Scientists react to news of a human case of influenza A being detected.
Dr Andrew Catchpole, Chief Scientific Officer at hVIVO plc, said:
What do we know from the information available?
“There are only few very details that have emerged so far but the key point here is that the influenza surveillance network that is very well established in the UK is working well and was responsible for identifying the infection. The infected person clearly had flu-like symptoms on presentation to their GP in order for the GP to trigger collecting and sample and sending it in to the UKHSA as part of the surveillance network but they are reported to have experienced relatively mild disease, which is good for the person concerned but that fact alone doesn’t really tell us anything about transmission potential of the virus or the risk to vulnerable groups. Details on the age and the overall health status of the infected person have not been released so we do not know if this was an otherwise healthy individual whom we would expect to recover from flu without intervention or if the person was in a proportion of the population defined as vulnerable in terms of risk of severe disease with influenza infection.
What do we know from existing evidence about this specific virus?
“The virus is reported to be very similar to a type of virus that is known to be circulating in pigs in the UK and yet to date the is the first reported case of human infection in the UK. Therefore, this tells us that it is a rare event that the virus that is currently circulating in pigs is able to cause human infection otherwise we would have seen more human infections before now. It is somewhat reassuring that this is not a brand new virus with no known prior origins as this helps us track the possible source of the infection. However, more details on the specific strain are needed to fully understand the risk it may pose to humans. All we know from the limited information so far is that the virus is related to a currently circulating pig virus but importantly we do not yet know if there are any material differences in the virus isolated from the person to the pig counterpart, which may alter the viruses ability to spread between people. Pigs are able to be infected with influenza viruses that originate from both human and avian species so they are a common source of newly emerging flu strains that infect people but typically such infections either do not spread between people or have very limited transmission before they die out as normally the viruses are not able to replicate and transmit efficiently in people.
What more information do we need to discover before knowing how significant this development is?
“We need to understand if there are any material differences of the virus isolated from the infected person and the parental pig virus, for example, viral genes switched with human influenza viruses would be of particular concern as this could lead to increased ability to transmit from person to person but this seems extremely unlikely from the information released to date. It will also be important to understand the age and health of the infected person before they became ill with the virus to understand if they are in the part of the population of known influenza vulnerability or an individual that would otherwise be expected to clear influenza virus without intervention. Tracing back exactly how the person became infected will be a key priority as this will help determine the likelihood of the virus already having been passed between other people and minimising any further transmission. Linking the infection back to direct contact with an infected pig will clearly be less concerning than if there has been no possible pig contact as the latter would mean that transmission between people of this pig-origin virus has already taken place.
Is it usual for routine flu surveillance to pick up viruses like this?
“The combined animal and human influenza surveillance network is the usual mechanism by which such virus infections are detected. Without this in place we would only become aware of infections if they either caused a very severe illness in an infected individual or once they had caused wide spread infections in the community. The relative frequency, however, of detections of animal-origin viruses causing human infections remains very rare.
Can we glean anything from the reported fact that the individual concerned experienced a mild illness and has fully recovered?
“Most of the population are expected to only have relatively mild illness from influenza infection with the majority of severe cases being the elderly, very young or with other existing comorbidities. On the one hand it is clearly good news that the person has fully recovered and only suffered a relatively mild illness and that this strain does not appear to be highly pathogenic. However, without understanding the infected person’s known risk for severe influenza it is not possible to really extrapolate much from this. For example, was the person in a group of the population known to be at risk for server disease or complications from influenza infection or otherwise healthy and expected to clear the virus naturally? Such details have not yet been reported. From a public health perspective, epidemics of respiratory viral infections are in main due to large numbers of asymptomatic or mild cases driving the transmission between people as they are not so sick that they are staying at home but instead continue to go about their normal business and come into contact with multiple other people and therefore spread the disease. Consequently, it is typically the flu viruses where the majority of cases are mild and not those that are highly pathogenic that have the greatest potential to cause localised epidemics or pandemics.
Is it time to panic or not yet?
“No, it is not time to panic yet. This is not an unprecedented event worldwide, albeit still rare. Almost always such cases cause very little between person transmission and are quickly cleared from the community. The concern only comes when there is clear evidence of sustained human to human transmission for a virus that has not previously been circulating in the community such that the community has little protective immunity. Currently, there is no such evidence for this latest event.”
Prof John Edmunds, Professor in the Centre for the Mathematical Modelling of Infectious Diseases, London School of Hygiene & Tropical Medicine, said:
“The detection of a novel influenza virus is unusual, but not unheard of. It can occur when influenza jumps across species, or two strains of flu recombine. Although these events usually result in viruses that struggle to transmit in their new hosts, there is always a danger that they can or that they may become further adapted (i.e. evolve to become more transmissible). Hence, these events are always serious and need to be carefully investigated. In this instance, it appears that the case was picked up from routine surveillance. This is worrisome as it might imply that the virus has already spread to some extent. Careful monitoring of the contacts of this case is now underway to establish if there has been any further transmission. The incident also demonstrates the importance of a strong surveillance and public health response system.”
