The MV Hondius, a Dutch-flagged polar expedition ship, is sailing for Spain’s Canary Islands after an outbreak that has so far produced seven confirmed or suspected hantavirus infections and three deaths, with an eighth case confirmed the following day in Switzerland in a passenger who had already left the ship. The virus has been identified as the Andes strain, the only hantavirus known to pass between people. The WHO has issued a Disease Outbreak News. 147 passengers and crew from 23 nationalities were aboard.
After six years of pandemic aftermath, many people are asking: is this the next COVID? I study the dynamics of infectious disease outbreaks, and the short answer is no. The biology of this virus makes large-scale spread extremely unlikely.
Two Cruise Ships, Two Viruses
Cruise ships pack people into confined quarters for weeks. They are about as favorable an environment for disease transmission as you will find outside a laboratory.
We have a direct comparison. In early 2020, COVID-19 swept through the Diamond Princess, a large Princess Cruises liner carrying roughly 3,700 passengers and crew. 634 tested positive within a month, an attack rate of 17%. Modeling studies estimated the reproductive number reached as high as 11 in the early, uncontrolled phase. One analysis projected that without intervention, 79% of the ship would have been infected.
The Hondius carried 147 passengers and crew. The first symptoms appeared on April 6. Five weeks later: eight cases. An attack rate around 5%, on a much smaller ship, over a longer period. The comparison is imperfect — the ships differ in size, demographics, ventilation — but the stark difference in spread is hard to explain away. Something about this virus limits its transmission in ways that SARS-CoV-2 did not.
What Limits Spread?
Three things.
The first is when people become infectious. COVID’s defining feature was presymptomatic transmission. An estimated 44% of secondary infections occurred before the index case showed any symptoms. People were shedding virus through ordinary breathing and conversation, days before they had reason to stay home.
With Andes virus, the picture looks different. Patients do develop viremia days before symptoms appear. But whether virus reaches saliva or respiratory secretions before symptoms is an open question — the largest prospective study of viral shedding enrolled patients at hospitalization, so pre-symptomatic fluids were never sampled. What we do have is the epidemiologic record, and it consistently points to transmission during the prodromal phase — early fever and body aches — through close and sustained contact. In the best prospective study of household contacts in Chile, sex partners of infected patients had a 17.6% risk of infection. Other household contacts: 1.2%. That pattern does not suggest a virus spreading silently through casual contact.
The second is speed. The median incubation period for Andes virus is 18 days, with a range of 7 to 39. COVID’s was about five to seven days. In documented clusters of person-to-person Andes virus transmission, secondary cases fell ill 19 to 40 days after contact with the index case. Each link in a transmission chain takes weeks. A virus that moves this slowly gives public health systems time to find cases and break the chain of transmission.
The third is that hantavirus cardiopulmonary syndrome is severe. Case fatality runs 35 to 40%. In a clinical series of 16 Andes virus patients in Chile, respiratory failure and hemodynamic instability appeared one to seven days after the onset of prodromal symptoms. Patients become too sick to travel and too sick to attend social gatherings — too sick to seed the kind of silent community transmission that made COVID uncontrollable in its first months.
We Have Seen the Worst Case
The closest precedent is the 2018-2019 outbreak in Epuyen, a small town in Chubut Province, Argentina. A single rodent-to-human infection sparked a chain of person-to-person cases: 34 confirmed, 11 dead. Three individuals who attended crowded social gatherings while symptomatic drove most of the transmission. The reproductive number before public health intervention was 2.12. After authorities imposed isolation and quarantine, it dropped to 0.96 and the outbreak burned out.
That is the track record. Even in the worst documented episode of Andes virus person-to-person spread, standard public health measures were enough to stop it.
What Is Worth Watching
The Hondius outbreak is not a pandemic threat. But the story is not over, and several things bear watching.
The reproductive number on the ship may turn out to be higher than the 2.12 estimated in Epuyen, given the close living quarters aboard a small expedition vessel. Some passengers still on board may be incubating the virus, so the final attack rate could exceed the current 5%. Approximately 40 passengers left the ship at St. Helena after the first death, and Dutch authorities have not said where they are now. Contact tracing across multiple countries will be harder than it was in a single Argentine province. And there may be transmission events outside the ship that have not yet come to light, from passengers who were incubating the virus when they disembarked.
None of this changes the pandemic picture. But Andes virus person-to-person transmission has previously been documented only in rural communities in southern Argentina and Chile, among people who share households and beds. A cruise ship with passengers from 23 countries is a different context, and a reminder that this virus has been studied too little.
