In the 1950s in the US it was noticed that after vaccination paralytic polio sometimes occurred. In the 1990s after it began happening in India and Africa it was confirmed: " Through
their laboratory work, they discovered that tissue injury
produced by an injection aided the poliovirus to infect
the body and readily journey to the spinal cord. For the
first time, health professionals working in polio endemic
regions had scientific evidence that pediatric injections
could incite paralysis.".