>>13611712There are two reasons. Tetanus for example is because we think immunity subsides after several years and your memory cells aren't as effective. That's actually debatable and more of an overprotective western medicine mindset, but it boils down to the vaccines actually are safe and effective so why not give a booster each decade or on exposure. Tetanus also caused by a bacteria's toxic byproduct, not a virus.
Yearly vaccines like the flu vaccine are developed each year to try to create immunity to the expected prevailing strain. These viruses mutate frequently and immune systems only have partial protection from previous exposure to similar viruses. As a side note, this is what makes novel diseases more dangerous since the population doesn't have any similar immunity to rely on. This is also why naturally acquired immunity us more robust than the covid mrna vaccines as mrna is building antibodies for one specific s-protien but natural immunity can target s or n proteins or use some other method. Then on reinfection there's a higher chance the immunity system has memory of some mechanism to fight the infection.
