title: Population Immunity against Serotype-2 Poliomyelitis Leading up to the Global Withdrawal of the Oral Poliovirus Vaccine: Spatio-temporal Modelling of Surveillance Data.
abstract: BACKGROUND
  Global withdrawal of serotype-2 oral poliovirus vaccine (OPV2) took place in April 2016. This marked a milestone in global polio eradication and was a public health intervention of unprecedented scale, affecting 155 countries. Achieving high levels of serotype-2 population immunity before OPV2 withdrawal was critical to avoid subsequent outbreaks of serotype-2 vaccine-derived polioviruses (VDPV2s).
  
  
  RESULTS
  In August 2015, we estimated vaccine-induced population immunity against serotype-2 poliomyelitis for 1 January 2004-30 June 2015 and produced forecasts for April 2016 by district in Nigeria and Pakistan. Population immunity was estimated from the vaccination histories of children <36 mo old identified with non-polio acute flaccid paralysis (AFP) reported through polio surveillance, information on immunisation activities with different oral poliovirus vaccine (OPV) formulations, and serotype-specific estimates of the efficacy of these OPVs against poliomyelitis. District immunity estimates were spatio-temporally smoothed using a Bayesian hierarchical framework. Coverage estimates for immunisation activities were also obtained, allowing for heterogeneity within and among districts. Forward projections of immunity, based on these estimates and planned immunisation activities, were produced through to April 2016 using a cohort model. Estimated population immunity was negatively correlated with the probability of VDPV2 poliomyelitis being reported in a district. In Nigeria and Pakistan, declines in immunity during 2008-2009 and 2012-2013, respectively, were associated with outbreaks of VDPV2. Immunity has since improved in both countries as a result of increased use of trivalent OPV, and projections generally indicated sustained or improved immunity in April 2016, such that the majority of districts (99% [95% uncertainty interval 97%-100%] in Nigeria and 84% [95% uncertainty interval 77%-91%] in Pakistan) had >70% population immunity among children <36 mo old. Districts with lower immunity were clustered in northeastern Nigeria and northwestern Pakistan. The accuracy of immunity estimates was limited by the small numbers of non-polio AFP cases in some districts, which was reflected by large uncertainty intervals. Forecasted improvements in immunity for April 2016 were robust to the uncertainty in estimates of baseline immunity (January-June 2015), vaccine coverage, and vaccine efficacy.
  
  
  CONCLUSIONS
  Immunity against serotype-2 poliomyelitis was forecasted to improve in April 2016 compared to the first half of 2015 in Nigeria and Pakistan. These analyses informed the endorsement of OPV2 withdrawal in April 2016 by the WHO Strategic Advisory Group of Experts on Immunization.
authors.affiliation: Department of Infectious Disease Epidemiology, St Mary's Campus, Imperial College London, London, United Kingdom.
authors.forename: Margarita
authors.identifier: http://orcid.org/0000-0001-5597-9285
authors.initials: M
authors.lastname: Pons-Salort
authors.affiliation: Department of Infectious Disease Epidemiology, St Mary's Campus, Imperial College London, London, United Kingdom.
authors.forename: Natalie A
authors.initials: NA
authors.lastname: Molodecky
authors.affiliation: Department of Infectious Disease Epidemiology, St Mary's Campus, Imperial College London, London, United Kingdom.
authors.forename: Kathleen M
authors.initials: KM
authors.lastname: O'Reilly
authors.affiliation: World Health Organization, Islamabad, Pakistan.
authors.forename: Mufti Zubair
authors.identifier: http://orcid.org/0000-0002-7113-1855
authors.initials: MZ
authors.lastname: Wadood
authors.affiliation: National Emergency Operation Centre, Ministry of National Health Services, Regulations and Coordination, Islamabad, Pakistan.
authors.forename: Rana M
authors.identifier: http://orcid.org/0000-0003-1720-7457
authors.initials: RM
authors.lastname: Safdar
authors.affiliation: National Primary Health Care Development Agency, Abuja, Nigeria.
authors.forename: Andrew
authors.initials: A
authors.lastname: Etsano
authors.affiliation: World Health Organization, Abuja, Nigeria.
authors.forename: Rui Gama
authors.initials: RG
authors.lastname: Vaz
authors.affiliation: World Health Organization, Geneva, Switzerland.
authors.forename: Hamid
authors.initials: H
authors.lastname: Jafari
authors.affiliation: Department of Infectious Disease Epidemiology, St Mary's Campus, Imperial College London, London, United Kingdom.
authors.forename: Nicholas C
authors.initials: NC
authors.lastname: Grassly
authors.affiliation: Department of Infectious Disease Epidemiology, St Mary's Campus, Imperial College London, London, United Kingdom.
authors.forename: Isobel M
authors.initials: IM
authors.lastname: Blake
mesh_terms: D002675:Child, Preschool; D060740:Disease Eradication; D017723:Drug Utilization Review; D014943:Global Health; D006801:Humans; D007109:Immunity; D015994:Incidence; D007223:Infant; D011051:Poliomyelitis / Q000453:epidemiology / Q000517:prevention & control*; D017955:Poliovirus / Q000145:classification / Q000276:immunology; D011055:Poliovirus Vaccine, Oral / Q000008:administration & dosage*; D012703:Serotyping
chemical_list: D011055:Poliovirus Vaccine, Oral