We sailed three times last year - member of our party in July came down with COVID, in November it was me, and in December our youngest DD came down with a normal cold. From Facebook groups we've joined folks have reported back and taking the number of people reporting COVID or flu divided by total number of families on the cruise the percentage is higher than I would have thought. I shared this on a Facebook Disney Cruising page and got "murdered" by the comments saying I was lying or making it up. We still plan to cruise - have two planned, but I go into it realizing that we have a good chance of contracting COVID.
We do as one of the posters mentioned and increase vitamin intake starting a couple weeks prior to the cruise.