Background The completion of poliomyelitis eradication is a global emergency for public health. In 2012, more than
50% of the world’s cases occurred in Nigeria following an unanticipated surge in incidence. We aimed to quantitatively
analyse the key factors sustaining transmission of poliomyelitis in Nigeria and to calculate clinical effi cacy estimates
for the oral poliovirus vaccines (OPV) currently in use.
Methods We used acute fl accid paralysis (AFP) surveillance data from Nigeria collected between January, 2001, and
December, 2012, to estimate the clinical effi cacies of all four OPVs in use and combined this with vaccination coverage
to estimate the eff ect of the introduction of monovalent and bivalent OPV on vaccine-induced serotype-specifi c
population immunity. Vaccine effi cacy was determined using a case-control study with CIs based on bootstrap
resampling. Vaccine effi cacy was also estimated separately for north and south Nigeria, by age of the children, and by
year. Detailed 60-day follow-up data were collected from children with confi rmed poliomyelitis and were used to
assess correlates of vaccine status. We also quantitatively assessed the epidemiology of poliomyelitis and programme
performance and considered the reasons for the high vaccine refusal rate along with risk factors for a given local
government area reporting a case.
Findings Against serotype 1, both monovalent OPV (median 32·1%, 95% CI 26·1–38·1) and bivalent OPV (29·5%,
20·1–38·4) had higher clinical effi cacy than trivalent OPV (19·4%, 16·1–22·8). Corresponding data for serotype 3 were
43·2% (23·1–61·1) and 23·8% (5·3–44·9) compared with 18·0% (14·1–22·1). Combined with increases in coverage,
this factor has boosted population immunity in children younger than age 36 months to a record high (64–69%
against serotypes 1 and 3). Vaccine effi cacy in northern states was estimated to be signifi cantly lower than in southern
states (p≤0·05). The proportion of cases refusing vaccination decreased from 37–72% in 2008 to 21–51% in 2012 for
routine and supplementary immunisation, and most caregivers cited ignorance of either vaccine importance or
availability as the main reason for missing routine vaccinations (32·1% and 29·6% of cases, respectively). Multiple
regression analyses highlighted associations between the age of the mother, availability of OPV at health facilities,
and the primary source of health information and the probability of receiving OPV (all p<0·05).
Interpretation Although high refusal rates, low OPV campaign awareness, and heterogeneous population immunity
continued to support poliomyelitis transmission in Nigeria at the end of 2012, overall population immunity had
improved due to new OPV formulations and improvements in programme delivery.Funding Bill & Melinda Gates Foundation Vaccine Modeling Initiative, Royal Society.Introduction In May, 2012, after more than 20 years of mass vaccination campaigns, the 65t