This large randomized crossover study of more than 429,000 adults aged 65+ found no meaningful difference in effectiveness between adjuvanted and high-dose influenza vaccines during the 2023–2024 flu season. Rates of PCR-confirmed influenza, influenza-related hospitalization/ED visits, and community-acquired pneumonia were nearly identical between groups, supporting current ACIP guidance that either vaccine is an appropriate preferential option for older adults.