[yuubi]

And time and again the smallpox vaccine also failed to protect.

> Historically, the vaccine has been effective in preventing smallpox infection in 95% of those vaccinated. — https://www.cdc.gov/smallpox/vaccine-basics/index.html

(Other sources have numbers from about 90% to 97%). However, this imperfect protection, in enough people, was enough to do the job.