Human-to-human transmission is not common, and the UN health agency reiterated that the risk to the wider public was low from a disease typically spread from contact with infected rodents.
A Dutch couple and a German national have died, while a British national was evacuated from the ship and is in intensive care in South Africa, officials said.
Three more people with suspected cases are still on board the Dutch-flagged MV Hondius.
The cruise ship hit by the deadly outbreak is marooned off Cape Verde, an island nation in the Atlantic off West Africa, which has not allowed the vessel to put passengers ashore.
The WHO said it had been told there were no rats on board.
People are usually infected by hantavirus through contact with infected rodents or their urine, their droppings or their saliva.
However, a limited spread among close contacts has been observed in some previous outbreaks with the Andes strain, which spreads in South America, including Argentina, and which the WHO believes could be involved in this instance.
Testing is under way.
The Hondius left Ushuaia in southern Argentina in March.
“We do believe that there may be some human-to-human transmission that’s happening among the really close contacts, the husband and wife, people who have shared cabins,” Maria Van Kerkhove, the director of epidemic and pandemic preparedness and prevention at the WHO, told reporters in Geneva.
“Some people on the ship were couples, they were sharing rooms so that’s quite intimate contact,” Van Kerkhove said.
The UN health body said its working assumption was that in the initial cases of the Dutch couple, who joined the ship in Argentina after travelling in the country, they were infected before joining the cruise.
Other cases may also have been infected whilst on bird-watching trips to islands where birds and rodents live as part of the cruise, it said.
Van Kerkhove said the focus now was to evacuate the two sick passengers still onboard and then for the ship to continue to the Canary Islands.
The third suspected case still on board only reported a mild fever at some point.
“We have heard from quite a few people on the boat,” she said.
“We just want you to know we are working with the ship’s operators. We are working with the countries where you are from. We hear you, we know that you are scared,” she said, adding they were working hard to get people home safely.
But while the WHO said the plan was for the ship to head to the Canary Islands, Spain’s health ministry said it had made no decision yet on receiving it and that this would depend on data collected from the ship. (Reuters)
