I think you're right -- I had Marek's on my brain.Vaccinate for flu? I read Dutch did some vaccinations on chickens.
The flu strain they get today is different tomorrow so vaccinating for flu wouldn't help.
Coccidiosis and Marek's vaccinations are the ones most common in hatcheries.
https://www.cacklehatchery.com/should-you-vaccinate-your-chickens/