The Marek's vaccine is generally given at hatch or within 24hrs of hatch. The "dose" is dictated by the manufacturer, so you'd have to read the information provided by whatever product you choose if you decide to try to vaccinate yourself. Example below of information sheet along with where to purchase.
Even though birds have never been outdoors, it doesn't mean they possibly have not been exposed to the Marek's virus somehow.
If you have a problem with Marek's within your existing flock, then it may be better to get vaccinated day old chicks, keep them quarantined for a couple of weeks before exposing them to your flock. This means you'd want to change cloths/not handle quarantine chicks after you've tended to existing flock(s), so as to lessen exposure until the vaccine "takes".
The vaccine does not prevent infection from the virus, it can help lessen the formation of tumors.
https://valleyvet.cvpservice.com/product/view/3690294
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