You Probably Don't Need To Stress About Hantavirus
Some facts about the new outbreak that's worrying the globe
As with any disease outbreak, this is a developing situation. It’s possible that the data will change, and we will have to start stressing. I’ll update this piece if there are any major changes to what’s happening with the disease, and please do leave a comment if there’s anything that looks wrong!
Since the start of the COVID-19 pandemic people have been understandably on edge about outbreaks of infectious disease. Everyone is on tenterhooks worrying about the next pandemic that will destabilize the world and kill millions of people. The latest disease to hit the news in a big way is hantavirus, which has infected at least 8 on a cruise ship in the Atlantic and killed 3 people so far.
This is, without a doubt, scary. There’s an eerie similarity to the early days of COVID-19, when we were hearing daily updates about people dying on a very different cruise ship. People are wondering if this is the next pandemic that we will have to live through.
Fortunately, while nothing is ever certain, the chances that hantavirus is going to infect the world is very small. The hantavirus outbreak will probably have no impact at all on the lives of most of the people who are reading this article.
What’s A Hantavirus?
The post-COVID focus on infectious disease has proven to be a fascinating experience for me personally. Every six months, I have to dig in my notes from my masters of public health to read up on another uncommon disease because suddenly its in the news.
Hantaviruses are a group of viruses that mostly infect animals. The majority of these pathogens rarely infect humans, and of the 30 or so that have caused human outbreaks, most die out very quickly. That’s because cases of hantavirus are almost always caused by exposure to animal feces—there are very few documented cases of human-to-human transmission. We will get back to that, because it’s important.
There are two very different syndromes caused by hantaviruses—one mostly impacts the kidneys, the other is a respiratory disease. The kidney syndrome has never been documented to transmit between humans, while the respiratory syndrome has in some cases. The respiratory disease is caused by hantaviruses from the Americas, while the kidney disease is mostly from viruses in other parts of the world.
Hantaviruses have a very wide range of disease severity. While the case fatality rate in documented outbreaks is quite high—somewhere in the region of 1/3 of people who are known to have an infection die—there are almost certainly many more cases out there that we don’t know about. One study in China found that the case fatality rate between 1950 and 1997 was only 3.5%, and even that is an estimate based on people who tested positive to the virus. There are likely people who had less severe disease who did not get tested, and therefore the true case fatality rate—or the infection fatality rate—is probably lower, at least for the two strains of hantavirus that cause infections in China.
There is some direct evidence that the true fatality rate from hantaviruses is much lower than the case fatality rates would suggest. In a 1999 outbreak in Panama, there were 11 cases of respiratory hantavirus and 3 deaths. This is a case fatality rate of 27%, which is of course very high. But the team looking into this outbreak did a big survey of all of the contacts of the hantavirus patients, looking for antibodies to the disease. An additional 40 people in these households had antibodies to the infection. That means that the true infection fatality rate—the percentage of infections that resulted in death—would be more like 5.9%. That’s still high, but it’s substantially lower than the initial figure, and there were probably even more cases out there that the team never identified.
Infectiousness
The other important thing to know about hantaviruses is that they don’t spread easily between people. In that 1999 outbreak in Panama, which was of respiratory disease, there were only two possible case of human-to-human spread. One doctor who cared for patients, and another operating room assistant.
In fact, the only hantavirus out of the 30 where we’ve directly identified human-to-human transmission is the Andes virus. This causes respiratory disease, and is responsible for a large outbreak in 2018/19. In this outbreak, 34 people were confirmed to have been infected, and 11 died. The initial outbreak happened when an infected person attended a party with more than 100 guests, and managed to pass on the disease to 6 other people.
In this outbreak, the disease spread was very slow. There are two main reasons for that:
The infectious period started when people became symptomatic. In 52% of cases, the investigation into this outbreak confirmed that transmission happened when the infected person became feverish. Many people didn’t pass on the disease at all, likely because they stayed home when they started to feel sick.
The incubation period for the infection was very long. The serial interval—the time from the first symptoms appearing in an infectious person to the time that those symptoms appeared in the next patient—was 23 days. That means that on average the disease took nearly a month to incubate and start spreading each time it infected a new person.
This is why, despite there being two months of uncontrolled disease spread before the Panamanian health authorities and the CDC started intervening, there were only 34 cases in this outbreak total. For context, in the initial days of COVID-19, two months of uncontrolled disease spread was enough to cause somewhere in the region of 1 million infections. This is primarily because COVID-19 had a serial interval of 2-7 days in early 2020, and can be passed on before people started experiencing symptoms.
Epidemiologically, it’s hard to know exactly how much contact is required for someone to get hantavirus. In that 2018/19 outbreak, 6 people were infected from the index case—the first person identified to have the virus—during a 90-minute attendance at a party. That’s not a great deal of close contact, meaning that we know at least in theory that the virus may be spread quite readily in high-risk settings.
The Current Outbreak
This all brings us to the current outbreak. As the World Health Organization is reporting, the cases are centered around a cruise ship which left Argentina in early April. Of the 147 passengers aboard, 8 have become unwell and sadly 3 people have already died. The cruise ship is currently quarantined off the Canary Islands on the west coast of Africa.
In addition, there were a number of passengers who disembarked the cruise ship in St Helena, and island in the Atlantic. These people have all already returned home, which means that there are potential cases spread across the globe.
The virus has also been confirmed to be the Andes strain of hantavirus. This is not ideal, because as I mentioned it is the only hantavirus that is known to cause human-to-human transmission. It is therefore quite likely that we will see at least a few more cases in the coming weeks.
For the people on the cruise ship, the next few weeks are probably going to be awful. My heart goes out to all of them, because they are stuck on a small ship with few facilities, waiting to see if they get a nasty virus that has already killed several people that they were travelling with. It’s not a great situation to be in.
For everyone else, however, the current situation is very low risk. Yes, the Andes virus can pass between humans, but it takes a very long time for that to happen. Unless there has been some major mutations that reduce the incubation time and make people infectious before they become symptomatic, it’s very unlikely that there will be an explosion in cases.
And to be clear, while such mutations are possible, I don’t think they’re very likely. Just look at the cruise ship data—the ship left Argentina, where the initial infections most likely happened, on April 1st. It is now May 7th as of writing, and there are only 8 confirmed cases so far. This fits perfectly with the 2018/19 outbreak—an index case bringing the virus onto the ship, and then infecting a number of people over the course of a week who are only now showing symptoms.
The most plausible explanation based on the data we have so far is that this outbreak will follow the same trend as it did in 2018/19. That means that there will probably be more people from the cruise ship who start getting sick over the next few weeks, but there will be very few additional cases after that. As long as the 23 people who disembarked the ship in St Helena don’t start going to parties if they get a fever, the risk of there being further transmission is extremely low.
It could happen. I know I’ll be keeping an eye on the situation, because it’s always good to be up to date. That being said, I would be very surprised to see a significant epidemic or even a large number of cases from this outbreak. It’s entirely possible that fewer than 30 people will eventually test positive to the disease.
Thanks for this, good to have accurate information.
And of course, typhus.