Their illness is their vaccination. Once they get it once, that's it for them - no more pox. YAY
We're in Houston and having the worst year of pox EVER here. It's dry pox, thankfully. But it's so nasty. Thankfully it looks like the scabs are dropping off now. (Incidentally that and mosquitoes are how it spreads.)
You'd think since we're so dry here we wouldn't have had it so bad as the chickens aren't on the ground the others had been on yet at all! ??
here's a good article from our very own Backyardchickens about pox - great pictures to help you make sure:
https://www.backyardchickens.com/LC-diseases-AvianPox.html
I've read that vitamin A will help by the way. You can provide it via non-iron Enfamil PolyViSol vitamins (that formula, the non-iron, is available at CVS). Three drops slowly drizzled into the side crack of the beak daily for a week then taper off.
You can also get beta carotene capsules and give them a drop of that - as beta carotene is more hard to overdose than regular A.
I also like wheat germ oil as it contains A, D, as well as E. One small cap full for four cups of feed mixed in well. Or put it in a travel sized sprayer and spritz on the food just before serving.
Just boost their nutrition and give them the best stuff they can get right now. The OACV in the water and a bit of yogurt daily won't hurt if you feel they're feeling poor.