Hi again, Flu here.
We flu viruses change in 2 ways.
The first is called antigen drift.
These are small changes to our Ha and Na pieces.
The old flu vaccines can't find our new looks.
So each year a new vaccine is needed.
The second is called antigen shift.
We jump between animals like birds or pigs.
We get ourselves brand new Ha or Na pieces,
so we look like brand new viruses.
People usually don't have immunity to these new viruses.
This is one of ways we get 'influenza pandemics'