infection rates and seriousness of infection along with death massively skews toward older and those with risk factors, mostly cardio vascular and diabetic. A 20 - 40 year old healthy person without those risk factors which would be fairly uncommon on a ship's crew and thus crew members would be extremely unlikely to be sick to a point where they were unable to perform their duties.

Rules that forced anyone who was infected or like so many other places where if you contact traced to someone that was infected you had to be quarantined, could explain it, but that was an overabundance of caution. In the event of a national emergency where that asset was needed, over abundance of caution is tossed and that crew would go to sea unless they were truly sick, which statistics of this virus show is extremely unlikely.