Norwegian Cruise Line Holdings will continue to follow the Centers for Disease Control & Prevention (CDC) conditional sailing order, although the US agency will deem such adherence voluntary as of Saturday.

The New York-listed owner of 28 ships announced on Friday that Norwegian Cruise Line, Oceania Cruises and Regent Seven Seas Cruises will uphold what the CDC now calls the "Covid-19 Program for Cruise Ships Operating in US Waters".

Chief executive Frank Del Rio said Miami-based Norwegian has shown full commitment to the conditional sailing order, which was extended by almost three months in late October, by requiring vaccinations for all passengers and crew since July.

“Our adoption of the voluntary programme provides guests, crew, travel partners and other stakeholders with the assurance that our brands will continue to meet and exceed the provisions laid out by the CDC and provide unparalleled health and safety protocols not found in any other sector of the travel and leisure space," he said.

Norwegian told the CDC on Friday that its three brands would follow the voluntary programme as part of its SailSafe health and safety programme.

Peer cruise majors Carnival Corp and Royal Caribbean Group did not immediately return calls seeking comment.

The Federal Maritime Commission praised the CDC's decision to make following the conditional sailing order voluntary when it released its two-year report on Covid's impact on the cruise sector on Friday.

"I commend the decision to allow the cruise lines to operate unimpeded so long as they employ best practices that are reasonable and customary for the hospitality and transportation industry," commissioner Louis Sola wrote in the 10-page report.

"The cruise lines continue to adapt to the circumstances presented by new variations of Covid-19 and modify their actions as needed to mitigate harm."