If the 2 that didn't get wet pox are a male and female, be sure to breed those 2 together this year and get as many chicks as possible from them, those two will pass on a wet pox resistant gene, which will be better for you and everybody that gets any eggs or chicks from you in the future.
It is not a wise idea to breed from fowl that get a disease easily and especially if they can't get over it without using medication. There are Sumatra breeders that will not buy birds if they know that the breeder vaccinates his fowl on a regular basis. I know it would be extremely difficult to get rid of 40 Sumatras and keep only the 2 that didn't get sick, but in the long run that would definitely be a great asset to your breeding program, not only would it be a great asset to your breeding program, but also to the Sumatra breeders around you and to those you sell your fowl too.
Vaccinations do not always make fowl get sick, but I have a feeling that we would have a lot less sick chickens if all of us would
not breed from fowl that have gotten sick. I have Sumatras here that have never been sick from anything, if they had showed anything at all I would not have bred from them. That's my goal in breeding my fowl and I will not intentionally buy chickens from others that don't do the same.
I'm not trying to make you look bad, I'm just sharing my chicken breeding experience, so that others don't need to make the same mistakes that I have made in breeding from fowl that need vaccinations and medications to stay alive.
Keep marching ya'll!