In the midst of the still ongoing SARS-CoV-2 pandemic, we now have another virus to worry about, the H5N1 influenza virus which has just caused one fatality and several cases in Cambodia.
What is the H5N1 virus?
We are all familiar with the flu virus, this is the same virus that causes typically mild illness during the winter months, with the most common versions we normally encounter being the H1N1 and H3N2 subtypes. Subtypes for the flu are somewhat similar to the different variants of SARS-CoV-2; when a new variant or subtype starts spreading, chances are that previous vaccination or infection will no longer be as protective. The same is true for flu, previous infection or vaccination with an H3N2 subtype will likely not offer much protection against infection with an H5N1 subtype.
However, the H5N1 flu virus is quite different from the types of flu we normally think of. First, these viruses usually spreads between birds, where it is often lethal, giving them the name Highly Pathogenic Avian Influenza Viruses (HPAI), which includes H5N1 and other similar flu subtypes. Outbreaks of these viruses can have devastating effects on both wild and farmed bird flocks. Thankfully though, the H5N1 virus doesn’t spread to people very easily and even then, further person-to-person transmission is quite rare.
How dangerous is it?
In the 26 years since the H5N1 flu virus first emerged, there have only been 868 cases of human infection. However, the concerning part is that of these 868 cases, 457 have died, giving an estimated case fatality rate of 53 per cent, which makes the H5N1 virus one of the most lethal that we know.
The critical concern here is that in recent years, H5N1 and other HPAI viruses have been spreading around the world, with repeated outbreaks in many countries, including the UK, which recently saw its first human case of H5N1 infection. The more cases of bird flu that we have in any region, the more chances that it will spread to people or other animals.
In 2022, the virus jumped from birds to farmed minks in Spain. There the virus caused severe illness in the minks, but also began to spread from animal to animal, something that hadn’t been seen before and may have been facilitated by the close confines the farmed animals are kept in.
The flu virus evolves quickly and so there were fears that rapid evolution of the virus in minks would result in a virus that could easily infect people. Thankfully, this didn’t happen, 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 enable better growth and transmission in mammals.
Can H5N1 infect humans?
With the growing number of H5N1 outbreaks in birds, and the large scale of the farmed animal industry, there are more and more chances for adaptive mutations to occur, raising the possibility of a human-transmissible H5N1 virus emerging.
If a human-transmissible version of H5N1 does emerge, there is a good chance that it would rapidly spread, as most people have no immunity to this subtype of flu, which could potentially result in a new pandemic. The big unknown though, is that if this H5N1 pandemic did arise, would the virus still be as lethal?
A pandemic of H5N1 with a 53 per cent fatality rate would be unimaginable; however, some studies have suggested that when the virus switches hosts and adapts to mammals the lethality drops down significantly. This is a glimmer of hope, but as we’ve seen over the past 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 to a human-transmissible version of H5N1 is not a sure thing. We’ve been tracking these viruses since 1996 and despite many chances, the virus has never made this jump. Even with the significant increase in avian H5N1 outbreaks in recent years, the number of human cases has remained very low. It’s very possible it will never happen, that the virus is too well adapted to birds and that limits its ability to infect people.
How can we stop it spreading?
However, hope is not a strategy to control the spread of an infectious disease. As such, we have a robust H5N1 monitoring network and many governments around the world respond rapidly to any outbreaks of bird flu, in order to minimise the chances for human spread. In addition, we have two antivirals that would likely offer some benefit against H5N1 infection and there are several licensed H5N1 vaccines.
Unfortunately, just like with SARS-CoV-2, the virus keeps evolving and so our existing H5N1 vaccines may not offer robust protection against the current versions of H5N1, though studies suggest they may still work very well and they also give us a very good starting point for making updated versions of the vaccines.
An additional wild-card in the emerging H5N1 situation is the SARS-CoV-2 pandemic. We know that even mild cases of SARS-CoV-2 can cause immune alterations that persist after resolution of the acute infection. We do not know how, and if, this altered immunity would affect human H5N1 infections or transmission, but it is an additional aspect that needs continual monitoring. At present, the situation poses very little threat for most people, but the situation needs careful monitoring and preparation, as it could change at any time with dire consequences.
Read more about viruses:
