April 12th, 2021
Anniversary of announcement that the polio vaccine developed by American physician Dr. Jonas E. Salk was "safe", potent and effective". Incidence of infantile paralysis or poliomyelitis declined by 95% following introduction of preventive vaccines. The first mass inoculations of children with the Salk vaccine began in Pittsburgh in 1954.
Source: www.historyofvaccines.org/content/articles/history-polio-poliomyelitis