June 11 (Reuters) - Shares in major U.S. cruise operators fell in premarket trading on Friday, as Royal Caribbean Group said late Thursday that two guests onboard its Celebrity Millennium ship had tested positive for COVID-19. Royal Caribbean, Carnival Corp and Norwegian Cruise Line Holdings Ltd all fell between 1% and 2%, after speedy vaccinations and a promise of a return to normalcy had allowed them each to gain about 21% to 38% this year. Celebrity Millennium was one of the first ships in North America to restart sailing last week following announcements from all the three cruise operators to restart trips from U.S. ports this summer. Cruise expert Stewart Chiron, who is on board the Celebrity Millennium ship, told Reuters life onboard on Thursday night went on as planned, with dinners, shows, evening entertainment and that no one on board was wearing masks. (Reporting by Praveen Paramasivam in Bengaluru; Editing by Maju Samuel) Our Standards: The Thomson Reuters Trust Principles.