27 vaccinated people test positive for Covid on a Carnival cruise ship

27 vaccinated people test positive for Covid on a Carnival cruise ship: Twenty-seven vaccinated people aboard a Carnival cruise ship that docked in Belize tested positive for Covid-19 this week. Only one of the positive cases was a passenger; the remaining 26 were crew members. All 27 people are experiencing mild or no symptoms.

I'm wondering what this means--are these breakthrough infections among the vaccinated, who have contracted the Delta variant?  Does the cruise ship require proof of vaccination among passengers?  Given the events of the past 10 years, I tend to think of cruise ships as floating petri dishes.  Looking at the infection rates in New York City, perhaps we are going to live in a less dense future.  However, Hong Kong has had 212 deaths.  Someone needs to write more about this.