Dr Ed Hutchinson, Senior Lecturer, Medical Research Council-University of Glasgow Centre for Virus Research, University of Glasgow, said:
“Viruses are heavily dependent on their hosts, so they usually find it hard to infect a different host species to the one they’re used to. Influenza A viruses are (by virus standards) unusually good at infecting a wide range of different species, but even they struggle at replicating in a new host species. A virus that jumps into a new host – for example from a pig into a human – usually doesn’t spread any further, causing what we call a ‘spillover infection.’ Because a spillover infection won’t spread more widely, this is mainly a concern if it makes the infected person ill: it’s good to know that in this case the individual recovered after a mild illness. However, it is very important to monitor spillover infections. This is because, on very rare occasions, these spillover viruses can become good enough at growing in their new human host to begin spreading from human to human. Very occasionally, this creates a new human-adapted virus that could cause a pandemic – although it is important to be clear that this report does not suggest that this particular spillover virus had got anything like that far.
“Influenza A viruses have a particular ability that helps them to get going in new host species. Human and animal influenza A viruses can ‘breed’ if they get into the same host, producing hybrid offspring that are well-adapted to growing in humans but which aren’t recognised by our immune responses to previous human influenza infections or vaccinations (a process called genetic shift). Because of this, it’s particularly important to monitor spillovers of influenza A viruses.”
Prof Peter Openshaw, Professor of Experimental Medicine, Imperial College London, said:
“Influenza affects many species, especially birds, humans, horses and pigs.
“H1N2 is one of the flu viruses currently circulating in pigs in the UK. There is a single human case in N Yorkshire detected as part of routine flu surveillance by UKHSA and RCGP. The illness was apparently mild and the person has fully recovered. The source of transmission has not been identified.
“This is newsworthy in that it’s the first time it’s been detected in humans in the UK, but it’s not the first human case. There have been a total of 50 human cases of influenza A(H1N2)v reported globally since 2005. Without evidence of human-to-human transmission it’s not particularly worrying; it shows surveillance is working.
“However, we know that seasonal influenza is now beginning to infect people in the UK – and for that reason it’s worth having the flu vaccine if it’s offered to you, even though it doesn’t protect you against this H1N2 pig virus strain.”
Prof Ian Brown, Director of Scientific Services, Animal and Plant Health Agency (APHA), said:
“Whilst this the first such GB detection of this strain of swine influenza in humans in context these events occur on a regular basis at global level. These viruses generally lack the ability to spread human to human and such events are usually explained by direct or indirect contact with pigs although to date the source of infection is unknown hence work is underway to track and understand. This particular sero group of influenza A virus has been in GB pigs for over 30 years and are often associated with herds suffering respiratory disease.”
Prof Punam Mangtani, Professor of Epidemiology at the London School of Hygiene & Tropical Medicine (LSHTM), said:
“Current information suggests that this individual presented with a mild illness and that tests confirmed this was the result of an infection by an influenza virus that circulates in pigs, known as a swine flu variant.
“Sequencing so far indicates some genetic differences to previous swine flu viruses that have spilled over to humans. It’s unusual to pick up such viruses, hence the need to check contacts and improve surveillance in that area to check whether there are more cases.
“Further genetic work, such as looking at its affinity to certain receptors in the respiratory linings of humans, may prove useful, as well as comparisons of amino acid sequences of the viruses in swine and this human case, which may help assess why this one human case might have occurred.
“Prompt action is being carried out by the UKHSA in active collaboration with their counterparts in veterinary health which will help to support good, high animal welfare standards and biosecurity while further information is obtained.”
Prof Ian Jones, Professor of Virology, University of Reading, said:
“It’s very unlikely the single case of H1N2 swine flu reported today represents anything more than has been seen in the past. And although a single case may not be representative, the fact the individual concerned had a mild infection that resolved without hospitalization is also in keeping with previous experience. We should remember that surveillance programmes can discover infections that might never have been noticed based on case numbers or severity, so a level of reasoned tolerance has to be applied. Not every new agent is a threat.”
Prof Paul Hunter, Professor in Medicine, UEA, said:
“Today the first human case of swine flu has been reported in the UK this season. It is apparently Influenza A H1N2. This subtype of influenza has been reported sporadically over recent decades from several different countries but has yet to cause a major outbreak in humans.
“In 2022, seven cases of human infection with influenza A(H1N2) were reported globally. With one case in Europe, The Netherlands https://www.ecdc.europa.eu/en/publications-data/zoonotic-influenza-annual-epidemiological-report-2022. One of these cases was hospitalised but the others had mild respiratory disease. Indeed most reports are of a relatively mild illness. As far as I can tell the last cases on influenza A(H1N2) to have been reported from the UK was in 2002.
“Some but not all cases of influenza A(H1N2) had a history of contact with pigs.
“Overall the evidence is that influenza A(H1N2) does not cause any more severe disease than other more commonly circulating types of influenza. Also person to person transmission, does not appear to be very efficient and sustained person to person transmission has not been reported so far.”
Prof Ian Jones: “No interests to declare.”
Prof Paul Hunter: “No COIs.”