Cruise lines that sail in the United States will soon be allowed to decide if they want to follow pandemic-era guidelines from the Centers for Disease Control and Prevention.

The “conditional sailing order,” a mandatory set of rules that cruise companies have had to follow since 2020, expires on Saturday. After that, the agency will transition to a “voluntary COVID-19 risk mitigation program” for ships that are registered in foreign countries and operate in U.S. waters.

These rules have included requiring vaccination for a certain percentage of passengers and crew; tests before boarding for passengers and regular testing for crew; and mask-wearing indoors unless eating or drinking.

The shift to a voluntary program comes after the CDC raised its health notice level for cruise ships, warning all travelers to avoid cruising as the omicron variant sent case numbers skyrocketing. According to the agency, cruise ships reported 14,803 coronavirus cases between Dec. 30 and Jan. 12. That’s 95 times the number of cases reported — 155 — between Dec. 1 and 14.

Even operators that don’t opt to follow the recommendations will be under the CDC’s authority in important ways, said Aimee Treffiletti, a captain in the U.S. Public Health Service and lead for the agency’s maritime unit. They will have to report coronavirus cases and be subject to the public transportation mask mandate that has been extended into March. The CDC will be able to conduct investigations into outbreaks and issue a no-sail order for specific ships.

Despite the recent spike in cases on cruises, the CDC is still not seeing severe outcomes on ships — which Treffiletti attributed to the coronavirus mitigation measures that cruise lines are following.

“Things like hospitalization, ventilator use, medical evacuations and deaths are not a major issue on board,” she said.

The CDC originally announced in late October that the cruise rules would become voluntary, before the omicron variant emerged.

Under the new voluntary program, the agency will continue to recommend the same measures that have been in place. Cruise lines have until Jan. 21 to opt in; those that participate will need to follow all of the recommendations, not just some of them.

The CDC said its maritime unit will work closely with ships that take part in the voluntary program, monitoring their coronavirus measures and cases. The ships that opt in will continue to appear on the CDC’s chart of foreign-flagged ships sailing in U.S. waters. Those vessels are assigned a color based on whether coronavirus cases have been reported in the past seven days.

“It’s really about transparency with the cruising public,” Treffiletti said. “They’ll be able to say that they’re in constant communication with the CDC, they’re reporting covid cases as well as other respiratory disease to the CDC on a daily basis, they’re following all the public health protocols and mitigation strategies to control the spread.”

Those that don’t participate will be marked as gray, meaning the CDC has not reviewed nor confirmed their health and safety procedures. Treffiletti said passengers should know that “there will be many unknowns” about what mitigation measures and protocols are being followed on those ships.

The Cruise Lines International Association pointed out that cruise companies operating in Florida have already been voluntarily following the conditional sailing order since this past summer, when a judge ruled it should be considered a nonbinding recommendation.

In a statement, the industry group said the CDC’s move to a voluntary program “recognizes the cruise industry’s unwavering commitment to providing some of the highest levels of COVID-19 mitigation found in any industry.”

“CLIA ocean-going cruise line members will continue to be guided by the science and the principle of putting people first, with proven measures that are adapted as conditions warrant to protect the health of cruise passengers, crewmembers, and destinations,” the statement said.

The shift away from mandatory requirements drew criticism from some in Congress.

Rep. Doris Matsui (D-Calif.) and Sen. Richard Blumenthal (D-Conn.) criticized the CDC’s decision to switch to a voluntary program on Thursday, urging the agency to extend the order and enforce it strictly, and calling on the industry to improve transparency and conditions for passengers who test positive on cruises.

“While the world battles the highest surge in COVID-19 cases to date, prioritizing and strongly enforcing measures that maximize the safety of all those on board cruise ships is critical,” they wrote in a letter to CDC Director Rochelle Walensky. “Prematurely transitioning to a voluntary program could allow companies to skirt necessary public health measures.”