Editing Polio vaccine (section)

===Attenuated===
Oral polio vaccines were easier to administer than IPV, as they eliminated the need for sterile syringes, so were more suitable for mass vaccination campaigns. OPV also provided longer-lasting [[immunity (medical)|immunity]] than the Salk vaccine, as it provides both [[humoral immunity]] and [[cell-mediated immunity]].<ref name="ReferenceZ">{{cite journal|vauthors=Wahid R, Cannon MJ, Chow M|date=May 2005|title=Virus-Specific CD4+ and CD8+ Cytotoxic T-Cell Responses and Long-Term T-Cell Memory in Individuals Vaccinated against Polio|journal=Journal of Virology|volume=79|issue=10|pages=5988–5995|doi=10.1128/JVI.79.10.5988-5995.2005|pmc=1091702|pmid=15857985}}</ref>

One dose of trivalent OPV produces immunity to all three poliovirus serotypes in roughly 50% of recipients.<ref name=PinkPages/> Three doses of live-attenuated OPV produce protective antibodies to all three poliovirus types in more than 95% of recipients. As with other live-virus vaccines, immunity initiated by OPV is probably lifelong.<ref name="Robertson_1993" /> 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 (epidemiology)|endemic]].<ref name=Peds/>  OPV does not require special medical equipment or extensive training. Attenuated poliovirus derived from the OPV is excreted for a few days after vaccination, potentially infecting and thus indirectly inducing [[Contact immunity|immunity]] in unvaccinated individuals, thus amplifying the effects of the doses delivered.<ref name="Nathan">{{cite journal |vauthors=Nathanson N, Martin JR |date=December 1979 |title=The epidemiology of poliomyelitis: enigmas surrounding its appearance, epidemicity, and disappearance |journal=American Journal of Epidemiology |volume=110 |issue=6 |pages=672–692 |doi=10.1093/oxfordjournals.aje.a112848 |pmid=400274}}</ref> Taken together, these advantages have made it the favored vaccine of many countries, and it has long been preferred by the global eradication initiative.<ref name="GPEI-OPV">{{cite web |title=OPV - Oral Polio Vaccine |url=https://polioeradication.org/polio-today/polio-prevention/the-vaccines/opv/ |access-date=9 August 2024 |website=Global Polio Eradication Initiative (GPEI) |publisher=}}</ref>[[File:Number of cVDPV cases.svg|thumb|upright=1.3|cVDPV cases (red line) outnumbered wild polio cases (blue line) for the first time in 2017]]
The primary disadvantage of OPV derives from its inherent nature. As an attenuated but active virus, it can induce vaccine-associated paralytic poliomyelitis (VAPP) in roughly one individual per every 2.7{{nbsp}}million doses administered.<ref name="GPEI-OPV" /> The live virus can circulate in under-vaccinated populations (termed either '''variant poliovirus''' or '''circulating vaccine-derived poliovirus''', cVDPV), and over time can revert to a neurovirulent form causing paralytic polio.<ref name="GPEI-OPV" /> This genetic reversal of the pathogen to a virulent form takes a considerable time and does not affect the person who was originally vaccinated. With wild polio cases at record lows, 2017 was the first year where more cases of cVDPV were recorded than the wild poliovirus.<ref>{{Cite web |title=Eradication of polio – Is Syria being left behind? |url=https://sphcm.med.unsw.edu.au/infectious-diseases-blog/eradication-polio-syria-being-left-behind |url-status=live |archive-url=https://web.archive.org/web/20181006114450/https://sphcm.med.unsw.edu.au/infectious-diseases-blog/eradication-polio-syria-being-left-behindd |archive-date=6 October 2018 |access-date=6 October 2018 |website=UNSW School of Public Health and Community Medicine |vauthors=Kunasekaran M}}</ref>

Until recent times, a trivalent OPV containing all three viral strains was used, and had nearly eradicated polio infection worldwide.<ref name="ReferenceA">{{cite journal |vauthors=Marin M, Patel M, Oberste S, Pallansch MA |date=January 2017 |title=Guidance for Assessment of Poliovirus Vaccination Status and Vaccination of Children Who Have Received Poliovirus Vaccine Outside the United States |journal=MMWR. Morbidity and Mortality Weekly Report |volume=66 |issue=1 |pages=23–25 |doi=10.15585/mmwr.mm6601a6 |pmc=5687270 |pmid=28081056}}</ref> With the complete eradication of wild poliovirus type{{nbsp}}2<ref>{{cite web |date=22 July 2019 |title=Poliomyelitis |url=https://www.who.int/news-room/fact-sheets/detail/poliomyelitis |url-status=live |archive-url=https://web.archive.org/web/20191017233318/https://www.who.int/news-room/fact-sheets/detail/poliomyelitis |archive-date=17 October 2019 |access-date=18 October 2019 |website=[[World Health Organization]] (WHO)}}</ref> this was phased out in 2016 and replaced with bivalent vaccine containing just types 1 and 3, supplemented with monovalent type{{nbsp}}2 OPV in regions where cVDPV type 2 was known to circulate.<ref name="GPEI-OPV" /> 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 two cases in 2016 to 1037 cases in 2020.<ref>{{Cite web |title=GPEI Strategy for the Response to cVDPV2 2020–2021 |url=https://polioeradication.org/wp-content/uploads/2021/03/GPEI-cVDPV2-nOPV2-Factsheet-20210312-EN.pdf |url-status=live |archive-url=https://web.archive.org/web/20210801223541/https://polioeradication.org/wp-content/uploads/2021/03/GPEI-cVDPV2-nOPV2-Factsheet-20210312-EN.pdf |archive-date=1 August 2021 |access-date=1 August 2021 |website=Polio Global Eradication Initiative}}</ref> 

A novel OPV2 vaccine (nOPV2), which has been genetically modified to reduce the likelihood of disease-causing activating mutations, was granted emergency licencing in 2021, and subsequently full licensure in December 2023.<ref name=":x22">{{Cite web |date=12 April 2024 |title=GPEI-OPV Oral polio vaccine |url=https://polioeradication.org/polio-today/polio-prevention/the-vaccines/opv/ |access-date=12 April 2024 |website=Global Polio Eradication Initiative - World Health Organization |language=en-GB}}</ref> This has greater genetic stability than the traditional oral vaccine and is less likely to revert to a virulent form.<ref name=":x2">{{Cite web |date=12 April 2024 |title=GPEI-OPV Oral polio vaccine |url=https://polioeradication.org/polio-today/polio-prevention/the-vaccines/opv/ |access-date=12 April 2024 |website=Global Polio Eradication Initiative - World Health Organization |language=en-GB}}</ref><ref name="GPEI-nOPV2" /> Genetically stabilised vaccines targeting poliovirus types 1 and 3 are in development, with the intention that these will eventually completely replace the Sabin vaccines.<ref>{{Cite web |last=Gadye |first=Levi |date=2023-06-14 |title=Two New Vaccines Join the Fight to Eradicate Polio {{!}} UC San Francisco |url=https://www.ucsf.edu/news/2023/06/425601/two-new-vaccines-join-fight-eradicate-polio |access-date=2024-08-10 |website=The University of California San Francisco |language=en}}</ref>