Polioviruses; wild and vaccine-derived, have been reported more in the northern states compared to the southern part of Nigeria with a record of active immunisation activities. The present case of VDPV in a southern state was the first of its kind to our knowledge, being a female and a one-month-day-old child. Majority of cases in the previous outbreaks were male children with an average age of two years. Poliomyelitis is a disease of children, but all ages can be affected and there is no sex predilection. The clinical and epidemiological characteristics of the case reported are consistent with what was reported in the literature. Available clinical data from this report showed that the confirmed case had various symptoms, including the inability to move the affected limbs, fever and severe pains in the intestine, with the case ending up being dead. This finding is consistent with previous reports of cVDPV1 and cVDPV2 infections, for which a cumulative 74% of cVDPV-associated AFP cases involved residual paralysis and fever typical of poliomyelitis. However, outcome data are not usually reported with the outbreaks. The final classification of the virus is still in doubt and we recognised this as a limitation of our investigation. The laboratory report we have access to did not provide the gene bank assession number as well as the serotype of the virus. If we consider that it is an iVDPV, the immunological status of the infant is not documented regarding immune pathologies or immune deficiencies and no blood sample was collected for testing blood immunoglobulins (Ig) levels.
WHO recommends clinical and biological investigations, including diagnosis of immune deficiencies before classifying a VDPV as iVDPV. More so, the vaccination profile of the case indicated that she was adequately vaccinated for her age, although she did not receive any dose of OPV during SIAs. Furthermore, there was no evidence of person-to-person transmission in the community to classify it as cVDPV. The outcome of the investigation on vaccination coverage conducted among the under-five children in the immediate environment of the case revealed that majority of them were vaccinated both in the last SIAs round and the round before it. Different from this report, earlier authors have attributed under-vaccination among children with the emergence of poliovirus infection. For instance, Kamadjeu et al. observed that 55% of all WPV cases had never received OPV (so-called zero-dose individuals) and almost 80% were under-vaccinated (i.e. they received <=3 doses of OPV) in Somalia. In addition, most patients with cVDPV2 and WPV were under-vaccinated in Nigeria. However, at the population level, the record of administrative OPV coverage in the LGA in the last four years revealed a drop in RI from 94% in 2012, 83% in 2013, 103% in 2014 to 67% in 2015. This drop in coverage rate, if it is real, may have contributed to the re-emergence of the infection in the LGA which indicated a population immunity gap that enables the development of the poliovirus. This finding alone is in accord with previous studies suggesting that the principal risk factor for VDPV emergence and spread is low population immunity.
Other authors reached similar conclusions using NP-AFP and NICS 2006 data to estimate coverage with type 2-containing OPV. Moreover, asides the drop in the RI in the affected LGA, the environmental characteristics of the case-patient may also contribute towards the emergence of the infection. We observed that the case had lived in a filthy environment with poor toilet and water facilities, which could aid the occurrence of the infection. Similar observations were also made in earlier reports. The immunisation response administrative data revealed that a substantial number of children were vaccinated in the affected ward while more than 100% coverage rates were achieved during the three rounds of the large-scale response in the states where they were conducted. It is expected that this will boost the herd immunity and prevent transmission. A similar response has been conducted in previous outbreaks in Nigeria with an encouraging outcome. For instance, after two rounds of tOPV SIAs conducted in affected northern states in Nigeria in May and August 2009, monthly incidence of cVDPV2 decreased sharply from 35 cases in 2009 to 0-3 cases during September 2009-June 2010.
The SIAs, being a mass vaccination campaign strategy aimed to administer additional doses of oral poliovirus vaccine (OPV) to each child aged <5 years, regardless of their vaccination history, may have proven effective in reducing the incidence of VDPV. The strategy has also been mainly used in many countries and has contributed to the 99% global reduction in the incidence of paralytic poliomyelitis observed since 1988 lunch of the global polio eradication initiative. We identified some crucial strengths in the public health response embarked upon during this polio event. The state and LGA team were committed and there was the timely arrival of training, logistics and social mobilisation funds. The national government and partner agencies adequately provided financial support and there was an excellent and efficient state cold chain system. Also, evening review meetings at state and LGAs helped in correcting gaps for the next day s work and there was intensive supervision by state team and partners during training and implementation. However, some gaps, including the non-release of counterpart funding by the state and LGA to aid logistics activities, late arrival of vaccines to the state, inadequate micro plans, poor team performance in the filing of the data tools and ineffective social mobilization, especially to schools and churches, were noted.