Paul Nuki Global Health Security Editor. Maeve Cullinan Global Health Security Reporter

The Andes strain of hantavirus behind the outbreak on a cruise ship is susceptible to “super-spreader” events, according to research backed by the US military.

Three people have died on the Dutch vessel MV Hondius and three others are sick with suspected hantavirus infection.

More than 140 people are stuck in quarantine and cannot disembark the ship because the rodent-borne virus is thought to be caused by a strain which can spread between humans.

The US Army Medical Research Institute of Infectious Diseases – historically known as the centre of the US biological weapons programme – now focuses on biosecurity and, in 2020, backed groundbreaking research that showed the Andes strain of hantavirus can spread rapidly between people.

The researchers, based at Fort Detrick, Maryland, also found that symptomatic individuals were capable of causing “super-spreader” events given the right social circumstances, such as those found on cruise ships.

“After a single introduction [of Andes hantavirus] from a rodent reservoir into the human population, transmission was driven by three symptomatic persons who attended crowded social events,” the researchers found in a 2018-2019 outbreak in Chubut Province, Argentina, which resulted in 34 confirmed infections and 11 deaths.

“Our findings traced the first person-to-person transmission event to a birthday party with approximately 100 guests,” said the researchers, whose work was published in the New England Journal of Medicine.

The “index patient” was found to have infected five others during “90 minutes” at the party. The second person infected was found to be the “likely source” for six further cases and, after his death, his wife infected an additional 10 people at his wake.

“It appears that inhalation of droplets or aerosolised virions may have been the routes of infection,” added the researchers.

Despite medical experts acknowledging that human-to-human spread of hantavirus is rare, these findings do not bode well for those on the cruise ship as it makes its way from Cape Verde to the Canary Islands, where it is to be evacuated.

Human-to-human transmission could explain why a British doctor fell ill after treating patients on the ship, and may account for the images of medical workers in full bio-protection gear on board the stricken vessel.

It also explains why the South African authorities are urgently trying to trace up to 114 passengers and crew who shared a flight with one of the sick patients from the cruise ship.

The woman disembarked the vessel and flew on a narrow-bodied Embraer E190 aircraft from the island of St Helena to Johannesburg on April 25, where she died a day later.

There are fears she may have infected others on the flight that lasted nearly four hours.

Foster Mohale, a spokesman for South Africa’s ministry of health, told The Telegraph that officials were working with the airline to contact passengers urgently. “We have started contact tracing,” he said.

The World Health Organisation (WHO) has been quick to respond to the outbreak, and with good reason.

The agency considers hantavirus, alongside other deadly pathogens such as the Nipah, Ebola, MERS and Marburg, as carrying a high risk of triggering a major international outbreak.

All are included on the WHO’s Pathogens Prioritisation list, a document targeted at scientists and pharmaceutical companies to encourage the development of new drugs and vaccines to tackle the world’s most pressing health threats.

Perhaps learning the lessons of Sars-Cov-2, where human-to-human transmission was recognised too late, Maria Van Kerkhove, the WHO’s director of epidemic and pandemic preparedness and prevention, said on Monday that there was evidence of human-to-human spread on the cruise ship.

“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,” Ms Van Kerkhove told journalists in Geneva.

Unfortunately, there is no licensed vaccine for the Andes strain of hantavirus, although several candidates are in clinical trials.

Vaccines for other hantavirus strains more commonly found in Asia have been approved for use in China and South Korea, but are not thought to be useful in this outbreak.

The US army is developing a vaccine targeting three hantavirus strains, including the Andes strain. The vaccine is now in phase 2a trials, meaning it is being tested in small groups of human participants after showing promising results in animal studies.

But none of this will come in time for those on the MV Hondius, where passengers can do little to protect themselves but wash their hands frequently and isolate themselves.

Jake Rosmarin, a US travel blogger on board the ship, told The Telegraph that passengers were in “good spirits” but taking precautions.

“Safety measures are in place, including social distancing and masking,” he said. “Passengers also have the option to have meals delivered directly to their cabins.

“Access to the outer decks is permitted for fresh air, while gathering in indoor common areas such as the lounge is currently restricted,” he added.

It is quite a change from a few days ago.

The cruise ship, which has room for 170 passengers in 80 cabins, provides plenty of opportunities for close contact and, potentially, super-spreader events.

According to an online advert for the MV Hondius, its main restaurant offers a “casual and social atmosphere, designed to encourage connection among guests”.

The “open seating” plan means guests can dine “with whom they choose” over “multi-course dinners”.

There are also a “wide variety of interactive workshops, exhibitions and performances” passengers can join.

“The camaraderie that develops on board is an important part of the Oceanwide experience,” it added.