Just as the cruise sector looked poised to rebound, Covid-19 has once again reared its ugly head.

Over the weekend, reports began circulating that ships on several cruise lines — Carnival Corp and Royal Caribbean Group among them — had suffered outbreaks of the respiratory virus on board.

According to Fortune magazine, Carnival reported a "small number" of positive tests on the Carnival Freedom, a claim called into question by passenger Ashley Peterson, who tweeted that there were hallways full of rooms with infected guests.

"The initial Covid cases on this ship started with the crew. We weren't given that information on how often the crew was tested or when they were tested and they weren't really enforcing masks unit everyone started getting Covid," Peterson said on CNN on Monday morning.

"I don't think I'll ever go on a cruise again at this point."

The magazine also reported Royal Caribbean's Odyssey of the Seas had been diverted back to Miami after 55 crew members and passengers test positive. All were said to be vaccinated.

Social media chatter further suggested outbreaks on other ships, including ones operated by Carnival and Royal Caribbean, as well as one operated by MSC Cruises.

All three companies and Norwegian Cruise Line Holdings were contacted for comment but did not immediately respond.

After limping through 2020 and 2021 thanks to the Covid-19 pandemic, cruise lines had spent the last several weeks trumpeting their strong forecasts for 2022.

Norwegian said it would be back to full capacity by April 2022, while Royal Caribbean said it expects a full-year profit for next year as it returns its core itineraries and boosts capacity.

Carnival chief executive Arnold Donald said last week that the cruise line was already operating at 90% capacity and that it logged $1.29bn in revenue for the fourth quarter of its fiscal year, which ended on 30 November.

The news boosted shares for the three US-listed cruise majors, helping Carnival, Royal Caribbean and Norwegian Cruise Line Holdings to add $2bn in market cap in a single day, even with investors selling off amid fears of the Covid-19 omicron variant.

On Monday, shares cooled off with Carnival dipping $0.25 to close at $20.95.

Meanwhile, Norwegian shares fell by $0.58 to $22.14 and Royal Caribbean dipped $1.97 to $78.46.