Do you also think I should be required to show a certificate of vaccination for MMR? Oh, wait - they didn't give "certificates" in the 60's when I had SEPARATE shots for measles (both kinds) or for when I suffered a case of the mumps.
How about proof that I got a flu shot this year if sailing during flu "season" as well?
Even if a vaccine is developed, there is no way the cruise lines will require "proof" that each passenger received it (or already recovered from the disease).