With how quickly things have been changing with real world data, I'm not sure the CDC's old guidelines are worth much.
The June cruise is bookable now,
https://www.celebritycruises.com/itinerary-search?dates=2021-06&ship=EG
I was waiting for that to see what their COVID protocols were:
For U.S. cruises and guests: Prior to booking, please consult all applicable U.S. Centers for Disease Control travel advisories, warnings, or recommendations relating to cruise travel, available at http://www.cdc.gov/travel/notices. If a certain threshold level of COVID-19 is detected onboard your cruise ship during your voyage, your voyage will be ended immediately and the ship will return to the port of embarkation, and your subsequent travel, including your return home, may be restricted or delayed.
For cruises and guests worldwide: Health and safety protocols, guest conduct rules, as well as regional travel restrictions and clearance to visit ports of call, are subject to change without notice based on ongoing evaluation, public health standards, and government requirements. Onboard and destination experiences, features, itineraries and guest conduct rules vary by ship and destination and are subject to change without notice.
Elsewhere on their site:
Announcing Our Return to Sailing
We’ve been hard at work developing new health and safety protocols to keep you Healthy at Sea.
We’ve partnered with medical and scientific advisors from our Healthy Sail Panel, our Global Head of Public Health and Chief Medical Officer; and local health and government authorities to guide us in the development of our protocols. These measures include reduced capacities and enhanced cleaning and sanitization.
All guests ages 16 years and older must be fully vaccinated with all Covid-19 vaccine doses administered at least 14 days prior to sailing. As of August 1, 2021, all US Guests age 12 and older must be fully vaccinated. For UK residents, all guests 18 and over must be fully vaccinated with all Covid-19 vaccine doses administered at least 14 days prior to sailing. Crew members onboard will be vaccinated.
As we return to sailing, your health and safety remains our top priority.
You can find the latest CDC travel advisory on cruising here.
Vaccines
All guests ages 16 years and older must be fully vaccinated with all Covid-19 vaccine doses administered at least 14 days prior to sailing. As of August 1, 2021, all US Guests age 12 and older must be fully vaccinated. For UK residents, all guests 18 and over must be fully vaccinated with all Covid-19 vaccine doses administered at least 14 days prior to sailing. Crew members onboard will be vaccinated.
Testing & Screening
Safe vacations start with prevention. Prior to boarding, all guests are required to complete a health questionnaire and guests ages 2 –15 will be required to take a complimentary antigen test at the terminal. In some homeport countries, testing may be required for all guests regardless of vaccination status. An additional test will be administered onboard, free of charge, if required for home country re-entry.
What health screening measures are being taken at the terminal on boarding day?
At the terminal, we'll continue screening measures to ensure everyone boarding the cruise, or working at the terminal, is in good health. All guest documents, including proof of full vaccination administered two weeks prior to sailing, and a negative SARS-CoV-2 test result must be presented. Prior to embarkation, check https://www.celebritycruises.com/health-and-safety and government sites for specifics as the requirements vary by port. Additionally, all guests, crew, visitors and port personnel will be required to complete a Health Questionnaire and have their temperatures taken in the terminal by trained staff using touchless, handheld thermometers. Guests and crew members with temperatures of 100.4 degrees Fahrenheit or greater will undergo a secondary screening. The secondary screening is conducted by a third-party medical professional and determines if a person is eligible to sail.
Do I need a COVID-19 vaccine to cruise? What documents count as proof of vaccination? How far in advance do I need to get my vaccine?
At this time, we are requiring guests 18 years and older to be fully vaccinated for COVID-19. Each guest must submit proof of full vaccination no later than boarding day, in the form of the original vaccination record document issued by either (1) the country’s health authority that administered the vaccination (e.g., U.S. CDC's Vaccination Record Card) or (2) the guest's medical provider that administered the vaccination. Electronic vaccination records will only be accepted for residents of those countries where electronic documentation is the standard issued form (e.g., a unique QR code). The vaccination record submitted to Celebrity Cruises must show that the guest is fully vaccinated. This means that the guest has completed the full cycle of required doses for the vaccine administered (e.g., received the second dose in a two-dose series), and that the guest has received the final dose at least 14 days before sailing.