CNN  — 

Royal Caribbean postponed the inaugural sailing of its cruise ship Odyssey of the Seas after eight vaccinated crew members tested positive for Covid-19, the company’s CEO said.

The positive tests come as major cruises lines are finally starting to set sail for the first time since the pandemic halted their movements.

The eight positive cases were identified during routine testing, Royal Caribbean President and CEO Michael Bayley said in a statement posted on Facebook.

“All 1,400 crew onboard Odyssey of the Seas were vaccinated on June 4th and will be considered fully vaccinated on June 18. The positive cases were identified after the vaccination was given and before they were fully effective,” Bayley said.

The company pushed back the Odyssey’s inaugural sailing from July 3 to July 31. It was set to sail from Fort Lauderdale, Florida, and make several stops in the Caribbean. A simulation cruise originally scheduled for late June will also be rescheduled.

“While disappointing, this is the right decision for the health and well-being of our crew and guests,” Bayley said.

Six of the eight crew members who tested positive did not display any symptoms, while two had mild symptoms. All eight were quarantined and are being closely monitored by the company’s medical team, Bayley said.

“To protect the remaining crew and prevent any further cases, we will have all crew quarantined for 14 days and continue our routine testing,” Bayley added.

So-called “breakthrough coronavirus cases, in which people get infected after vaccination, are expected. The cases result in milder Covid-19 illness than in unvaccinated people, a CDC study published this month shows.

Fully vaccinated people are more than 90% protected against infection, with partially vaccinated people 81% less likely to become infected than those who haven’t had been inoculated, according to the ongoing study of essential workers who got the Pfizer/BioNTech or Moderna vaccine.

Royal Caribbean’s vaccination policy requires all guests 16 years of age or older leaving from US ports to be fully vaccinated, excluding ships leaving from Florida.

Starting August 1, all guests 12 years of age or older must be fully vaccinated. Younger travelers who are not yet eligible for the vaccine may sail with a negative test result and must follow certain protocols, the company said.