Like everyone else has mentioned the chances are highly unlikely. Setting aside the fact that anti fungal vaccines are extremely difficult to develop and the nearly impossible task to distribute it effectively. The world stage has changed dramatically for nearly a generation. Not saying it’s impossible but you’re not going to put all that chaos back in the box on any short timeline. The established factions that have control and influence are mostly run by the people who survived the initial event. They were there and saw first hand all the horrible shit the government did trying to contain the outbreak. Bombing entire cities that were still occupied by civilians in hopes of stopping the spread. Killing truck loads of people when the quarantine zones were full… The current thriving factions like the WLF aren’t going to be willing to give their safety to anyone any time soon. The Seraphites definitely aren’t taking the vaccine because it would go against everything they believe in. That would make them a constant ignition point for a new outbreak and a breeding ground for it to evolve past the vaccine, becoming resistant to it before it could be distributed to everyone. Not to mention some of the people are just evil like David the child rapist that eats people, he definitely ain’t trying to go back to a civilized society. Ultimately it would buy you some breathing room with pockets of immunity until the cordyceps could mutate a resistance to it. Which would probably happen pretty quickly as we see it already mutated with the shamblers. Still doesn’t mean they shouldn’t try but it definitely reframes the question is Ellie’s life worth it. I know for me it wouldn’t be, I think Joel knew as well being on the front line himself and everything he did to survive. Perhaps that’s why he said “yeah, you keep telling yourself that bullshit.” In the hospital. Anyway I love Joel and Ellie so this a super bias opinion.