(CNN) — A "No Sail Order" for cruises has been extended until July 24 by the Centers for Disease Control and Prevention due to cruises' unique aspects that can cause coronavirus outbreaks.
The CDC website says it has "reason to believe that cruise ship travel may continue to introduce, transmit, or spread Covid-19."
"We have been learning a lot over the last four months about this virus and the pandemic, about the unique aspects and some of the situations that can cause very large outbreaks," says Dr. Martin Cetron, CDC Director for Global Migration and Quarantine.
Dr. Cetron spoke with CNN about how recent coronavirus outbreaks on the Diamond Princess and the Grand Princess helped the CDC understand how stealthily the virus can spread in a cruise ship environment, and what it will take to safely sail again.
Asymptomatic cases pose serious concern
Cruise ships pose risks because they are closed space environments with three distinct demographics prone to illness -- elderly guests whose average age is in the 70s, people with underlying medical conditions, and the ship's crew.
The Diamond Princess was the first cruise ship with known coronavirus patients on board, and the virus spread very easily among the guests. Nearly half of the cruise ship's passengers and crew who tested positive for the novel coronavirus were asymptomatic at the time they were tested, according to a report from the US Centers for Disease Control and Prevention.
That combination of asymptomatic or mildly symptomatic cases "can lead to this explosive and relatively unrecognized outbreak," says Dr. Cetron.
The Grand Princess off the California coast in March.
While the ship's crew is generally a younger, healthier population, they are also susceptible to contracting the virus because of the joint living activities on a vessel over several weeks and because their living quarters aren't as spaced out, limiting their ability to social distance. That situation was seen on the Grand Princess cruise ship, which was quarantined off the coast of California.
"Unlike the passengers, they stay on board from one sailing to the next ... so they can serve as a reservoir: both the asymptomatic, and the mildly infected who don't recognize that they have any serious concern," says Dr. Cetron.
By staying on board and sailing week to week, the crew also poses an ongoing risk of introduction, amplification and dissemination beyond the cruise ship environment.
Creating safe sailing during a pandemic is 'a tall order'
The CDC usually posts travel health notices for countries and other international destinations, but this time it's different.
"These are the first times we listed conveyances as part of our travel health notices advising people not to travel. Normally that's a geographically based thing," says Dr. Cetron.
Part of the "No Sail Order" requires a ship to submit comprehensive plans to prevent, detect, respond and contain Covid-19 infections.
"The lines now have to develop a comprehensive plan for resuming operations and sailing in a safe manner that takes into account the unique high risk setting of cruise ships in the era of this Covid pandemic," he says.
He added that the CDC will continue to work with the cruise lines to make sure they understand the nature of the risk and ensure all necessary public health procedures are in place.
"It's a tall order to try to make a cruise ship environment safe for sailing in an era of this virus continuing to circulate on the globe," he added.
The CDC hasn't yet determined when it will be safe to resume sailing, but its chief responsibility is to protect public health, says Dr. Cetron.
"This order will expire in 100 days, but there's nothing that stops CDC from providing a continuation or a new order or adjusting the orders based on the threat picture at the time," he added.
The CDC has not developed specific guidance for cruise travelers yet, according to Dr. Cetron. Its current focus is on ensuring cruise lines are developing and implementing plans to prevent, detect, respond to, and contain Covid-19 onboard and ensuring cruise lines have the necessary public health procedures in place when sailing does resume.
The CDC has also made sure that during this period of suspended operations, crew members still stuck in cruise ships know there's a process in place for them to safely disembark and return home.
When will it be safe to sail
Dr. Cetron says Covid-19 is "an insidious virus (that) transmits in stealth mode by people who don't even know they have it."
Restarting the cruise business will require thinking about how to reduce density, similar to the reopening of businesses on land. And social distancing will be part of that plan.
"Safe guidelines for how to maintain social distancing and gradual resumption of activities recognizing that we're still early in this global pandemic," says Dr. Cetron.
"There's much more to come as this virus continues to circulate before we have a vaccine. We have to think about how we alter all of our environments in order to create safer spaces."
The CDC is monitoring the situation closely but doesn't have enough information about when it will be safe for cruise ships to resume sailing, he said.
"We have not advised cruise lines about the specific time of when they can resume cruise travel for passengers, it is premature to do that," he added.