Norwegian Cruise Line Holdings is doubling down on safety this year. Wall Street has taken the under on that bet.

The cruise line operator said this past week it is extending its guest vaccination requirement, which had been due to expire at the end of October, for all of its sailings through the end of the year. Because children under the age of 12 can’t currently get a Covid-19 vaccine, Norwegian’s policy means that these children aren’t allowed on its ships for now.

Fears that the Delta variant could hamper cruise lines’ long-awaited recovery have weighed on the sector lately. But Norwegian’s stock has been a particular underperformer this year, falling more than 7%, while shares of Carnival Corp. and Royal Caribbean Group have logged slight gains over that period.

Investors may be on the wrong side of this trade for multiple reasons. For one thing, Norwegian’s passengers are typically older, rendering the fact that young children can’t yet get vaccinations less relevant. For another, Norwegian’s brands skew more high end than those of its peers.

Luxury matters right now. Based on his conversations with large travel agencies, Truist Securities analyst Patrick Scholes said in a July note that recent bookings for 2022 were up significantly from those made two years earlier for markets that skew toward premium-price sailings like Alaska, Europe and the Mediterranean. Meanwhile, his channel checks showed bookings for more mass-market cruises, like those to the Caribbean, were down as much as 25% over the same period.

Those data points seem to suggest consumers are booking cruises for bigger, “bucket-list” trips rather than shorter weekend-like getaways, he said. If true, that is good news for Norwegian, which, along with Carnival, seems to have historically had relatively lower exposure to the Caribbean and many voyages to Alaska and Europe.

Norwegian’s mandate extension comes despite the fact that the Centers for Disease Control and Prevention is essentially requiring 95% of crew and 95% of passengers to be fully vaccinated for ships to begin sailing without first conducting test cruises. Meanwhile, Royal Caribbean said as of earlier this month it expects 90% of all its guests to be vaccinated, even while it doesn’t require it of children under 12.

“Could enough passengers prefer a ship that is 100% vaccinated to a ship that is 90-95%, that it could give [Norwegian] better pricing and volume?” UBS analyst Robin Farley asked in an August note.

The answer may vary by geography. Florida, a major U.S. cruise hub, has banned businesses from requiring proof of a Covid-19 vaccine, something Norwegian has successfully challenged for now. But Ms. Farley worries about Norwegian losing out on the family summer travel market from Florida, estimating 15% to 20% of passengers on summer Caribbean cruises are children.

Still, given the Delta variant’s continued proliferation, a 10% difference in vaccination rate could start to seem a lot more meaningful in consumers’ eyes, especially in the event of a major outbreak on any ship. It is also worth noting that many ports outside the U.S. have their own vaccination requirements, meaning that cruise lines that don’t require vaccinations of all passengers may need to offer different experiences for different passengers.

Test your investing savvy against our Heard on the Street writers to predict the best-performing stock for the rest of 2021.

According to a July survey conducted by cruise review site Cruise Critic, 55% of respondents said they would be less likely to book a cruise that offered a different passenger experience based on vaccination status, with 48% saying they would instead look for a ship where everyone was required to be vaccinated.

With many schools finally back in session, it is likely that bucket-list trips are being booked more frequently by travelers without children, anyway. Though after a year-plus working at home alongside their kids, it isn’t crazy to think parents who are planning such trips might be dreaming of their own escape.

On balance, Norwegian’s bet looks both safe and sane.

SHARE YOUR THOUGHTS

Would you go on a cruise right now? Why or why not? Join the conversation below.

Write to Laura Forman at laura.forman@wsj.com