What happened

Shares of cruise-line stocks are down again on Tuesday, with Carnival Corporation (NYSE:CCL) sliding 4.6% in 11 a.m. ET trading, and rivals Royal Caribbean (NYSE:RCL) and Norwegian Cruise Line Holdings (NYSE:NCLH) not far behind.

And once again, it's news regarding the omicron Covid-19 variant (also known as variant B.1.1.529) that's upsetting investors today.

Collage showing a cruise ship, a man in a face mask and a microbe.

Image source: Getty Images.

So what

As MarketWatch reports today, Moderna CEO Stéphane Bancel is warning that his company's Covid-19 vaccine may suffer "a material drop" in effectiveness at protecting patients from the new omicron variant of the SARS-CoV-2 coronavirus. Separately, Regeneron has warned that its antibody treatment to cure patients who have already contracted the disease may likewise be less effective against omicron.   

These comments, by the way, are opposite to the apparently good news that we heard yesterday -- that doctors in South Africa, at the epicenter of the omicron wave, are finding that the new variants cause only "very, very mild symptoms" in infected individuals. As such, they're sowing a new wave of doubt in cruise investors today and sparking the sell-off in Carnival Corporation stock.

Now what

And yet, is it possible that both facts are true? Could it be the case that omicron will evade immunization from vaccines developed to fight the original SARS-CoV-2, and that antibodies against that original variant won't work as well against the omicron variant? Could it also be true that that's not a disaster scenario -- because the omicron variant simply isn't as deadly a disease as was the first variant of Covid-19?

It's a rosy scenario to be sure, but it might turn out to be the truth.

For now, probably the most important thing for investors to keep in mind is that it's going to take scientists several weeks of testing and analysis to confirm precisely how well omicron evades prior immunization and how serious the symptoms are in the event a vaccine fails to prevent it.

Until those facts get nailed down, expect more turbulence in the travel and tourism sector.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.