Where do you live? I think most vaccines are for chickens 8 weeks and older. I don’t vaccinate for that, but it is not prevalent in my area. In the southern US, TX, FL, LA, or tropical areas, they have it pretty bad. Birds who have had chicken fowl pox are immune to it later, but not chicks, and they get it worse.
It comes in a vial of 1000 doses, and you have to purchase ice packs and a cooler from the seller, plus 2 day shipping, to guarantee staying cold, which raises the cost. Here is a good place to buy from:
https://www.jefferspet.com/products...MIiZf3nb_X9AIV7WxvBB3omgTEEAQYAiABEgINBfD_BwE