I know that we need to vaccinate at 1 day old and it can be done at day 18 in the egg. But what if the chicks is older? I read that it's possible to vaccinate older but mabye it will be less effective... I also read that we can give the vaccine at maximum 16 weeks old. And why it will not be good after 16 weeks? I would like to have a scientific explanation
Outside United States
To be effective, Mareks vaccine must be given at hatch.