One of those catch 22's.
We don't vacinate for this, but take care of where we buy any stock from, and run a sort of "closed herd" putting new stock in isolation.
If you don't have it and don't introduce it (sheep can also carry it), then vacination is not needed, but pigs can carry it sub-clinically (show no signs) and only show it when stressed from other illness, or pass it on to suseptible pigs.
With 45 sows if we vacinated against everything we would make no money, so make a judgement call.
Can now feel everyone going down with it as I write !