Between 1970 and 1984, Romania participated in the safety trials of an oral vaccine sponsored by the WHO. It turned out, that, for unexplained reasons, the incidence of the vaccine-associated paralytic poliomyelitis (VAPP) is 5-17 higher in Romania than in other countries. It was first believed that the vaccine in Romania was too virulent, but replacing it with a different vaccine did not change the VAPP incidence. It turned out that 87% of paralyzed children received intramuscular injections 30 days before the onset of paralysis (17 injections on average), as compared to 51% of non-paralyzed children (3 injections on average). Each injection increased the risk of paralysis by 13%. Paralysis was observed 8 times more often in those who received one injection, and 182 times more often in those who received over 10 injections. Overall, 86% of VAPP cases were caused by injections.