Very glad the pig responded to the jab.
We had a similar situation with one of our 3 pigs last year - suddenly she became listless and uninterested in food, though still drinking. No obvious rash (mind you difficult to tell with an OSB x GOS) but definitely an unhappy pig. Vet came out, decided it was most probably erisypelas and gave her an injection with the the comment 'she will either be better tomorrow or dead' - she got better.
The vet said that generally erisypelas is normally present in the soil and it is really a matter of chance (pig slightly stressed/below par) given that the other 2 pigs were in the same paddock as the infected one but had no issues.
I asked about vaccinating the other and he basically said there was no point. Our pigs were also due to have gone to the abattoir but we had to delay until the withdrawal period for the antibiotics had passed.