[choward]

That doesn't make any sense. The worse the virus is the less you should need to "sell" the vaccine.

If it was a rushed vaccine to prevent you from becoming a zombie you wouldn't need to sell it.

If it was a vaccine for the common cold you would need to sell it.

[mombul]

Of course, you're right, it doesn't make sense and that's not how it should work. Unfortunately, science and medicine are fields that are largely driven by money. Laboratories don't really care about a vaccine for a disease in Africa that nobody will be able to afford or invest in. They will turn to better markets. There are multiple reasons, the most obvious one is that they're private companies, their goal is to make profit. The second one is that it's a field that requires a lot of resources. So you need a big ROI.

One of the reasons the vaccine for COVID-19 was produced so quickly is not because of some sort of conspiracy, like they had the antidote all along or something. It's just that when there's a real buck to make, it becomes a race between these big pockets companies.

That's one side of it and I don't know enough to cover every aspect of it. Also, it's just my opinion of course and how I see this.

Edit: I'm doubtful about a zombie vaccine being given for free. Most likely in a Walking Dead type scenario, where the economy would completely go down, there wouldn't be any financial incentive to keep the searches going, and no money to back up the researches either. So we'd be scr*wed I think. :o)