I asked Meyer Hatchery about this today and they responded. It's very important to vaccinate on hatch day before they come in contact with other birds from other flocks as that's how this spreads rapidly. I know it's possible to vaccinate later (few days old) but not advisable as its not as effective and they could've already been exposed. I don't know how to copy and paste via cell phone to show you exactly what she wrote.Thanks for the advice. I just ordered some today.