Lot still unknown about hantavirus: Dr Soumya Swaminathan
Dr Soumya Swaminathan said the Andes hantavirus outbreak on a cruise ship is likely to remain localised. She said the episode highlights the need for strong tracing, isolation and preparedness.
Q: Hantavirus is not new, and neither is the Andes strain. So what changed? How did we get to a situation with 11 infected cases and 3 deaths on a single cruise ship?
A: We have a lot of viruses, as you know, and the Andes strain of Hantavirus is the one strain known to spread from person to person. Normally, Hantaviruses spread from rodents to humans – it is a zoonotic infection. You come into contact with rodent urine, stool, or an aerosolised version of that, and you get infected. But this particular strain, predominantly found in Latin America, has been described to spread from human to human, though only after close physical contact.
A cruise ship is an ideal environment for an outbreak, particularly when transmission requires close physical contact or an airborne route. Many of us will remember the Diamond Princess – the first ship off the coast of Japan where a COVID outbreak was identified, one of the earliest signs that SARS-CoV-2 spread through the air.
In this case, compared to the total number of people on the ship, very few have been infected, and mostly those who shared a cabin or were in close physical contact for a prolonged period. This is not the rapid airborne spread we saw with COVID. The concern for people around the world is understandable – is this the beginning of something that can spread globally and become a pandemic? The answer is no. This virus is very different from measles. Yet, as the incubation period is long, we may need to wait six to eight weeks before all doubt is cleared. But this will be a contained outbreak, and there is no risk to the wider public.
Q: One of the suspected cases was a flight attendant who cared for the infected Dutch couple on their flight to the cruise ship. That detail has alarmed many people. How does someone get infected in that setting, and how concerned should we be about others who were on that flight?
A: I do not have information on what protective equipment either the infected passengers or the flight attendant were using at the time – whether they were wearing masks, gloves, and so on. What we do know is that the Andes virus infects the respiratory system and presents much like influenza. The virus particles most likely travel in respiratory droplets when someone speaks or coughs. The flight attendant was in close contact with this person on board the flight, and that is the most plausible explanation.
What this also means is that everyone seated near that patient is now being tracked and tested. Contact tracing will be critical here. I would draw a parallel to SARS-1, which infected many people, including in hospital settings, but because it did not spread from asymptomatic individuals, it was possible to isolate symptomatic cases and contain the outbreak through careful contact investigation. There were close to 800 deaths and roughly 11,000 infections, but it was contained within months. I believe the same outcome is possible here, because passenger logs from both the ship and subsequent flights allow authorities to trace, call, and screen every exposed individual.
Q: Looking at the cases we have, most symptomatic patients are serious – one French individual is hospitalised in critical condition. Yet, there is an American who is completely asymptomatic. What does that range tell us about how this virus behaves?
A: Medicine and public health always have exceptions. COVID taught us things about viral transmissibility that we did not previously know, and every outbreak like this is an opportunity to learn. While the Andes virus is believed to spread mostly from symptomatic individuals and asymptomatic infection is not widely observed, that does not rule out the possibility that certain individuals experience minimal or no symptoms at all.
What is encouraging is that we have diagnostics, and viral sequencing was completed very quickly. Labs in South Africa, Switzerland, and elsewhere have all sequenced the virus and confirmed it is the same strain – and it has not mutated significantly. There is no indication that it is becoming something different from the original Andes virus.
Q: What is the R factor of this virus, and how does it compare to something like COVID or influenza?
A: The R factor depends on the intrinsic transmissibility of the virus itself, but also on the stage of infection of the index case, what precautions are in place – ventilation, protective equipment, proximity – and whether there is any exchange of biological fluids. At the beginning of an outbreak, you may see higher transmission because people are not yet aware of what is happening. That number falls as awareness and precautions increase, and I expect far fewer secondary cases going forward.
To be direct: the intrinsic transmissibility of this virus is much, much lower than SARS-CoV-2, measles, or influenza.
Q: You mentioned the case fatality rate is 20 to 40 percent – that is an extraordinarily high number. What is the virus actually doing to the body that makes it so lethal, and does that differ across strains?
A: Once infected, the case fatality rate of 20 to 40 percent is enormous. For COVID, it was 1 percent or less. A 20 to 40 percent fatality rate puts this in the same range as Ebola. People are dying of severe pneumonia and pulmonary oedema.
