When are cruise lines still requiring COVID testing? Here's what passengers should know.

After the Centers for Disease Control and Prevention ended its COVID-19 cruise ship program last month, many cruise lines changed their health and safety protocols and dropped some testing requirements.

Major brands like Royal Caribbean International and Carnival Cruise Line rolled back their COVID-19 rules and removed testing for vaccinated passengers on some voyages, while other lines scrapped them for all sailings.

Tests are still required in a number of cases, though. Policies may vary depending on local requirements at various destinations. Here's what passengers need to know about cruise protocols, from testing to face masks and social distancing.

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When do cruise lines still require COVID-19 testing?

► Carnival Cruise Line will require vaccinated passengers to test only on trips 16 or more nights or where required by local rules, including on voyages to Canada and Bermuda, beginning Sept. 6, according to a press release.

Unvaccinated travelers or those who do not show proof of vaccination will be required to "present the results of a negative PCR or antigen test taken within three days of embarkation." In the U.S., guests under 5 are exempt from vaccine and testing rules.

►Royal Caribbean will require vaccinated guests 5 and older to test on U.S. cruises 10 nights or longer beginning Sept. 5, according to its website. They must test within three days of boarding. Unvaccinated guests age 5 and up need to take a test within three days of sailing on all trips. The changes are subject to local regulations.

►Celebrity Cruises will require vaccinated passengers to test only on voyages that are 10 or more nights where local rules permit starting Sept 5., according to its website. They must do so within three days of sailing. Unvaccinated guests 5 and up must take a test within three days prior to their trip for U.S. sailings.

►Norwegian Cruise Line Holdings Ltd. will drop all pre-cruise requirements for vaccinated passengers age 12 and over as of Sept. 3, according to a news release.

Guests age 12 and older who are unvaccinated or do not show proof of vaccination will still need to test within 72 hours before boarding. The company said it "continues to strongly recommend all guests be up to date on vaccination protocols and test at their convenience prior to travel." Changes are also subject to local requirements.

► MSC Cruises will no longer require fully vaccinated U.S. residents to take a test before boarding on sailings from U.S. ports to the Bahamas and the Caribbean as of Sept. 1. Unvaccinated passengers age 2 and older still need to show proof of a proof of a "lab-administered negative COVID-19 viral test negative test" taken within three days of their cruise, according to its website.

Passengers 2 and older who are not U.S. residents must "show proof of a negative antigen or RT-PCR lab administered test" taken within three days of their sailing, regardless of vaccination status, according to its website. The line's policies vary by destination.

►Princess Cruises will require travelers on voyages 16 nights or longer to "take a supervised test within three days of embarkation (guests 5 and older)," as of Sept. 6, according to a news release. That rule also applies to guests on full Panama Canal transits and certain other voyages.

Passengers who are unvaccinated need to show a negative result from a self-test taken within three days of embarkation on sailings 15 nights or less. Unvaccinated children under 5 are exempt. The changes are subject to local regulations.

►Holland America Line will not require testing for vaccinated guests on sailings 15 nights or shorter starting Sept. 6, according to a news release. Unvaccinated passengers age 5 and up must show a negative result from a medically supervised test or self-test taken within three days of embarkation.

Those changes are subject to local restrictions, and exclude full Panama Canal transits and certain other sailings.

All guests 5 and up must provide "a medically supervised COVID-19 test with written negative result" on cruises 16 nights or more. Travelers need to test within three days of embarkation, and unvaccinated travelers must still request an exemption.

►Disney Cruise Line will no longer require fully vaccinated guests traveling on the Disney Dream, Disney Fantasy, Disney Wonder and Disney Wish to take a test before their cruise beginning Sept. 23, according to its website. For sailings on the Disney Magic, the change will take effect Nov. 7.

Travelers who are not fully vaccinated must continue to show proof of a negative test test taken one to three days before their trip, though they will no longer need to take an additional test at the terminal on embarkation day.

What kind of COVID-19 tests do cruise lines require?

Cruise lines still set criteria for the tests they accept. Carnival said it will accept self-administered tests as well as lab-administered tests from unvaccinated guests on voyages that are 15 nights or less beginning Sept. 6, according to a news release.

Royal Caribbean will also allow passengers on U.S. cruises to take a home test or one "professionally administered by a health professional" starting Sept. 5 according to its website. Celebrity will accept self-tests beginning on the same date, as well.

Those changes are subject to local requirements.

Norwegian Cruise Line, meanwhile, will require "proof of a medically supervised negative PCR or Antigen test" for unvaccinated passengers 12 and up, according to its website. Travelers should check with their cruise line for their specific rules.

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What kind of on-board testing requirements do cruise lines have?

Cruise lines still have on-board protocols in place, as well. For its part, Carnival requires passengers who are exposed to or in close contact with anyone who tests positive for COVID or displays symptoms to be tested, along with their close contacts, according to its website. Those passengers may have to quarantine in their rooms until medical staff clears them.

Other cruise lines have similar protocols in place, like Norwegian, which advises passengers who have COVID symptoms while sailing to contact the ship's medical center, which is equipped to test on board, per its website. Royal Caribbean Group's namesake brand, Royal Caribbean International, also offers testing to travelers who feel sick, and its medical lab "allows for rapid, accurate onsite PCR testing with results in under an hour, alongside a multitude of other evaluative tests," its website reads.

What other protocols do cruise lines have in place?

► Masks: Carnival highly recommends guests wear masks in public indoor areas except when eating or drinking, and "when in large congregate events outdoors where physical distancing cannot be maintained," per its website. The cruise line also recommends masking during embarkation and debarkation and requires masks in the medical center. Crew members also wear masks indoors at all times.

On Royal Caribbean International, masks are optional for passengers who are vaccinated. They are recommended for unvaccinated kids. MSC has a similar policy. On Princess sailings, masks are recommended but not mandatory in most areas of the ship, though the cruise line notes on its website that "you may need to wear them in select venues or certain situations." Masks are optional on all Norwegian voyages.

►Cleaning: Many cruise lines continue to emphasize heightened cleaning on board. Norwegian, for example, has "enhanced cleaning and sanitation protocols" in place with EPA-approved disinfectants, and is making use of medical-grade air filters, according to its website. Royal Caribbean similarly boosted its cleaning regimen, including cleaning frequently used places like elevators and stairways every two hours. Crew must also take continual training classes and "refreshers," per its website.

MSC is also utilizing "hospital-grade disinfectants, electrostatic sprayers, increased cleaning frequency and a focus on high-traffic, frequently-touched areas," according to its website.

►Physical distancing: Cruise lines may recommend social distancing during parts of trips. Royal Caribbean International recommends distancing in select parts of the ship indicated by signage.

Cruise lines may also change their policies as needed.

"Our member lines continuously monitor health conditions onboard and employ health and safety protocols commensurate with those conditions," Anne Madison, a spokesperson for Cruise Lines International Association, told USA TODAY in an email. "This may result in cruise lines adjusting protocols when appropriate in consultation with medical and other health experts in order to continue to make science-driven and medically informed decisions."

Passengers can find more information on ship protocols on their cruise line's website.

This article originally appeared on USA TODAY: When cruise lines are still requiring testing