The virus behind Marek's disease in chickens hasn't gotten less severe over time, it's gotten much, much more severe, as an example. HIV does not seem to have gotten less severe, nor smallpox.
Here's some introductory material:
https://www.pbs.org/newshour/science/tthis-chicken-vaccine-makes-virus-dangerous
https://www.nationalgeographic.com/science/article/leaky-vaccines-enhance-spread-of-deadlier-chicken-viruses
https://www.youtube.com/watch?v=haRMKsI-jKY
https://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.1002198
https://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.1002236