Variolation already exists back then, but with a survival rate of 1-2%. What Edward Jenner did was to confer scientific status on the idea of vaccination and investigated cowpow as a much safer method of inoculation against smallpox.

Heck, decade earliers, George Washington inoculated his troops against smallpox against the wishes of the continental congress.

For one thing, a very different disease. For another, how long did it take him?

And depending upon how you count, it took decades to develop a polio vaccine.

I would not bet on a COVID vaccine being developed and tested in about a year in 1990.