ACIP will presumably endorse this change at its meeting on Jun 26-27, making GSK the first RSV-vaccine company with a recommendation for the 50-59 group.
The clinical trial referenced above was a “bridging study” rather than an efficacy study. I.e., the trial showed that adults age 50-59 given Arexvy had a non-inferior safety profile and immune response compared to adults age 60+ in a separate arm of same trial.