Yes, but that is where you use epidemiology. If you track down each person in the local area infected with the mutant strain and follow their clinical course you can work this out. You can also do serology to make sure the immune response is similar, etc.

There will always be the possibility that the attenuated strain varies is some major but subtle way from the non-attenuated strains, but this is not something ever seen before with other viruses. If you demand to know everything and the answers to all possible risks up front medical science doesn’t advance.