WHO warns hantavirus most infectious at onset as cruise ship deaths reach 3

People infected with the Andes hantavirus are most contagious at the very start of illness, the World Health Organization said Sunday, as about 150 passengers and crew from the cruise ship MV Hondius prepared to quarantine after an outbreak aboard the vessel killed three people.

The Andes strain is the only hantavirus known to spread from person to person, and its incubation period of up to 42 days means new cases could surface well into June — a risk that includes 17 American passengers who returned home from the Dutch-flagged vessel without a mandatory quarantine order.

"The first few moments of illness is when the infectiousness is highest," said Olivier Le Polain, WHO's deputy director for epidemiology and response analysis, at a social media briefing Sunday, according to The Korea Times. "If WHO recommends quarantine it is because actually people are infectious at the very start of illness," he added, underscoring why early isolation is critical despite the difficulty of identifying symptoms in real time.

The MV Hondius, a Dutch-flagged vessel, arrived in waters near Tenerife early Sunday after the outbreak. Three people aboard died from the virus. Approximately 150 passengers and crew disembarked, including nationals of Germany, Britain, Switzerland, Greece, Australia, France and the United States.

The Andes strain poses a particular concern because it is the only hantavirus variant capable of direct human-to-human transmission, The Korea Times reported. The average time from infection to the appearance of symptoms is about three weeks, but the maximum incubation period stretches to roughly 42 days, making early detection especially difficult since initial signs — fatigue and mild fever — are easy to overlook.

Several nations moved swiftly to isolate returning passengers. Germany, Britain, Switzerland and Greece imposed 45-day quarantine periods, slightly exceeding WHO's recommended six-week window. Australia and France required two to three weeks of isolation. The 17 American passengers, however, returned home without a mandatory quarantine order — a decision WHO Director-General Tedros Adhanom Ghebreyesus said "may have risks," according to The Korea Times.

Le Polain warned that the virus's lengthy incubation period creates an extended window of uncertainty. "The hantavirus's long incubation period means that we can see cases again coming up in the next few days, perhaps even next week," he said, according to The Korea Times.

As of Sunday, health authorities across Europe and Australia were continuing to monitor returning passengers. With 17 Americans home without a required quarantine, WHO officials urged the United States to reconsider its approach and warned that further cases could emerge in the days ahead.