Merck has announced it will give back the rights of an adult respiratory syncytial virus (RSV) vaccine it has been developing in a collaboration with Moderna. The New Jersey-based pharmaceutical company will instead switch gears to pursue an RSV program of its own antibody candidate, which is currently in a phase 2 trial.

The vaccine candidate, mRNA-1172, uses a Merck lipid nanoparticle for delivery. Moderna has noted that it has more data for its own lipid nanoparticle than for Merck’s.

Moderna’s RSV vaccine candidate entered phase 1 testing last year. The company announced it has just begun dosing its candidate for infants. Merck will work to complete phase 1 testing before it formally cedes rights to the vaccine.