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Scientists comment on a case of human-to-human case of Swine Flu reported in Spain.
Dr James Hay, Research Fellow, Pandemic Sciences Institute, University of Oxford, said:
What do we currently know about this situation?
“This is a very recently reported case, so I have no further information beyond what has been posted publicly.
“There has been a confirmed case of swine influenza A/H1N1 in Catalonia. People may be familiar with the A/H1N1 viruses which cause seasonal influenza in humans, alongside A/H3N2 (the subtype of influenza which has dominated this year’s influenza season) and influenza B. Sequencing this particular influenza virus showed it belongs to the Eurasian swine lineage, confirming its origin from pigs. Human A/H1N1 and swine A/H1N1 viruses are related and biologically similar, but as the name suggests, they are genetically distinct and circulate in different species.
“The patient has already recovered, and contact tracing and testing efforts have found no further infections. This suggests that there has been no further transmission, which is reassuring.
Can we be confident it is an example of human-to-human transmission and if so, how concerning is this?
“It is hard to say without further information, but if contact with swine has been definitively ruled out, then it is likely the patient was infected through human-to-human transmission. It seems that none of the patient’s contacts had exposure to pigs either, which suggests a short chain of infections back to the original source in pigs, though there may be other explanations. Overall, this single case in isolation is not cause of concern. If this particular strain of influenza was able to transmit well between humans, more cases would likely have been found through contact tracing efforts.
How often does human-to-human transmission happen?
“It isn’t unusual for people working with pigs to become infected with swine flu viruses, though it is less common to see human-to-human transmission. However, swine influenza cases with no direct link to pigs are occasionally reported, so this is not particularly unusual. For example, there was a case of influenza A/H1N2 originating from pigs detected in the UK back in November 2023. Similar to the present situation, that patient was only mildly sick and recovered without any further transmission.
Does this change the level of risk to people in Spain/Europe?
“No, there is no change in risk level to the public. We know that these viruses circulate in swine all the time, and sporadic cases like this happen relatively frequently. Public health authorities are well versed in quickly responding to these cases, and the fact that no further infections have been detected is reassuring.
Any other comments?
“Situations like this demonstrate the importance of good epidemiological surveillance so that sporadic infections from animals to humans can be detected and dealt with quickly. From the information I have seen, this sounds like a typical case study of a surveillance system working as intended.
“One small comment is to highlight that this lineage of swine flu is distinct from the A/H5N1 in birds and cattle that we have been hearing about over the past couple of years. It is less of a barrier for swine flu viruses to infect humans, as our biology is more similar to a pig than it is to a chicken, so these sporadic infections with swine flu are to be expected.”
Dr Chris Illingworth, Senior Lecturer in Virology, University of Glasgow, said:
What do we currently know about this situation?
“I don’t know any more than is described in the newspaper reports, but I can provide some context. The claim that this is swine influenza will most likely have come about through genome sequencing identifying the virus as being one that originates from pigs. Genome sequencing is routinely carried out on influenza viruses across the world.
Can we be confident it is an example of human-to-human transmission and if so, how concerning is this?
“Viral genome sequencing will establish with high confidence that the virus found in a person will have originated in a pig. If the person in question has had no contact with pigs, it is very likely that human-to-human transmission has occurred.
“Small-scale human-to-human transmission of a non-human influenza virus is of itself not a great concern. For context, around 60 cases of animal-to-human transmission of influenza were detected in 2022. What would be a concern is the discovery of more cases of swine influenza in the same region of Spain in the days and weeks to come, which could suggest that the virus is beginning to spread more generally. Evidence of the broader spread of the virus would suggest that the virus could have the potential to cause a novel pandemic.
How often does human-to-human transmission happen?
“Animal-to-human transmission of influenza happens fairly often. Multiple cases of this are detected each year, and the actual number of cases is probably much larger than the number of those detected. In some of these cases, human-to-human transmission occurs, but nearly every time, the outbreak ends without having any wider consequences. About once every 30 years we get a situation where this leads to a new pandemic: The most recent were 1918, 1957, 1968, and 2009. In the absence of further cases, this is likely not going anywhere, but the detection of more cases could indicate an event of concern.
Does this change the level of risk to people in Spain/Europe?
“In so far as only one case has been discovered, I would say this doesn’t change the level of risk to people in Spain / Europe.
Any other comments?
“Spillover events, in which animal viruses infect humans, behave something like a lottery. Spillover events are common, perhaps happening somewhere in the world every day, and a single detected case of an animal virus in a person is almost always nothing to worry about. Yet every now and then, perhaps once every few decades, a virus hits the jackpot, with devastating consequences for human health and wellbeing. While a case of swine flu in Spain is almost certainly an event that will soon be forgotten, it is a reminder that our societies exist continuously a few steps away from catastrophe. How we plan, how we shape our health systems, our social fabric, and our scientific infrastructure, will all play a critical role in how we fare the next time we have to face the worst together.”
