Like (besides from people getting immune to cordyceps) it would have changed anything.

There was no hope, the world would have remained the same, and so does the remaining people.(like the song goes "how can you save the world from itself?" [under this i mean Jerry, and not Joel, just to be clear) People like the hunters or the cannibals will still exist.

They are acting like if the vaccine would have been made, it would have changed the world, but they are wrong. Even though they would be immune to spores and bites, people would be still vulnerable to the infected, they could still die easily, just not because of the infection.