A lot of the effective vaccines work on pathogens with a long infection cycle. A vaccine primed immune system is able to completely squelch an infection before it's life cycle is complete. Think rabies, you can vaccinate people after infection and it works. Things like measles and smallpox have long life cycles. Point trying to make, people exposed often are often infected but don't become symptomatic or infectious if they've been vaccinated.
Then you got other things like diphtheria and tetanus vaccines which are toxoid vaccines. They protect against the toxins not the bacteria itself. Tetanus toxin is so toxic that a small latent infection that your immune system doesn't know is there can kill you. Point, vaccines are a very broad category.
Difficulty with covid is it's got a respiratory route of infection and transmission. But unlike most respiratory viruses also causes systemic infection[1]. Which influenza generally doesn't. Point there is it's difficult to get herd immunity but the downsides of covid makes it a worthwhile effort.
[1] That which doesn't kill you can leave you a shell of who you were.
Then you got other things like diphtheria and tetanus vaccines which are toxoid vaccines. They protect against the toxins not the bacteria itself. Tetanus toxin is so toxic that a small latent infection that your immune system doesn't know is there can kill you. Point, vaccines are a very broad category.
Difficulty with covid is it's got a respiratory route of infection and transmission. But unlike most respiratory viruses also causes systemic infection[1]. Which influenza generally doesn't. Point there is it's difficult to get herd immunity but the downsides of covid makes it a worthwhile effort.
[1] That which doesn't kill you can leave you a shell of who you were.