There is another strain of Hantavirus that causes more of a haemorrhagic disease and is actually associated with a lower case fatality rate. That strain is more prevalent in Europe and Asia. The Andes strain, the one we are dealing with now, is primarily a Latin American virus and is the more lethal of the two in terms of respiratory impact.
Q: Given all of this – limited spread but very high lethality – is there any scientifically valid case for pandemic concern?
A: The short answer is no. One can be very confident it will not become a pandemic. But should you be concerned? Yes – and for the right reasons. Each of these outbreaks is an opportunity to test preparedness. What would India do if this ship had docked at one of our ports? Are health authorities ready? Do the protocols exist? Are the communication lines with other countries and health agencies in place?
This also reminds us that the next pandemic could appear at any time. Viruses are mutating constantly, and the question is not if but when. We have to ask ourselves whether we are better prepared than we were before COVID. The good news is that surveillance in India today is strong.
Q: Historical data show a pattern – Argentina 2014, another cluster in 2018-19 with 11 deaths, and as recently as 2025, 225 cases and 59 deaths were reported across the Americas. Does that trajectory worry you, or does it actually reinforce the containment argument?
A: The pattern is clear. The initial human infection almost certainly comes from exposure to an infected rodent. One person gets infected, and in a gathering or close setting, they infect one or two more. That is an outbreak. But in every documented case, it has been localised and time-bound. Unless this virus radically changes its fundamental properties, we do not expect a different trajectory now.
What has changed is our ability to respond. The tools we have today to research and track viruses during an active outbreak are far more powerful than what existed even a decade ago.
Q: Two Indians are among the crew. Both are currently asymptomatic, but the incubation period can stretch to six or eight weeks. The UK has already mandated 45 days of self-isolation for returning passengers. What should India be doing right now, and what would you personally recommend to these two individuals?
A: I would wait for the official government advisory, but for asymptomatic individuals, self-isolation is entirely appropriate, with regular check-ins by phone or video call. We managed an enormous amount of isolation infrastructure during COVID – for two people, this should not be a challenge at all.
The precise call – self-isolation at home or in a designated facility – is one every country must make based on its own protocols and risk assessment. I am confident the Indian authorities will advise accordingly and act swiftly.
Q: Hantavirus is not new, and neither is the Andes strain. So what changed? How did we get to a situation with 11 infected cases and 3 deaths on a single cruise ship?
A: We have a lot of viruses, as you know, and the Andes strain of Hantavirus is the one strain known to spread from person to person. Normally, Hantaviruses spread from rodents to humans – it is a zoonotic infection. You come into contact with rodent urine, stool, or an aerosolised version of that, and you get infected. But this particular strain, predominantly found in Latin America, has been described to spread from human to human, though only after close physical contact.
A cruise ship is an ideal environment for an outbreak, particularly when transmission requires close physical contact or an airborne route. Many of us will remember the Diamond Princess – the first ship off the coast of Japan where a COVID outbreak was identified, one of the earliest signs that SARS-CoV-2 spread through the air.
In this case, compared to the total number of people on the ship, very few have been infected, and mostly those who shared a cabin or were in close physical contact for a prolonged period. This is not the rapid airborne spread we saw with COVID. The concern for people around the world is understandable – is this the beginning of something that can spread globally and become a pandemic? The answer is no. This virus is very different from measles. Yet, as the incubation period is long, we may need to wait six to eight weeks before all doubt is cleared. But this will be a contained outbreak, and there is no risk to the wider public.
Q: One of the suspected cases was a flight attendant who cared for the infected Dutch couple on their flight to the cruise ship. That detail has alarmed many people. How does someone get infected in that setting, and how concerned should we be about others who were on that flight?
A: I do not have information on what protective equipment either the infected passengers or the flight attendant were using at the time – whether they were wearing masks, gloves, and so on. What we do know is that the Andes virus infects the respiratory system and presents much like influenza. The virus particles most likely travel in respiratory droplets when someone speaks or coughs. The flight attendant was in close contact with this person on board the flight, and that is the most plausible explanation.
