You’re both right. It’s preventative, not prophylactic.

We’re strict with the latter because they’re administered to healthy adults. That doesn’t apply here. If you took every coronavirus mRNA vaccine side effect and multiplied the severity and frequency by two orders of magnitude, this vaccine would still be worth it for many with melanoma.

The reason preventative treatments are held to a higher standard is because they are given to a very large number of people who might never get the disease anyway. Hence, a 1 in 1000 risk is significant. If you are treating people who have had cancer, their risk of getting it again in much higher than 1 in 1000.