What will cruising look like? Industry offers clues, but there’s still no timeline.

Cruise companies are stepping up their pressure on U.S. health authorities to allow them to cruise again amid the COVID-19 pandemic.

On Monday, Royal Caribbean Group and Norwegian Cruise Line Holdings submitted health and safety protocols to the U.S. Centers for Disease Control and Prevention as part of the agency’s request for public comments about how to safely resume cruises. The protocols include testing all passengers and crew for COVID-19 before boarding, requiring masks and expanding medical capabilities on cruise ships.

The cruise industry’s lobbying group, Cruise Lines International Association, sent a separate, less-specific list of recommendations to the CDC on Monday, vowing that all cruise companies will require testing before boarding, masks and social distancing; improve ventilation and medical capabilities; and limit shore excursions. Monday was the final day to submit public comments to the CDC about cruising during the COVID-19 pandemic.

The CDC has banned cruises in the U.S. until Oct. 1, and most cruise lines have said they will not resume cruises until at least Oct. 31.

“We’ve learned a lot in six months,” said Royal Caribbean Group Chairman Richard Fain. “Our job was to find a way going forward to learn from the past, and not to repeat it.”

Norwegian Cruise Line Holdings CEO Frank Del Rio said it was “absolutely” safe to resume cruises again, citing the announced protocols.

“There is not one silver bullet,” he said. “It’s layer on top of layer on top of layer. .. .We’re going to test it, make adjustments along the way.”

A spokesperson for the CDC said the agency has not requested plans to resume passenger cruises from the companies.

“Currently, CDC does not have enough information to say when it will be safe for cruise ships to resume passenger operations,” the spokesperson said in an email. “CDC will continue to work with cruise lines to ensure that all necessary public health procedures are in place before cruise lines begin sailing with passengers.”

Carnival Corporation CEO Arnold Donald said it typically takes 30 days to get a laid-up ship running again.

The 74 recommended protocols submitted to the CDC on Monday by Royal Caribbean Group and Norwegian Cruise Line Holdings were hashed out by a panel of experts, including several who used to work for the CDC. The panel said it is impossible to eliminate the risk of COVID-19 spread at sea, but with their recommendations, that risk can be minimized. The panel did not determine an infection threshold on land that would make it safe to resume cruises but instead focused on what companies can do to bolster health and safety on ships.

“Getting down to zero risk is not likely,” said former Utah Gov. Mike Leavitt, who chaired the Royal Caribbean Group and Norwegian Cruise Line Holdings panel. “Therefore we have to have the component of mitigating spread and ability to respond.”

Some of the recommendations include:

• All passengers should be tested for COVID-19 between five days and 24 hours prior to boarding

• Crew should be tested in their home countries before leaving to join a ship and then again at the end of a seven-day on board quarantine period, ideally using PCR tests.

• Both passengers and crew should have their temperatures taken daily.

• Cruise companies should visit only ports that agree to evacuate and repatriate sick people on board.

• All ship heating and air-conditioning systems should be upgraded to MERV 13 filters, similar to those used by hospitals.

• Ships should lower doctor-to-passenger ratios on board.

• Crew should live in single cabins whenever possible and be allowed limited shore leave.

• Passengers who don’t attest that they agree to protocols not be allowed to cruise.

A protocol that was floated in March as cruise companies scrambled to try to avert a shutdown was barring passengers over 70 years old, who are particularly vulnerable to the effects of COVID-19. That was scrapped by the panel. Passengers who are at higher risk will instead be advised to consult with their doctors before cruising.

Carnival Corporation, the largest cruise company in the world, endorsed the CLIA proposals, which include testing – already being conducted on its Costa Cruises ships in Italy.

“This has been probably the most difficult period in our industry’s 50 year history,” said CEO Donald. “We are on a path with the industry to resume cruise operations in the U.S. using the knowledge from our advisors and in full cooperation with the authorities.”

Preventing COVID-19 spread on cruise ships is exceptionally difficult, health experts say. At least 110 passengers and crew members have died from COVID-19, at least 38 in Florida, according to a Miami Herald investigation, and at least 86 ships have been affected — approximately one-third of the global cruise fleet.

Cruise companies have struggled to contain COVID-19 outbreaks among crew members on their ships during the industry’s pause. On several occasions, even after months of isolation at sea, crew members tested positive upon returning to their home countries. CDC data obtained by the Miami Herald via a Freedom of Information Act request shows at least seven ships in U.S. waters during the month of August reported COVID-19 or COVID-like illnesses to the CDC that month. Carnival Corporation and Virgin Voyages pulled their ships out of U.S. waters in June and are no longer reporting illnesses to the agency. Norwegian Cruise Line Holdings is reporting from just three of its ships.