Andrew Read, one of the author's on the paper that showed vaccines made Marek's disease worse in chickens, said he didn't have this concern for SARS-CoV-2 vaccines.

He never actually said why not though.

https://theconversation.com/vaccines-could-affect-how-the-coronavirus-evolves-but-thats-no-reason-to-skip-your-shot-165960

It's strange. Considering his expertise, I'd expect him to be a tad more concerned. Single strand RNA means high mutation rate - and highly specific (spike only) vaccines surely increase the risk?