system
Given the following post, respond as if you're commenting on Reddit:
human
Why do some vaccines only need to be given once, while others such as the flu jab need to be done yearly?
gpt
The flu virus is highly unstable, anytime a person have two types of virus at the same time, he can create a third hybrid type that wouldn’t respond as well to the vaccine. 

That is why it is given yearly, of the several hundred subtypes, each year your get the ones most likely to affect the next year. If it is a match, great you don’t get the disease, but usually you get a slightly different version, in which case the vaccine works but not fully, you get a milder flu than an unvaccinated person.