Sarah Newey Global Health Security Correspondent. Paul Nuki Global Health Security Editor

When Maria Van Kerkhove first heard about a lethal and unexplained outbreak on a polar cruise, her mind jumped to bird flu – and her heart sank. Was this the beginning of the H5N1 pandemic that scientists have long predicted?

The signals were alarming. The index cases had been bird watching in Argentina, where avian influenza has been detected in farmed poultry and wildlife, and the ship had made further stops in a series of remote and ecologically diverse ports. Worse, there was already a cluster of three cases. If it was bird flu it had almost certainly started to spread between people.

There was relief when the results came back negative. A specialist lab in South Africa confirmed the patients had not been stricken with bird flu, nor legionnaires disease. The pathogen was Andes hantavirus. 

For Dr Van Kerkhove, the director of the Department of Epidemic and Pandemic Preparedness and Prevention at the World Health Organization (WHO), the results felt like a bullet dodged. 

“It’s strange but yes, I was relieved that it wasn’t a zoonotic influenza, because of the link with the first three cases,” she told the Telegraph from her office in Geneva. 

“If it had turned out to be a zoonotic influenza, and we already knew there were three cases epidemiologically linked … that would have been a bit more concerning.”

Ask most scientists about the pandemic threat they fear most, and H5N1 is top of the list. The avian virus, not dissimilar to the pathogen that caused the Spanish flu pandemic a century ago, is a deemed major risk as it mutates rapidly, has a high mortality, and infects a wide range of mammals – from pigs and seals to humans. 

“Avian influenza A viruses, particularly the H5N1 subtype, pose a persistent threat, with pandemic potential due to their genetic recombination capacity, interspecies adaptability, and high lethality,” scientists wrote in the Viruses journal last summer.

“Conservative estimates suggest that such a pandemic could result in up to 350 million deaths globally, severely disrupting healthcare systems, societal structures, and economies.”

But while the world may have escaped a worst case scenario for now, the Andes virus still poses very real risks – and the response has just entered a “make or break” phase, says Dr Van Kerkhove. 

Soon, it will be clear if the outbreak is going to burn out or spark a new transmission chain. 

Dubbed “rat virus” because it comes from rodents, there are no vaccines or treatments to combat the pathogen. Andes also has a mortality rate as high as 50 per cent and is the only hantavirus capable of human-to-human transmission, including “super spreader” events. 

The MV Hondius cruise proved the petri dish for such contagion: so far 11 people have contracted the disease, including three fatalities. 

The remainder of those on board – 88 passengers and 59 crew from 23 different countries – disembarked the stricken ship in Tenerife. Roughly 30 others, who left MV Hondius before the outbreak was fully understood, have also been traced. 

“The challenge right now is how the quarantine unfolds,” said Dr Van Kerkhove. “This is a critical period, sort of a make or break situation.”

Since the WHO first learned of the outbreak on May 2 – having been informed by the UK Health Security Agency – a massive global public health operation has been underway to curb the spread and treat sick patients.  

The WHO has recommended that everyone spends at least 42 days in quarantine facilities or self-isolation in their home countries, with access to frequent testing and medical care. The success or otherwise will likely emerge soon: despite an incubation period as long as six to eight weeks, it is thought that most people develop symptoms within a fortnight of exposure.

“We could have more cases, but that doesn’t mean the outbreak is expanding – it means there’s more testing among [known contacts],” said Dr Van Kerkhove. “It’s good that we haven’t had any further deaths, but I am worried about the woman in France who’s in ICU. From what we hear, she’s not doing well. Others in hospital are getting better.”

She added that she is confident in the global effort to halt onward transmission, noting that  the outbreak has stress-tested the post-Covid pandemic infrastructure. The response – involving 28 countries across multiple regions – has been characterised by camaraderie, solidarity and vigour.

Argentina sent hantavirus tests to Europe; labs in South Africa, Senegal, the Netherlands and Switzerland collaborated on sequencing; and Spain’s efforts to safely disembark passengers in Tenerife was “incredible”. Although America’s exit from the WHO has “cut off” technical collaboration and “strained” relations, even there information channels remain open. 

“No one cares about anyone’s politics… we have a common goal: to stop this outbreak,” Dr Van Kerkhove said. “It’s amazing what people can do when they put their mind to it… no one wants a repeat of what we went through with Covid. Not that this was going to be a repeat of Covid.”

Since the hantavirus first emerged almost a fortnight ago, the WHO has been keen to stress this is not the next pandemic. But they have also learned from past mistakes: six years on, many still criticise the UN agency – and Dr Van Kerkhove – for being overly reassuring about the potential for human-to-human and airborne spread in the early days of Covid-19. 

“Experts have come out of the woodwork again to speak about this, and people are speaking with such confidence. Yet we, as WHO, have tried to learn – have learned – how important it is to speak about uncertainty with nuance… and I personally have really taken that to heart.

“We don’t know everything about transmission. We’ve taken a precautionary approach in all of this, in every operation that we have, assuming that human to human transmission is more efficient than it probably is. But we just don’t know… in this outbreak we made the decision early on that everyone on the ship was considered high risk.” 

There are other unknowns too, chief among them: how did the Dutch couple first contracted Andes hantavirus? Initial theories that a bird watching trip near a rubbish dump in Ushuaia, where the MV Hondius left port in early April, have been largely dismissed. Argentinian scientists are now racing to sample rodents along the Dutch couple’s travel route. 

There are also questions about preparedness – the outbreak has highlighted the glaring lack of vaccines and treatments for Andes hantavirus, despite the known risk. 

The WHO lists the virus as a priority pathogen for research and development, while a 2020 paper in the New England Journal of Medicine warned the virus could trigger super-spreader events. Scientists say it is not the only known danger that has been neglected, in terms of both global scientific focus and funding. 

“I think this will spur on some research and development – the question is, will it be sustained? And how will we actually be able to see it through to see better diagnostics, better therapeutics, a vaccine, to help for the next time?” said Dr Van Kerkhove. 

Yet the Andes hantavirus is not the WHO official’s biggest concern. Every week, her department deals with outbreaks and tracks new risks that could erupt.The last fortnight has been unusual in intensity – largely because of how many countries are involved – but Dr Van Kerkhove never really thought it would spiral out of control. 

“Respiratory diseases [are] still my biggest concern – a flu pandemic, another coronavirus pandemic. My concerns are also around arboviruses [like dengue and Zika] because the vectors are moving further north and further south, and we’re seeing transmission in areas that never experienced it before.

“I think outbreaks like this are hantaviruses, Ebola, Marburg – they’re self limiting because they’re not efficiently transmitted. They can be devastating when they do emerge, but they don’t have that potential like… Covid or flu.”

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