Shares of cruise lines including Royal Carribean  were trading higher on Thursday after the Centers for Disease Control said Americans could relax and party on the high seas again as soon as this July – if they are fully vaccinated.

Cruising from U.S. ports could restart in mid-summer in American waters, the CDC said late Wednesday in a letter to the cruise industry obtained by USA Today.

“We acknowledge that cruising will never be a zero-risk activity and that the goal of the (Conditional Sailing Order)’s phased approach is to resume passenger operations in a way that mitigates the risk of COVID-19 transmission onboard cruise ships and across port communities,” Aimee Treffiletti, head of the Maritime Unit for CDC’s COVID-19 response within its Global Mitigation Task Force for COVID-19, said in the letter.

Among other things, the Conditional Sail Order requires cruise lines to establish agreements at ports where they intend to operate, implement routine testing of the crew and develop vaccination strategies.

The letter comes on the heels of a month of twice-weekly meetings with cruise industry representatives. During those meetings, the industry and the health agency discussed the Conditional Sailing Order.

72-Hour Flash Sale! Don’t miss this chance to join Jim Cramer’s Investment Club and become a smarter investor.

In a statement about the letter, spokesperson Caitlin Shockey provided USA TODAY with a more specific timeline, saying cruises could begin passenger voyages from the U.S. as soon as mid-July, depending on cruise lines’ pace and compliance with the CDC’s proposed framework.

That was enough to prompt investors to bid up shares of cruise operators, with shares of Royal Caribbean up 2.57% at $89.72 in premarket trading on Thursday, and shares of rival Carnival up 2.3% at $28.44. Norwegian rose 2.91% at $31.11.

Last week, Goldman Sachs upgraded Norwegian Cruise Line to a buy and raised the price target on the stock amid what the firm sees as a strong post-pandemic rebound in cruising demand.

Cruise lines in particular bore the brunt of bad public relations at the onset of the pandemic amid close-quarters outbreaks that highlighted COVID’s contagiousness.