What happened 

Shares of Carnival (CCL -0.06%) were up 21.7% in the first half of 2021, according to S&P Global Market Intelligence. The company benefited from reopening optimism as vaccines against COVID-19 have been administered worldwide. 

However, Carnival has lost most of that momentum so far in July, and the stock is now down 3.5% year to date.

A digital stock ticker board with the word comeback written in.

Image source: Getty Images.

So what 

Coronavirus vaccines have allowed an easing of mobility restrictions worldwide, and Carnival is starting to plan to resume sailings. 

It announced that 42 ships, or over 50% of its capacity, are scheduled to resume sailing by Nov. 30, great news for the cruise ship operator, which was devastated during the pandemic.

Customers are responding enthusiastically. Bookings for its ships in 2022 are ahead of 2019 levels, with a lot of pent-up demand after limited travel options for many months.

Now what 

Recently, however, the momentum for recovery has slowed down because of an announcement from Gov. Ron DeSantis of Florida denying cruise ship operators the ability to require all passengers to show proof of vaccination. The ramifications of such a decision could be that guests will have to pay for COVID-19 travel insurance if they want to board ships leaving Florida.

That's bad news for the stock because the added costs could discourage travelers from booking a trip in the first place -- a troublesome sign for a company trying to get itself on a firm footing. 

But Carnival plans to leave from ports in Los Angeles and San Francisco later in the year, apparently without any challenges to its required proof of vaccination for all guests departing from California ports.

Overall, with vaccination rates slowing down in the U.S. and the surge in cases caused by the Delta variant, it could be a little while longer until it's smooth sailing for Carnival stock.