The Centers for Disease Control and Prevention is loosening a major COVID-19 protocol for US cruise ships but still hasn't said when they will be able to resume sailing.

All staff and travelers aboard the ships should be vaccinated for COVID-19, but the cruise companies don't need to mandate vaccines to resume travel safely, the CDC said Monday.

Masks will be worn on board by staff and passengers, as well as at "all US transportation hubs, including seaports and ferry terminals", the agency said.

There was no immediate reaction from the Cruise Lines International Association, the world's largest cruise industry trade association, but "with no timing component, and only incremental in nature, this update disappointed the industry", said Brandt Montour of JPMorgan Chase in in a research note.

The CDC guidance came on the same day that Norwegian Cruise Lines asked the agency to let it sail from US ports starting July 4, saying its vaccination requirement for passengers and crew matches the CDC guidance that international travel is safe for fully vaccinated individuals.

Norwegian Cruise Line Holdings CEO Frank Del Rio, in a letter to CDC Director Rochelle Walensky on Monday, said the company would start cruises at 60 percent capacity in July and then increase by 20 percent capacity every 30 days.

He also said the company would comply with protocols developed by a team of medical experts hired by the company last year, including antigen tests for passengers prior to boarding, mask-wearing and enhanced ship-board medical teams.

Norwegian and other major cruise lines haven't sailed in the US since coronavirus outbreaks on ships brought voyages to a halt last year.

Cruise ships were some of the first super-spreader sites for the coronavirus in 2020 and have been docked since.

Florida's Republican Governor Ron DeSantis has threatened to sue the federal government if the CDC doesn't allow cruises to restart by the summer.

On Friday, the CDC issued its latest update on protocol for cruise lines to sail. The new recommendations include increasing from weekly to daily the reporting of coronavirus cases, implementing routine testing of all crew based on a ship's COVID-19 status and making contractual arrangements with medical facilities on shore for passengers who may fall ill during a voyage.

On Monday, Norwegian said the vaccination requirement issued last week by the CDC eliminates the need for the agency's conditional-sailing order, which requires cruise operators to conduct mock sailings and apply for a certificate at least 60 days before offering passenger cruises.

Once cruise lines have prepared their ships, they must give 30 days' notice to the CDC before starting test cruises and will have to apply for a conditional sailing certificate 60 days before a planned regular voyage.

Cruise ships in the US have been docked for more than a year because of the pandemic and can restart operations only by following the CDC's Framework for Conditional Sailing Order, issued in October to ensure that cruise ships build the onboard infrastructure needed to mitigate the risks of the coronavirus.

Norwegian and other major cruise lines haven't sailed in the US since coronavirus outbreaks on ships year. Norwegian said the vaccination requirement issued last week by the CDC eliminates the need for the agency's conditional-sailing order, which requires cruise operators to conduct mock sailings and apply for a certificate at least 60 days before offering passenger cruises.