Hantavirus likely to be fully contained but may take time, Hanage says

“William Hanage. File photo by Kris Snibbe/Harvard Staff Photographer Health Hantavirus likely to be fully contained but may take time, Hanage says Disease much deadlier than COVID but a lot harder to spread Alvin Powell Harvard Staff Writer May 12, 2026 8 min read The public health focus surrounding the deadly hantavirus outbreak on the Dutch cruise ship MV Hondius is now turning to preventing onward transmission, as the 18 American passengers of the vessel arrived in the U.S. on Monday and most of the rest of the travelers are either en route to or back in their home countries. The World Health Organization reports eight cases and three deaths as of May 8. Hantavirus is deadlier, case-by-case, than COVID, but is significantly harder to spread, according to William Hanage , professor of epidemiology at the Harvard T.H. Chan School of Public Health . In this edited conversation, the Gazette spoke with Hanage, who is also associate director of the Chan School’s Center for Communicable Disease Dynamics , about the outbreak and his expectation that it will be fully contained, though that will likely take some time. Eighteen people, including one who tested positive, are being monitored in the U.S. How dangerous is this? There have been three deaths so far, which sounds like a lot for the number of exposures we know about. The numbers that float about are around 40 percent base fatality. But it’s always difficult to be sure, because there could be milder cases that we don’t recognize amid the chaos of an outbreak. But it’s certainly toward the more severe end of the range. They talk about it being passed by close contact. What is close contact? What we know is that it seems to require reasonably extended contact with a person who is sick and is shedding virus. One of the cases of transmission was to the doctor on the cruise ship who was attending the index case. Not knowing anything about the precise course of illness, it’s easy to see how a doctor treating an extremely sick patient on a cruise ship is going to be in contact with them, probably in a poorly ventilated space, for quite some period of time and without knowledge of the personal protective equipment that they should be using. They simply didn’t know what it was. So, I think that case is not surprising. It does indicate, however, that healthcare staff should take care while treating people who have hantavirus to minimize the potential for transmission. How concerned are you about the next few weeks to months? Is there a time frame that is critical in giving an indication of which way this outbreak is going to go? I think that we will learn a great deal over the next month about whether or not any significant transmission chains have been seeded by the people who are currently being chased up. I think we can expect more cases. How many is not clear. One thing which I’m going to say right now is that I’m absolutely confident that this is going to be limited and contained. The number of people in the world who should be worried about this now is in the low hundreds, if not less. The only question is how long it will take to contain it. What does the transmission pattern — starting on a cruise ship, people getting onto airplanes, and then to different countries, illustrate about the potential health consequences of our connectedness today? Infectious diseases are our companions, and their spread reflects the contacts that we make between us. There’s a classic paper that I’ve seen referred to many times considering three generations from the U.K. A person’s great-grandfather never traveled more than 10 miles from the town he was born in. That person’s father traveled from that town to London and a few places around the country. But his son had traveled to every continent. This really shows the challenges today in responding effectively to outbreaks like this. It’s made more complicated by the fact that different countries may have access to different resources. Consider, for instance, how the U.K. has used paratroopers to deliver materials to help care for suspected cases on the remote island of Tristan da Cunha. It also is a bit of a call back to COVID, because one of the reasons that was so challenging is that the different countries had different priorities. Before vaccines and widespread immunity, COVID had the capacity to flatten healthcare systems provided it’s given free reign, but the number of severe cases would be dependent on how many old people there are in the population. But some countries have a very different age structure and fewer resources. So, for instance, South Africa has a relatively young population and you have to consider COVID rates alongside something like tuberculosis. After Omicron emerged, South Africa essentially started to switch its focus to tuberculosis. That illustrates the way in which different countries have different resources and different priorities, which makes coordinating a response more difficult. That’s one of the reasons why we need organizations like the WHO. I know that hantaviruses spread among rats, but what else is important to know about the virus? Most hantaviruses do not transmit human-to-human. When I saw the headline about this, I thought, “There’s a cruise ship where there was lots of contact with rodent feces? That’s a weird cruise ship.” Then I heard it came from Argentina, and it made more sense, because the only hantavirus we know of that’s capable of human-to-human spread is the Andes strain. Is transmission to humans very common with that strain? It’s not common, though there have been outbreaks. There’s a bit of controversy about how much human-to-human transmission occurs because it’s always very difficult to rule out a common exposure. That’s made more difficult by the fact that it can have a quite long incubation period. For instance, when you would see cases of COVID-like symptoms developing four or five days apart, you would think, “That’s a transmission chain.” But in the case of hantaviruses, it can be weeks. That can make it difficult to know if two people develop illness a few weeks apart whether it’s due to a common exposure as opposed to transmission. We’re turning right now to an outbreak that happened in 2018 and that was pretty thoroughly investigated. In that outbreak there were four rounds of transmission, from the index case to secondary, tertiary, quaternary before it was eventually contained. For a disease like this, the most effective way to control and contain it is going to be quarantine. And that quarantine is going to have to be quite long in order to be secure and effective. In the 2018 Argentina outbreak, they implemented enforced self-quarantine and put some limitations on large gatherings, which are key to outbreaks and transmission in many cases. So in order to stop it, people who may have been exposed but aren’t necessarily sick will have to be patient enough to sit in their house for several weeks? That’s right. Another thing that is important here is that everything we know indicates that people are infectious and most likely transmit as they develop symptoms and once they’ve developed symptoms. That’s actually a good thing, from the point of view of control. It means it’s very, very unlikely to cause anything more than a limited outbreak. What are your symptoms if infected? If you have hantavirus pulmonary syndrome, then you have very severe, rapidly progressing respiratory symptoms. Gastrointestinal symptoms have also been involved in at least one case. The onset is quite acute and will include a fever, one of the first indicators of infection. Of course, fevers are a symptom which is shared with many other infections. That’s one of the reasons why you’re hearing about people with symptoms who are being treated as potential cases but which on further investigation turn out to not be caused by hantavirus. How did this outbreak get started? The index patient came aboard in Argentina. Where they acquired it is not clear. There’s been focus on a bird-watching trip to a landfill site where it’s known that rodents — among which the virus is endemic — can be found. That being said, the trip occurred not very long prior to the index case developing symptoms, so it’s possible there was either an unusually short incubation period or that it could have been picked up in the weeks prior to that. How does this compare to, say, COVID or measles or flu, other illnesses that are going around today? It’s much less routinely infectious than any of those. COVID is capable of transmitting before people develop symptoms or when they have very few or no symptoms. The same is true with flu. Measles is extraordinarily transmissible and hangs in the air in aerosol particles for a long time. It is among the most contagious viruses we know and is only held in check by vaccination. This is comparably much less transmissible. The outbreak that is most immediately reminiscent is the original Severe Acute Respiratory Syndrome, or SARS, which also had transmission that was linked to symptom development or the onset of symptoms, and that was also driven by a few super spreading events. It’s very easy to imagine many opportunities for transmission on a cruise ship, but it’s much harder to see the outbreak continuing for a long period once it has been identified.

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