The vaccine, mRNA-4157, is being tested along with Merck's blockbuster cancer immunotherapy Keytruda, in a mid-stage trial. Data on the vaccine is expected in the fourth quarter of this year.

Merck will pay $250 million to exercise the option, as agreed under a 2016 agreement, Moderna said

The vaccine is designed to generate T-cells, a key part of the body's immune response, based on the mutational signature of a patient's tumor.