Objective:

To compare the effectiveness of adjuvanted and high-dose influenza vaccines in adults aged 65 years or older during the 2023-2024 influenza season, focusing on their impact on confirmed influenza cases.

Key Findings:

• No statistically significant difference in effectiveness between adjuvanted (4 cases per 1,000) and high-dose vaccines (4 cases per 1,000), indicating comparable performance.
• Adjusted relative vaccine effectiveness was 1.5% for polymerase chain reaction-confirmed influenza, with a notable 9% estimate for hospitalization that warrants further evaluation.

Interpretation:

Both adjuvanted and high-dose vaccines are effective and can be recommended for older adults, with no clear superiority of one over the other; ongoing analyses will provide further insights.

Limitations:

• Potential for imperfect adherence to the randomized crossover schedule, which may affect results.
• Study may not generalize to uninsured populations or regions outside Northern California, limiting broader applicability.
• Inability to assess testing propensity based on symptom severity could underrepresent milder cases.

Conclusion:

The study supports the continued use of both vaccine formulations for adults aged 65 years or older, pending further analyses to confirm findings.