How a human avian flu pandemic could evolve

As the SARS-cov-2 pandemic is still ongoing, we have another virus to fear, the H5N1 flu virus, which has just caused death and a number of infections in Cambodia. What is H5N1 virus? We all know the influenza virus, a virus that causes typically mild diseases during the winter months, with the most common versions that we normally encounter are the subtypes H1N1 and H3N2. Influenza subtypes are somewhat similar to different variants of SARS-cov-2; when a new variant or subtype begins to spread, it is likely that the previous vaccination or infection is no longer so protective.

 The same applies to influenza, a previous infection or vaccination with an H3N2 subtype will probably not offer much protection against infection with an H5N1 subtype. However, the H5N1 flu virus is very different from the kinds of flu we usually think about. First, these viruses usually spread among birds, among which it has a high rate of lethality, giving them the name of highly pathogenic avian influenza virus (HPAI), which includes H5N1 and other similar influenza subtypes. Outbreaks of these viruses can have devastating impacts on livestock and wild bird herds. Fortunately, however, the H5N1 virus does not spread very easily to people and even in this case, further transmission from person to person is quite rare.

In the 26 years since the first appearance of the H5N1 influenza virus, there have been only 868 cases of human infection. However, the worrying part is that of these 868 cases, 457 died, with an estimated death rate of 53%, making the H5N1 virus one of the deadliest we know. The fundamental concern here is that in recent years, H5N1 and other HPAI viruses have spread around the world, with repeated outbreaks in many countries, including the UK, which has recently seen its first human case of H5N1 infection. The more cases of avian influenza we have in any region, the greater the chances of it spreading to people or other animals. In 2022, the virus passed from birds to farmed minks in Spain. There the virus caused serious diseases in mink, but it also began to spread from animal to animal, a phenomenon that had never been seen before and that may have been facilitated by the narrow boundaries in which farm animals are kept. The influenza virus evolves rapidly, so there were fears that the rapid evolution of the virus in mink could result in a virus that could infect people. Fortunately, this has not happened yet, but we are seeing more and more cases of mammals infected with H5N1, from seals to bears, with some of these infections showing evidence of mutations that allow better growth and transmission in mammals. Can H5N1 infect humans? With the increasing number of H5N1 outbreaks in birds and the large scale of the livestock industry, there is an increasing likelihood of adaptive mutations occurring that increase the chance of an H5N1 virus emerging that is transmissible to humans. If a human-transmissible version of H5N1 emerges, there is a good chance that it will spread quickly, as most people do not have immunity to this influenza subtype, which could potentially cause a new pandemic. The big question, though, is that if this H5N1 pandemic happened, would the virus still be as deadly? A H5N1 pandemic with a mortality rate of 53% would be unimaginable; however, some studies have suggested that when the virus changes host and adapts to mammals, lethality decreases significantly. This is a glimmer of hope, but as we have seen in recent years, it is impossible to predict exactly what a virus will do or how it will evolve in nature. The good news is that this evolution towards a human-transmissible version of H5N1 is not certain. Despite continuous monitoring since 1996, no scientist has ever seen the virus make this leap. Even with the significant increase in avian H5N1 outbreaks in recent years, the number of human cases has remained very low. It is very likely that it will never happen, that the virus has adapted too well to birds, and this limits its ability to infect people. But hope is not a strategy against the spread of an infectious disease. Despite this, a strong H5N1 monitoring network and many governments around the world respond quickly to outbreaks of avian influenza in order to minimize the chances of human spread. In addition, there are several antiviral drugs that could offer some benefit against H5N1 infection and there are several authorized H5N1 vaccines and studies suggest that vaccines could still work very well and also give us an excellent starting point for to make updated versions of vaccines. Currently, this is a minimal threat to most people, but it requires careful follow-up and preparation, as it could change at any time with disastrous consequences.

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