Population Immunity against Serotype-2 Poliomyelitis Leading up to the Global Withdrawal of the Oral Poliovirus Vaccine: Spatio-temporal Modelling of Surveillance Data

Abstract

BackgroundGlobal 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).Methods and FindingsIn 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.ConclusionsImmunity 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.

Estimated Effect of Inactivated Poliovirus Vaccine Campaigns, Nigeria and Pakistan, January 2014–April 2016

“…Campaign coverage was suboptimal in Pakistan during much of 2014–2015; in September 2015, in areas at high risk for polio, just 39% of union councils were estimated to have achieved >80% campaign coverage, although this improved to 62% in November 2015 ( 18 ). Estimates of campaign coverage during January 2014–June 2015, which were based on the vaccination histories of children reported with nonpolio acute flaccid paralysis, also suggest that coverage among undervaccinated communities in Pakistan was poorer than that in Nigeria ( 19 ). Suboptimal vaccination coverage means that fewer children will benefit from a boost in intestinal immunity by IPV, not only as a result of smaller numbers receiving the vaccine, but also because fewer children will have been mucosally primed by OPV given in earlier campaigns or though routine immunization systems.…”

Dynamics affecting the risk of silent circulation when oral polio vaccination is stopped

“…From Table 1 we see there is some increase in vaccination level needed to get to our fixed prevalence of first infections as we progress through fast-shallow to slow deep waning scenarios. Data and modeling of vaccination rates has greatly increased in recent years ( Pons-Salort et al, 2016 ). But given the difficulties in determining what the effective levels of vaccination have been in populations over longer times, the waning patterns will not be identifiable from standard polio program data.…”

“…Patterns of vaccination vary greatly in the real world and extensive modeling work has demonstrated how important capturing the past pattern of vaccinations accurately is to making valid predictions of current and future risks that arise from inadequate vaccination programs in the past ( Duintjer Tebbens et al, 2014 , Duintjer Tebbens et al, 2013b , Duintjer Tebbens et al, 2013c , Duintjer Tebbens et al, 2013d , Duintjer Tebbens et al, 2015 , Duintjer Tebbens and Thompson, 2014 , Duintjer Tebbens and Thompson, 2015 , Pons-Salort et al, 2016 , Thompson, 2013 , Thompson and Duintjer Tebbens, 2014a , Thompson and Duintjer Tebbens, 2014b ; Thompson et al, 2015b , Thompson et al, 2015a Thompson et al, 2013a , Thompson et al, 2013b , Thompson et al, 2013c ; Thompson and Tebbens, 2012 ). But we want to show why these models will not adequately capture the risk of prolonged SC.…”

Sailing in Uncharted Waters: Carefully Navigating the Polio Endgame

“…Third, multiple campaigns with tOPV were conducted in the run-up to the switch, especially in high-risk areas such as Nigeria and Pakistan, to reduce the risk of development of new vaccine-derived polio virus (VDPV) 2 lineages and to interrupt any undetected lineages already in circulation. In the accompanying paper [ 7 ], Isobel Blake and colleagues nicely illustrate the correlation between low population immunity to type 2 and the risk of cVDPV2 cases in Nigeria and Pakistan; they also describe the encouraging increase in type 2 immunity that resulted from the tOPV campaigns conducted in these countries prior to the switch. Their immunity forecasts have provided confidence to the WHO Strategic Advisory Group of Experts that the proposed withdrawal of OPV2 in April 2016 should go ahead.…”