Just an FYI, the “flu” vaccine, depending on which shot you get, protects against 3-4 strains of the flu. The WHO decides each season which strains of flu are likely to be most prevalent that year, informs the vaccine manufacturers, and that year’s flu vaccine is made to order. But it doesn’t protect against any strain others than those it was designed for. That’s why even people who get their annual flu shot still sometimes get “the flu.” And, of course, it doesn’t protect if you don’t get the shot.
If you want to get technical, influenza “Type A” strains cause most of the human illnesses. The influenza A virus can be subdivided into different serotypes based on the type of hemagglutinin and neuraminidase. The "H" in "H1N1" for example refers to hemagglutinin (HA) and "N" in "H1N1" refers to neuraminidase (NA). There are 16 different types of HA and 9 different types of NA, therefore, there are potentially 144 different subtypes of influenza A viruses. Among them, two subtypes of influenza A, H1N1 and H3N2, most commonly infect humans. For each subtype virus, the hemagglutinin gene mutates all the time and hence there are many variants of the same subtype viruses, and hence the need to change the virus strain for seasonal flu vaccines on an annual bases.
https://www.sinobiological.com/research/virus/influenza-hemagglutinin-subtypes