There is a higher rate of mutation in flu virus antigens than in tetanus bacteria, so you need to change the antibodies you are producing (through vaccination) every year for the flu.

Infections that enter the body through the bloodstream (like tetanus) are much less toxic than infections that enter the body through binding to epithelial cells (like the flu).

The flu is everywhere but bacterial infections are incredibly rare, so you don’t need to be as careful in maintaining your immunity against them.

Antibody production against viruses drops off about 10 times faster than antibody production against bacterial infections, so you need to be vaccinated 10 times more frequently against the flu.