The Centers for Disease Control and Prevention updated its COVID-19 guidance regarding cruise ships this week amid the U.S. ongoing no-sail order, set to expire July 24.

This week's update included a new color-coded system, plus instructions about the use of commercial transportation for crew disembarkation, according to CDC spokesman Scott Pauley.

Cruise ships are assigned a "green," "yellow" or "red" designation, related to the presence of COVID-19 on the ship. Green ships have no confirmed COVID-19 cases or similar illnesses for 28 days; yellow ships are awaiting test results; and red ships have confirmed or similar illnesses in the last 28 days. Crew transfers and repatriation via commercial means are now allowed on green ships but not on yellow or red ones.

Here's what each designation means in detail:

Green (currently unaffected by COVID-19, per the cruise line):

• No confirmed COVID-19 cases or COVID-like illness for 28 days (per a medical professional)
• If there have been ship-to-ship transfers in the past 28 days, previous ship must have no COVID-19 or similar illnesses the past 28 days prior to transfer
• Land-based crew who embarked must have quarantined for 14 days after getting on the ship
• Ships have to send an enhanced data collection form to the CDC each week, or the status turns to red

Yellow

• One or more COVID-like illnesses on the ship pending a polymerase chain reaction (PCR) test
• Status changes to red if the test comes back positive, to green if negative
• Status also changes to red if crew not tested, or if results not ready within one week of reported case
• The ship will change to a red classification, too, if this isn't the case: If there have been ship-to-ship transfers in the past 28 days, previous ship must have no COVID-19 or similar illnesses the past 28 days prior to transfer; and land-based crew who embarked must have quarantined for 14 days after getting on the ship

Red

• Either one or more cases of COVID-19 or COVID-like illnesses within past 28 days;
• Or: Ship had ship-to-ship transfers from a vessel with cases of COVID-19 or COVID-like illnesses within past 28 days;
• Or: Land-based crew embarked and weren't quarantined for 14 days;
• Or: In past 28 days ship didn't submit one or more enhanced data collection forms

Each designation also means there are certain requirements and recommendations for asymptomatic crew members aboard. For example: Crew members on green ships are recommended to stay in single-occupancy cabins with private bathrooms, while that is required on yellow and red ships. The closure of all crew bars, gyms or other group settings is not required on green ships, but it is the case on yellow and red ships. Handshaking is discouraged on all ships.

If a confirmed green ship wishes to use commercial transportation to disembark crew, it has to submit a plan to the CDC. The CDC will inform the ship if it has made "green" clearance, and will post a list of these ships on its website. Commercial travel won't be allowed if the ship status changes to yellow or red.

USA TODAY has reached out to several major cruise lines and the Cruise Lines International Association for comment. Royal Caribbean is reviewing the new guidance.

"We are studying the CDC’s latest update. We will continue to work with CDC and other authorities toward our shared goal of getting our crew safely home," Royal Caribbean spokesperson Jonathon Fishman told USA TODAY.

Cruise ship operators say they are anxious to get crews home.

The Cruise Line International Association indicated its members are doing their best to abide by CDC rules and to repatriate crews. Executives at individual companies, however, are far more direct.

"We are doing everything we possibly can for our crew, and we are frustrated as they are about the difficulty in getting them home," Frank Del Rio, CEO of Norwegian Cruise Line, told USA TODAY. "Money is literally no object. We are willing to charter flights home, wherever home may be."

The problem, he said, is "we are dealing with a regulatory landscape that seems to change daily and forces us to adjust our plans just as frequently."

When will cruise ships sail again?

Major cruise companies, including Norwegian, Royal Caribbean, Carnival and Disney, have extended their sailing suspensions just past the July 24 expiration date.

British cruise line P&O Cruises, a Carnival subsidiary, is canceling all its sailings until Oct. 15 because of the coronavirus pandemic – after a move by Canada to ban cruises and ferries carrying more than 100 people with overnight capacity in its waters through Oct. 31.

Royal Caribbean also announced it would cancel sailings to Alaska, Canada and New England through October.

"As a result of the Government of Canada’s decision to extend its restriction on cruise ships carrying more than 100 guests until Oct. 31, Royal Caribbean 2020 sailings to Canada, New England and Alaska must be canceled," Fishman confirmed to USA TODAY.

"We are working with public health agencies and governments to ensure a safe return to service, and look forward to setting sail from and to these destinations next year," he added.