Through continued study on an additional 26 monkeys, Brodie ultimately concluded that administration of live virus vaccine tended to result in humoral immunity while administration of killed virus vaccine tended to result in tissue immunity.Soon after, following a similar protocol to Kolmer, Brodie proceeded with self experimentation upon himself and his co-workers at the NYC Health Department laboratory. Brodies progress was eagerly covered by popular press as the public hoped for a successful vaccine to become available. Such reporting did not make mention of the 12 children in a New York City Asylum who were subjected to early safety trials. As none of the subjects experienced ill effects, Dr. Park, described by contemporaries as "never one to let grass grow under his feet," declared the vaccine safe. When a severe polio outbreak overwhelmed Kern County, California it became the first trial site for the new vaccine on very short notice. Between November 1934 - May 1935, over 1,500 doses of the vaccine were administered in Kern County. While initial results were very promising, insufficient staffing and poor protocol design left Brodie open to criticism when he published the California results in August 1935. Through private physicians, Brodie also conducted a broader field study, including 9,000 children who received the vaccine and 4,500 age- and location-matched controls who did not receive a vaccine. Again, results were promising. Of those who received the vaccine, only a few went on to develop polio.
With wild polio cases at record lows, 2017 was the first year where more cases of cVDPV were recorded than the wild poliovirus, a trend that is expected to continue.To combat this, the WHO in 2016, decided to switch from the trivalent polio vaccine to the bivalent polio vaccine. This vaccine no longer contains the type 2 polio virus because it was eradicated in 1999. Contamination concerns
In 1960, the rhesus monkey kidney cells used to prepare the poliovirus vaccines were determined to be infected with the simian virus-40 (SV40), which was also discovered in 1960 and is a naturally occurring virus that infects monkeys. In 1961, SV40 was found to cause tumors in rodents. More recently, the virus was found in certain forms of cancer in humans, for instance brain and bone tumors, pleural and peritoneal mesothelioma, and some types of non-Hodgkin lymphoma. However, SV40 has not been determined to cause these cancers.SV40 was found to be present in stocks of the injected form of the IPV in use between 1955 and 1963. It is not found in the OPV form. Over 98 million Americans received one or more doses of polio vaccine between 1955 and 1963 when a proportion of vaccine was contaminated with SV40; an estimated 10–30 million Americans may have received a dose of vaccine contaminated with SV40.