I saw the headlines about this last night and thought, “See? This is what happens when you don’t let cruise lines require vaccine passports. The unvaccinated come aboard and before you know it there’s an outbreak.”

Except … it wasn’t the unvaccinated who brought the virus onto the ship. That’s the catch. Everyone aboard this “test” cruise had been vaccinated, including the two infected passengers.

Which way does that cut for requiring vaccine passports?

“Today two guests sharing a stateroom onboard Celebrity Millennium tested positive for Covid-19 while conducting the required end of cruise testing,” the cruise line company said. “The individuals are asymptomatic and currently in isolation and being monitored by our medical team.”

All guests were required to show a negative Covid-19 test and proof of vaccination 72 hours before its departure on Saturday, the company said.

“Celebrity Millennium is sailing with fully vaccinated crew and guests and following comprehensive protocols that align with our destination partners and exceed CDC guidelines to protect the health and safety of our guests,” Celebrity Cruises said. “This situation demonstrates that our rigorous health and safety protocols work to protect our crew, guests and the communities we visit.”

The cruise, which set sail from St. Maarten, is operating at one-third capacity to test how well the safety protocols implemented by Royal Caribbean work to limit infections. In addition to proof of vaccination and a recent negative test, passengers have to complete the cruise line’s own health questionnaire and health screening document, which needs to be authorized in advance. While aboard, various steps are being taken to limit physical contact: The buffets are no longer self-serve, for instance, and safety tutorials have been made virtual instead of being presented to a gathered audience. Complimentary COVID tests are available on the ship and there’s an “isolation” cabin with its own ventilation system where infected people can be moved. That’s where the passengers who tested positive are staying right now, per the Times.

In spite of all those precautions, they had two positive tests on day one. Cruise shares tumbled on Wall Street this morning as investors absorbed the news.

But is it really that surprising or discouraging? The two infected passengers are traveling together so either they were infected before they came aboard and their tests missed it or one infected the other while aboard because they’re in constant close contact. Both are asymptomatic, which means the vaccines are doing their job. And if there are no further infections, that’ll be circumstantial evidence that either the protocols are working to limit outbreaks or that the CDC is right that vaccinated people don’t transmit the virus very much after being infected — or both.

If there are a bunch of new infections in the coming days, on the other hand, then the CDC might need to revisit its conclusions about how often vaccinated people are infecting others. Because those aboard are isolated from the rest of the world, cruise ships are a natural “laboratory” from which scientists can lean how a virus spreads within a population. The outbreak on the Diamond Princess last spring was one of the world’s first and best chances to see how efficiently SARS-CoV-2 transmitted in close quarters and how lethal it might be. If there’s an outbreak now on the Celebrity Millennium and case counts start rising, scientists will scramble to try to figure out how. Are the mRNA vaccines less effective at preventing transmission than we thought? Were the two people who tested positive infected with the Indian variant, which is extra contagious and vaccine-resistant?

None of that matters much for vaccinated people so long as no one (or practically no one) aboard becomes severely ill. If you’re vaxxed and the worst you have to fear on a ship is being infected asymptomatically, well, who cares? Climb aboard and enjoy your cruise. If anyone does become severely ill, though, then demand for cruises might collapse. And, paradoxically, the pressure to allow cruises to require vaccine passports will get more intense, not less. At first blush that doesn’t make sense: If vaccinated people are bringing the virus aboard, what good does requiring proof of vaccination do? But the answer is obvious. If even vaccinated people are transmitting the virus, that’s all the more reason to keep vulnerable unvaccinated people off the ship. The two people who tested positive will probably shake off their infections without any difficulty because they’ve been immunized. Someone who hasn’t might not be so lucky.

Still, this is another reminder a la the outbreak on the Yankees’ coaching staff a few weeks ago that the CDC’s initial estimate about the rate of “breakthrough infections” in vaccinated people was probably low, and possibly way low. They recorded 5,800 infections in 66 million vaxxed people in mid-April, a rate of just .008 percent or one in every 12,500 or so people. Fast-forward to yesterday and there are already two infections on a ship carrying about 600 people. The reason so few breakthrough infections are being found in the general population, I assume, isn’t because they’re not happening but because vaccinated people very, very rarely getting tested anymore unless they’re forced to — like, say, if they work for a professional sports team or they’re taking a cruise. But that’s also a sign of the vaccines’ success: Even if infections among the vaccinated are much more common than we think, they may be going unnoticed because they fail to produce any symptoms that would clue a vaccinated person into the fact that they’re infected and need to be tested.

I’ll leave you with Scott Gottlieb making the case for vaccine passports for cruise lines this morning.

"The cruise lines should be requiring vaccinations on each ship," says @ScottGottliebMD. "They should try to achieve 100% vaccination among passengers and certainly crew–to try to create a protective bubble around that experience at least for the time being." pic.twitter.com/oqIX9u11XR

— Squawk Box (@SquawkCNBC) June 11, 2021