well OK. So flu has two major antigens, H (haemaglutinin) and N (neuraminidase.) think of them as the sticky outy bits on the surface of the virus. They are the bits your immune system recognises. This is why flu is named like H1N1, or H5N7. The numbers are broad subtypes of H and N,
Now flu can mutate in two ways. Firstly is the way all viruses do, by just little mutations - basically like Chinese whispers (pun intended.) every replication produces little errors so our say H1N1 flu is a little different each year. But it’s broadly H1N1. This process is called
antigenic drift. Once you’ve had any of them you’ll be a little bit immune to all . You might still get sick but it’s unlikely to be catastrophic
The second way flu mutates is
antigenic shift. This is generally what creates new flus that cause pandemics. because flu doesn’t just circulate in humans, it also circulates in pigs and water fowl and birds. So all sorts of stuff is sloshing around in birds and pigs.
If one of the following happens, it can cause reassortment and shift:
1. A new bird flu, let’s say H5N1 jumps bird to human - it’s totally novel.
2. a human or pig gets infected with two types of flu at once, they swap antigens and a brand new flu pops out. Jumps to humans or spreads in humans.
3. Bird flu jumps to another species like a pig then to humans.
All these events are like shuffling the deck - now you’ve got H5N1, or H7N9. different combinations of an H and an N. And it’s new and nobody has any immunity and if it can spread person to person, we are fucked.
the people who create the flu vaccine each year look at sentinel samples to see what strains are circulating and they make a guess as to which few go into the vaccine that year. They also test to see if new strains are arising. Although it tends to be obvious, because you get massive die off of animals. Or people