Prof Ed Hutchinson, Professor of Molecular and Cellular Virology, MRC-University of Glasgow Centre for Virus Research (MRC CVR), said:
“Reporting indicates that an elderly male patient in Catalonia was found to be infected by a strain of influenza virus that normally only infects pigs. The patient has now recovered, but the case is being carefully investigated because there is no obvious way in which this person could have been directly infected by a pig. One alternative explanation – as yet unproven – is that the virus was passed on via another person. This would be concerning, as most infections of humans with swine flu come directly from pigs and do not spread any further. When influenza viruses move from other animals into humans they have the potential to adapt to spreading efficiently among humans, which can create pandemics (as happened with a pig virus for the 2009 ‘swine flu’ pandemic) – although it is important to be very clear that so far we don’t have positive evidence for even limited human to human transmission in this case.
“While it is right that this is being taken seriously, it is important to note that cases like this are not unprecedented. In 2023 there was a similar case in the UK, where an older adult with no known contact with pigs acquired a (different) strain of swine flu and then recovered (https://www.gov.uk/government/news/ukhsa-detects-human-case-of-influenza-ah1n2v). That case went no further and hopefully the same will be true here – we are getting better and better at detecting cases like this and they may be more common than we think. However, even if the risks of a wider outbreak remain extremely low it is still good to know that the Catalan health authorities are investigating this carefully.”
Prof Ian Brown, Group Leader, Pirbright Institute, said:
“Swine influenza viruses do occasionally spillover to humans and it is not uncommon for direct links to pigs being inapparent. Detailed epidemiological enquiries will be required including a full traceability of the recent contacts of the infected person before any conclusions can be drawn on true origin and importantly onward transmission risk to other people. In general these viruses lack the capability to efficiently transmit amongst humans but genomic data when available could offer insights as to why this event happened. It should be noted elderly persons may be more vulnerable to infection especially if they have underlying conditions. The situation needs careful and urgent monitoring but without more information it would be premature to consider the risk level in Spain and wider Europe has changed.”
(from SMC Spain) María Iglesias-Caballero, Virologist at the Reference Laboratory for Influenza and Respiratory Viruses of the National Microbiology Centre – Carlos III Health Institute, said:
“We have requested serum from this patient because sometimes, as we use very sensitive diagnostic techniques, the cases are not always true infections. Serum has been requested to confirm this. That is pending.
“The samples we have available have been sent to a WHO centre and to the Centre for Research and Innovation in Catalonia (CRIC), so everything is perfectly coordinated within the network. It is true that this is a peculiar case, but it is under control. Catalonia is one of the autonomous communities that does very well in surveillance, sequencing a lot, and this is the only case we have.
“So we believe that if there really were sustained circulation, we would have detected it, and that is not the case. But even so, what we can seriously attest to is that we are monitoring it. We have coordination from the autonomous community, central government, Europe and internationally.”
(from SMC Spain) Aitor Nogales González, Senior scientist at the CSIC at the Animal Health Research Centre (CISA), National Institute for Agricultural and Food Research and Technology (INIA), said:
“It is not unusual for a swine flu virus to infect humans. Many cases go undetected because there is no epidemiological surveillance in this regard, while others go unnoticed because they are asymptomatic or because specific tests are not performed. These infections are usually mild or even asymptomatic, or cause symptoms similar to those of common flu, and are not usually transmitted between humans, or do so with very low efficiency.
“The virus responsible for the 2009 pandemic, the last influenza pandemic to date, was particularly complex, combining genetic segments from swine, avian and human influenza. Its origin was traced and it was confirmed that the jump to humans came from pigs. However, it was a virus that had been recombining and evolving for years before emerging massively in humans.
“In contrast, in the current case, pending final confirmation of the analyses and genetic sequences, the available data suggest that it is one of the swine flu variants circulating today. There is no indication, for now, that we are dealing with a particularly novel virus or one that behaves differently than expected. Therefore, the risk to the human population is considered low or very low.”
(from SMC Spain) María Montoya, Researcher in the Viral Immunology: Therapies and Vaccines group at the Margarita Salas Biological Research Center, said:
“I’m not surprised. Often, when flu is circulating, if there are no symptoms, it is not analysed in detail unless there are complications or someone goes to hospital and is tested. The fact that the patient has not had contact with animals means that they have been infected through contact with another person, who in turn may have had contact with animals or with another person in contact with animals. The thing is, flu viruses can be dangerous, they can recombine, as happened with the first pandemic of the 21st century. In addition, there may be a serious health problem in people who have a weak immune system for various reasons, both congenital and due to illness. The fact that it has been detected makes us appreciate the importance of detecting viruses that are circulating in Spain, because with this information we can be better prepared for possible complications later on.”
Declared interests
Dr Chris Illingworth: No conflicts of interest.
Dr James Hay: I have received payment from Gerson Lehrman Group (GLG) for providing expert consulting advice. I have no industry links, and my research is funded in full by the Wellcome Trust.
Prof Ed Hutchinson: https://www.gla.ac.uk/schools/infectionimmunity/staff/edwardhutchinson/
Prof Ian Brown: IB receives research funding to study avian influenza in multiple hosts. IB is a member of the UK scientific committee ‘New and Emerging Respiratory Threats Assessment Group’. IB participates in UKHSA led fora that assess the risk to human health from avian influenza viruses.
For these experts, no reply to the request for DOIs was received.