Leaked confidential data. A major stock sell-off.

Beneath the chaos surrounding Moderna and Merck 's latest data on their personalized cancer vaccine, oncologists see something else: an important milestone in melanoma treatment.

The companies on Thursday released detailed results from a phase 3 trial on their mRNA-based shot after partial data leaked on social media platforms earlier that day.

The vaccine, known as intismeran autogene, reduced the risk of melanoma recurrence or death by 42% when given alongside Merck's immunotherapy Keytruda, compared with Keytruda alone.…