CDC monitoring 94 cruise ships for COVID. Here's how to check whether your ship is on the list.

The Centers for Disease Control and Prevention is monitoring or investigating 94 ships for COVID-19 as of Monday, as it warns Americans to "avoid cruise travel" regardless of vaccination status. 

There were more than 5,000 COVID cases on board cruise ships sailing in U.S. waters the last two weeks of December, according to the CDC, but the fact that the public health agency is monitoring or investigating a specific ship does not mean there is a widespread COVID-19 outbreak on board. Even with stringent vaccination, testing and masking, among other protocols, it is fairly common for coronavirus cases to emerge among passengers and crew on cruise vessels. 

While the CDC said the rise in cases since the identification of the omicron variant prompted it to issue a level 4 warning about cruising on Thursday, the CDC long anticipated cases on board ships, in part because of the very nature of cruising.

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David Daigle, a spokesperson for the CDC, told USA TODAY last Tuesday that the health agency acknowledges it is "not possible" for cruising to be a zero-risk activity amid the pandemic. A person's chance at contracting coronavirus is higher on cruise ships because the virus spreads more easily between people spending time in close quarters on the vessels.

"(The) CDC has been managing COVID-19 outbreaks on cruise ships since this summer when cruise ships started sailing with passengers," Daigle continued, noting that the agency is working with ships to keep passengers and crew safe on board with mitigation measures in place. 

The MS Hamburg is seen docked in Buenos Aires on Nov. 29, 2021. Argentina ordered all passengers on board to isolate following the detection of a coronavirus case, according to the Ministry of Health.

Understanding that the CDC has been dealing with cases since cruises resumed over the summer, what does it mean when it marks a cruise ship as under investigation or as being monitored on its running list? 

What it means for a ship to be monitored or under 'investigation' by CDC

Just one case of coronavirus or COVID-19-like symptoms could prompt the CDC to monitor or investigate a ship, which is why it's important to understand how the CDC determines ship status and where to find the latest ship-specific information. 

The CDC's "Cruise Ship Color Status" chart is kept available to the public on its website and is updated regularly. Ships are broken into five color-coded classifications: Green, Orange, Yellow, Red and Gray.

"Ship color status is determined using surveillance data from the previous 7 days – regardless of voyage dates – and CDC investigation findings," the CDC says on its website.

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Green status means there have been no reported cases of coronavirus or COVID-19-like illnesses on board. As of Monday morning, there were 16 ships with green status, and some of those ships are on crew-only voyages. 

Orange status means the ship has reported cases among passengers and is being monitored but levels don't meet the threshold for CDC investigation. There were only three ships with orange status as of Monday morning.

On restricted voyages, or cruises carrying paying passengers, just one case among crew merits a yellow status, prompting a CDC investigation.

Yellow means reported cases meet the CDC's threshold for investigation. The vast majority of ships are currently categorized as yellow.

The threshold is higher for passengers on restricted voyages; the number of passengers with COVID-19 or COVID-19-like illnesses over the preceding seven days must equal or surpass 0.10% of the total number of passengers on board to merit investigation. So if a ship has 5,000 passengers, it would need at least five cases over seven days to be classified as yellow.

That percentage includes cases occurring within passengers in the five-day period after disembarkation that the health agency is notified about by state or local health departments, the CDC said.

Red status means there is "sustained transmission" of coronavirus or COVID-19-like illnesses on board or there is a potential for cases to "overwhelm" the ship's medical center. There were no ships with red status Wednesday.

Gray status means the ship's health and safety protocols have not been reviewed or confirmed by the CDC. There were no ships marked as gray Wednesday.

Every major cruise line has detailed procedures for handling coronavirus cases on board and protecting fellow passengers and crew.

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Ship statuses are posted with 'lag' to CDC site

While the CDC's chart is a useful resource while observing the cruise industry and COVID-19 and for travelers to decide whether to go on their cruise or to cancel, there is a caveat – the site is not perfectly aligned to what is happening on a ship at any given time.

"It is important to know that there is at least a two-day lag in the posting of an updated color status," the CDC's Daigle told USA TODAY.

Daigle laid out the progression of how cruise ship color codes are updated by the CDC. It's a three-day process.

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On day one, cases are detected or identified on the ship. On day two, cases are reported to the CDC, data is pulled from REDCap and the color status is assessed by the agency. On day three, the color status is updated on the agency's public website. 

And the lag may be more than two days at times. Posting doesn't happen on weekends or during federal holidays. As a result, "there may be a longer lag between case detection and posting," Daigle said.