What this also means is that everyone seated near that patient is now being tracked and tested. Contact tracing will be critical here. I would draw a parallel to SARS-1, which infected many people, including in hospital settings, but because it did not spread from asymptomatic individuals, it was possible to isolate symptomatic cases and contain the outbreak through careful contact investigation. There were close to 800 deaths and roughly 11,000 infections, but it was contained within months. I believe the same outcome is possible here, because passenger logs from both the ship and subsequent flights allow authorities to trace, call, and screen every exposed individual.
Q: Looking at the cases we have, most symptomatic patients are serious – one French individual is hospitalised in critical condition. Yet, there is an American who is completely asymptomatic. What does that range tell us about how this virus behaves?
A: Medicine and public health always have exceptions. COVID taught us things about viral transmissibility that we did not previously know, and every outbreak like this is an opportunity to learn. While the Andes virus is believed to spread mostly from symptomatic individuals and asymptomatic infection is not widely observed, that does not rule out the possibility that certain individuals experience minimal or no symptoms at all.
What is encouraging is that we have diagnostics, and viral sequencing was completed very quickly. Labs in South Africa, Switzerland, and elsewhere have all sequenced the virus and confirmed it is the same strain – and it has not mutated significantly. There is no indication that it is becoming something different from the original Andes virus.
Q: What is the R factor of this virus, and how does it compare to something like COVID or influenza?
A: The R factor depends on the intrinsic transmissibility of the virus itself, but also on the stage of infection of the index case, what precautions are in place – ventilation, protective equipment, proximity – and whether there is any exchange of biological fluids. At the beginning of an outbreak, you may see higher transmission because people are not yet aware of what is happening. That number falls as awareness and precautions increase, and I expect far fewer secondary cases going forward.
To be direct: the intrinsic transmissibility of this virus is much, much lower than SARS-CoV-2, measles, or influenza.
Q: You mentioned the case fatality rate is 20 to 40 percent – that is an extraordinarily high number. What is the virus actually doing to the body that makes it so lethal, and does that differ across strains?
A: Once infected, the case fatality rate of 20 to 40 percent is enormous. For COVID, it was 1 percent or less. A 20 to 40 percent fatality rate puts this in the same range as Ebola. People are dying of severe pneumonia and pulmonary oedema.
There is another strain of Hantavirus that causes more of a haemorrhagic disease and is actually associated with a lower case fatality rate. That strain is more prevalent in Europe and Asia. The Andes strain, the one we are dealing with now, is primarily a Latin American virus and is the more lethal of the two in terms of respiratory impact.
Q: Given all of this – limited spread but very high lethality – is there any scientifically valid case for pandemic concern?
A: The short answer is no. One can be very confident it will not become a pandemic. But should you be concerned? Yes – and for the right reasons. Each of these outbreaks is an opportunity to test preparedness. What would India do if this ship had docked at one of our ports? Are health authorities ready? Do the protocols exist? Are the communication lines with other countries and health agencies in place?
This also reminds us that the next pandemic could appear at any time. Viruses are mutating constantly, and the question is not if but when. We have to ask ourselves whether we are better prepared than we were before COVID. The good news is that surveillance in India today is strong.
Q: Historical data show a pattern – Argentina 2014, another cluster in 2018-19 with 11 deaths, and as recently as 2025, 225 cases and 59 deaths were reported across the Americas. Does that trajectory worry you, or does it actually reinforce the containment argument?
A: The pattern is clear. The initial human infection almost certainly comes from exposure to an infected rodent. One person gets infected, and in a gathering or close setting, they infect one or two more. That is an outbreak. But in every documented case, it has been localised and time-bound. Unless this virus radically changes its fundamental properties, we do not expect a different trajectory now.
What has changed is our ability to respond. The tools we have today to research and track viruses during an active outbreak are far more powerful than what existed even a decade ago.
Q: Two Indians are among the crew. Both are currently asymptomatic, but the incubation period can stretch to six or eight weeks. The UK has already mandated 45 days of self-isolation for returning passengers. What should India be doing right now, and what would you personally recommend to these two individuals?
A: I would wait for the official government advisory, but for asymptomatic individuals, self-isolation is entirely appropriate, with regular check-ins by phone or video call. We managed an enormous amount of isolation infrastructure during COVID – for two people, this should not be a challenge at all.
The precise call – self-isolation at home or in a designated facility – is one every country must make based on its own protocols and risk assessment. I am confident the Indian authorities will advise accordingly and act swiftly.
