DW Ever been confused by the names for bird flu or Spanish flu H1N1? Type A influenza has 130 known subtypes, but which ones cause the flu in people? Here’s what the numbers mean.
Avid virus-watchers will know that influenza viruses that make the news often feature the letters “H” and “N” in their names, such as H5N1 or H9N2.
Those are examples of “Type A” influenza viruses, classed as highly infectious pathogens and a significant threat to influenza viruses that make the news.
However, there are also B, C and D types, with various subtypes and lineages, many of which are colloquially known as avian/bird, cow and swine flues.
You can forgive yourself for feeling confused because it is confusing, and it’s not immediately clear which types of flu risk public health to the extent of the Spanish flu of 1918 or a COVID-style pandemic.
So, here’s DW’s guide to help you navigate the maze of influenza code. We’ll start with those four types of the virus.
The four types of influenza
As mentioned above, there are four types of influenza: A, B, C and D.
Types A and B cause seasonal, epidemic outbreaks of influenza among humans during winter months. But only type A is known to cause pandemics.
Type A influenza viruses often originate in aquatic birds and spread among 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 (consistently present) in humans, birds and pigs. Annual flu vaccines help protect us against strains of H1N1 viruses, among others.
The A(H1N1)pdm09 viruses, adapted from H1N1, caused the Spanish Flu pandemic of 1918, which killed 20-50 million people. The variant has been genetically traced to flu outbreaks in swine populations (swine flu) as recently as 2009.
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.
A major outbreak of H5N1 is also currently ongoing in birds and cattle in the US, but there is no evidence yet that it is spreading in humans.