1948
A breakthrough came in 1948 when a research group headed by John Enders at the Childrens Hospital Boston successfully cultivated the poliovirus in human tissue in the laboratory. This group had recently successfully grown mumps in cell culture. In March 1948, Thomas H. Weller was attempting to grow varicella virus in embryonic lung tissue. He had inoculated the planned number of tubes when he noticed that there were a few unused tubes. He retrieved a sample of mouse brain infected with poliovirus and added it to the remaining test tubes, on the off chance that the virus might grow. The varicella cultures failed to grow, but the polio cultures were successful. This development greatly facilitated vaccine research and ultimately allowed for the development of vaccines against polio. Enders and his colleagues, Thomas H. Weller and Frederick C. Robbins, were recognized in 1954 for their efforts with a Nobel Prize in Physiology or Medicine. Other important advances that led to the development of polio vaccines were: the identification of three poliovirus serotypes (Poliovirus type 1 – PV1, or Mahoney; PV2, Lansing; and PV3, Leon); the finding that prior to paralysis, the virus must be present in the blood; and the demonstration that administration of antibodies in the form of gamma globulin protects against paralytic polio.
Three doses of live-attenuated OPV produce protective antibodies to all three poliovirus types in more than 95% of recipients. OPV produces excellent immunity in the intestine, the primary site of wild poliovirus entry, which helps prevent infection with wild virus in areas where the virus is endemic. The live virus used in the vaccine can rarely shed in the stool and can rarely spread to others within a community. The live virus also has stringent requirements for transport and storage, which are a problem in some hot or remote areas. As with other live-virus vaccines, immunity initiated by OPV is probably lifelong.The trivalent (against wild types 1, 2, and 3) OPV has been used to nearly eradicate polio infection worldwide. Led by the Global Polio Eradication Initiative, 155 countries switched to use the bivalent (against wild types 1 and 3) between 17 April and 1 May 2016. The bivalent OPV is more effective against types 1 and 3, but does not cover type 2. The United States as of 2017 continues to recommend the use of a trivalent version, but a fully inactivated version. The switch to the bivalent vaccine and associated missing immunity against type 2 strains, among other factors, led to outbreaks of circulating vaccine-derived poliovirus type 2(cVDPV2), which increased from 2 cases in 2016 to 1037 cases in 2020.