All seafood (prawns, lobsters, crabs) that are bought pre-cooked are frozen out at sea. It can be dangerous to buy pre-cooked thawed out versions of these because you don't know how they have been stored, when they were defrosted or if they have been frozen more than once. It's safest to buy raw seafood & cook it yourself or to buy frozen seafood from a proper seafood place (not somewhere like a supermarket).

I had heaps of prawns last Christmas & everything was fine. I bought them from work frozen, got them home frozen & ate them within 2hrs of them being thawed. I had ham as well, but ate it when it was freshly cut on Christmas day. I'll be enjoying the leftovers this year though LOL!