system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What do the 'H's, 'N's, and numbers mean in bird/swine flu viruses? Eg; H1N1, H7N9, etc...
gpt
H and N stand for the names of two proteins on the surface of virus cells (Hemagglutinin and Neuraminidase). What they do is not particularly important, except that they have to be on the surface of every flu virus for it to survive, and antibodies are able to target them. Different versions of the different surface proteins mean new antibodies are needed, so a new vaccine is necessary for viruses with different versions of the surface proteins. So vaccines generally work for a specific H/N combination.

TL;DR: Surface proteins on the flu that are important for antibody targeting.

Edit: spelling.