The virus behind Marek's disease in chickens got much more severe because of the leaky vaccine used on chickens.

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

And here are a couple of papers:

  https://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.1002198

  https://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.1002236