Oral vaccine. The one developed by Sabin which I got as a kid. Today the polio vaccine used in the U.S. is an improved version of the injected polio vaccine developed by Salk which, unlike the Sabin vaccine, is based on inactivated viruses. It cannot cause polio, which is why it is the preferred vaccine in the U.S., but the Sabin vaccine, although it can cause polio in very rare cases, provides better protection against wild polio which is why it is the preferred vaccine in Third World countries where polio is or recently was epidemic.