It's probably pretty easy to make a "vaccine" to prevent infection with a virus that isn't real or to prevent a disease that doesn't exist.

In the past, one technique used to convince the public that a vaccine is effective is to redefine "cases" of a disease once the vaccine is rolled out. Right now a "positive test" appears to be meaningless, so it will be quite simple to "prove" a vaccine "prevents" COVID-19.

I wonder if this is why President Trump was VERY confident of speedy vaccine development.