Also two regions in Brazil are interested in the incompletely-tested Russian vaccine. If it backfires, that could cause resistance in the population against a successful one. That's a good time for a right-wing negationist governant to be anti-commie.
Unless the vaccine kills a lot of people, I don't see more resistance to future vaccines being likely (that is, more resistance than there is at present). If it fails and gives them coronavirus, it's not a big deal. If it fails and does nothing, it is also not a big deal. It's not like the same risk of giving them polio or smallpox or something.
If it fails and sensitizes people to the virus, so that they have more severe reactions to it, it could be a very big deal.
If vaccination causes people to relax other hygiene practices due to a false belief that the vaccine protects them, then that will make the next wave worse.
tldr?
You test vaccines by waiting to see if the vaccinated group gets sick less often than the control group, so you need a place with a high rate of public transmission. It's also helpful if that place has doctors with expertise in vaccine trials. Brazil has both.