Have you ever been confused by the names bird flu or Spanish flu H1N1? There are 130 known subtypes of type A influenza, but which subtypes cause flu in people? Here’s what the numbers mean.
Virus watchers may know that influenza viruses in the news often have names that contain the letters “H” and “N,” such as H5N1 or H9N2.
These are examples of “Type A” influenza viruses — classified as highly contagious pathogens and which pose a significant threat to the influenza viruses that are in the news.
But there are also types B, C and D, which have various subtypes and lineages, several of which are colloquially known as avian/bird, cow and swine flu.
Also read: WHO confirms human case of bird flu in India
You can forgive yourself for being confused — because it is confusing: And it’s not immediately clear which type of flu poses a public health threat to the level of the 1918 Spanish flu or a COVID-style pandemic.
So, here’s DW’s guide to help you navigate the labyrinth of influenza codes. We’ll start with those four types of the virus.
four types of influenza
As mentioned above, there are four types of influenza: A, B, C, and D.
Type A and B Type A causes seasonal, epidemic outbreaks of influenza in humans during the winter months. But only type A causes epidemics.
Type A influenza viruses often originate in aquatic birds and spread to bird species – which is then known as avian influenza or bird flu. But they can also spread to other mammals if the virus has the right mutations.
Some strains of the type A virus H1N1 are endemic (constantly present) in humans, birds and pigs. Annual flu vaccines help protect us from other than H1N1 virus strains.
The H1N1-adapted A(H1N1)pdm09 virus caused the 1918 Spanish flu pandemic, which killed 20-50 million people. This variant has been genetically traced to a recent flu outbreak in pig populations in 2009 (swine flu).
At the time of writing in June 2024, only two subtypes of influenza A – A(H1N1)pdm09, and A(H3N2) – were co-circulating (at the same time) among humans.
There is currently a large outbreak of H5N1 in birds and cattle in the United States, but there is no evidence yet that it is spreading to humans.
Type C Influenza viruses can infect humans and other mammals, such as pigs, but cause mild illnesses in people. Flu caused by type C viruses is rarer than those caused by type A viruses.
And Type D Influenza viruses mainly affect cattle. They can also spread to other animals, but no infections in humans have been reported.
Subtypes of influenza A viruses
Now let’s unravel those “HxNy” codes. We’ll focus on type A influenza because they are the ones that use this specific denomination and pose the greatest threat to human public health.
Type A influenza viruses are classified according to the two types of proteins found on the surface of the virus:
Hemagglutinin (H or HA)
Neuraminidase (N or NA)
Hemagglutinin and neuraminidase are found in all types of influenza and they work as a viral team: hemagglutinin helps the virus attach to the cell it infects, and neuraminidase releases the virus to infect other cells – in simple terms, this is how the virus spreads through your body.
These two proteins determine the “infectiousness” and “pathogenicity” of viruses – that is, how dangerous they are to your health.
There are 130 known H+N combinations. But since viruses are good at “recombining” – a process by which viruses swap genetic information – many more combinations are likely. Recombination in viruses can occur when, for example, two subtypes of a virus infect the same host (a person or non-human animal) at the same time.
This includes the fact that new H+N subtypes emerge in the wild. For example, until recently, researchers spoke of 16 hemagglutinin subtypes in influenza, but now two more subtypes are known to exist: H17 and H18, which are found in two species of bats.
When H and N are unknown numbers, H takes the variable “x” and N takes the variable “y”.
Type B influenza denomination
Type B influenza viruses are named after their lineage. And there are two: Yamagata and Victoria.
The names of type B influenza viruses are not as diverse as those of type A, as the H+N subtypes in type B influenza viruses do not appear to be as distinct within their lineage.
However, they can be further defined into clades and subclades, or groups and sub-groups.
For example, the B/Victoria influenza virus includes the V1A clade and subclades V1A.1, V1A.2, and V1A.3. B/Yamagata includes the Y1, Y2, and Y3 clades, but has no known subclades.
Types of Influenza to Watch Out For (If You’re Human)
Influenza viruses that originate from animals do not often affect humans, but when they do, we speak of zoonotic transmission. This is the same for any disease where there is animal-to-human-animal transmission or vice versa.
Five subtypes of avian influenza A viruses cause human infections — H5, H6, H7, H9, and H10 viruses.
Most zoonotic forms of influenza – from birds to humans – are A(H5N1) and A(H7N9).
A(H5N6) – known as highly pathogenic avian influenza (HPAI) – and A(H9N2) – known as low pathogenic avian influenza (LPAI) – have also caused human infections.
As mentioned above, type C influenza viruses cause only mild illnesses and type D influenza viruses do not affect humans at all.
