Sanofi to sell most of its Regeneron stake, with much of it being re-purchased by Regeneron, but both stressed the move would not hurt their long established and very profitable drug-development collaboration.

Pfizer is up against AstraZeneca and Sanofi in developing competing approaches to tackling the unmet need in respiratory syncytial virus (RSV). But earlier failures mean all players will need much more data to sway regulators.

US demands for 30,000 subject studies and harmonized protocols mean a robust result could arrive soon – but recruitment could still be a challenge.