The US government lowered ratings on Covid-19 safety for crewmembers for 20 cruiseships destined for US waters after reducing the testing period.

The Centers for Disease Control and Prevention (CDC) since late January has given weekly statuses to 68 vessels, most of which are owned by cruise majors Carnival Corp, Royal Caribbean Group and Norwegian Cruise Line Holdings.

Each ship is rated either green, orange, yellow or red to reflect readiness to safely transport crewmembers for commercial travel in US territories after filing Covid-19 response plans and showing no cases.

Green indicates that a vessel is fully ready to take on crew, while orange and yellow reflect that a ship is not fully Covid-19 safe but still ready enough to transport them.

Red means that a ship's Covid-19 safety measures do not meet CDC requirements to safely carry crew.

Week after week, the CDC gave almost every vessel a green rating and a red status to a few at most.

That is until this week when the CDC lowered the 12 May status on 15 "green" ships to orange, one to yellow and another four to red.

The downgrades happened as a result of reducing required colour-status duration to 14 days from 28 days, based on more onboard testing, screening protocols and data reporting, the CDC said.

The three ships that received red status this week were Carnival Corp-owned: Carnival Cruise Line's 2,130-berth Carnival Elation (built 1998), 1,972-berth Noordam (built 2006) and 3,142-berth Caribbean Princess (built 2003).