the observed waning of vaccine effectiveness over time. Multi-country surveillance and VE assessment remain essential to guide vaccination policies and optimize public health outcomes.
Conclusion
The 2023 Southern Hemisphere influenza vaccine provided significant protection against outpatient influenza-like illness across diverse settings, reinforcing the value of seasonal vaccination programs. Ongoing monitoring of vaccine effectiveness and timing of vaccination is important to sustain optimal protection.
References
Australian Sentinel Practices Research Network (ASPREN) 2023 -- Influenza Surveillance Data
Viral Watch Sentinel Influenza Surveillance Program, South Africa 2023
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
