Estimating historical impacts of vaccination against influenza B/Yamagata in the United States to inform possible risks of re-emergence in the absence of vaccination

Influenza B/Yamagata viruses have not been detected globally since 2020 and were removed from U.S. 2024/25 seasonal influenza vaccines. We inferred impacts of vaccination against B/Yamagata from 2016/17–2019/20 by combining B/Yamagata prevalence data with model-based estimates of disease burden prevented by vaccination against all influenza B viruses. B/Yamagata comprised approximately 16–22% of U.S. positive virus specimens in 2016/17 and 2017/18, compared to 1% in 2018/19 and 2019/20. Across all seasons, we estimated that vaccination against B/Yamagata prevented 4.15 million symptomatic illnesses, 58,500 hospitalizations, and 4070 deaths, and that 22.9 million B/Yamagata-associated symptomatic illnesses, 340,000 hospitalizations, and 25,100 deaths would have occurred without vaccination. Vaccination prevented the most B/Yamagata hospitalizations among adults ≥65 years but prevented the greatest percentage of B/Yamagata hospitalizations among children 6 months–4 years. Our results may help assess the potential impact if B/Yamagata were to recirculate in the absence of vaccination.