We currently have an epidemic of bird flu which can pass between birds and from birds to humans but cannot pass between humans. There is a great fear that the virus will evolve into one which can pass between humans causing a pandemic resulting in millions of deaths.

We cannot develop a vaccine for the evolved virus until it comes into existance and then we will have great difficulty producing enough of it before the virus spreads to millions or perhaps billions.

Now many people do not believe in evolution so, for them, it is impossible for the virus to evolve into one which can pass between humans. Therefore, they do not have to be vaccinated against bird flu.
So, taking this into consideration, we may not have a problem producing enough vaccine in time.

Bev and Jerry