SEROPREVALENCE OF ANTIBODIES TO POLIOVIRUS BY NUMBER OF VACCINE DOSE RECEIVED IN NIGERIAN CHILDREN AMINU M,1 AHMAD A A1 and OGUNRINDE G O2 Dept. of Microbiology, A.B.U Zaria, Nigeria Dept. of Paediatrics, ABUTH, Zaria, Nigeria

INTRODUCTION Since the WHA committed itself to global eradication of polio in 1998, the number of polio cases have fallen by over 99% (Dove, 2002). In 2002, the number of polio endemic countries dropped to seven( from highest to lowest); India, Nigeria, Egypt, Pakistan, Afghanistan, Niger and Somalia.- Nigeria is implementing the four strategies recommended by WHO for achieving polio eradication. Since 1996, Nigeria has embarked upon mass immunization campaigns and has implemented 2 rounds of NIDS and SNIDS. OPV is being administered twice every year to children under five years irrespective of their previous immunization status. Many of these target children have received more than five doses

AIMS AND OBJECTIVES This serological survey was therefore conducted in Kaduna State, Nigeria, a polio endemic area to determine the effect of mass immunization and number of OPV dose received on prevalence of antibodies to poliomyelitis in children

MATERIALS AND METHODS Study population/Sample collection: -1087 school children aged 1-11 years were selected in 2 schools each in 11 out of 23 Local Government Areas (LGA) in Kaduna State, Nigeria (fig. 1) .

Information on vaccination history, personal data and socio-economic status of children were obtained using structural questionnaires sent to caretakers Blood samples were collected by venepuncture in the selected children from Sept. to Dec. 1999 Sera separated from clotted blood by centrifugation and transferred into eppendorf tubes and stored at -20°C until analyzed

Neutralization Test - Neutralization antibody titres against poliovirus (PV) Types 1, 2 and 3 were measured at the National polio lab. Ibadan by means a modified micro neutralization assay described by Richardson et al 1995, as outlined. - In the manual for virological investigation of polio ( WHO, 1997) - Reference sera, poliovirus challenge viruses and HEP-2 cells provided by the lab were used for the analysis. - Antibody titre were expressed as the highest original dilution of serum completely inhibiting any cytopathogenic effect. - Specimens with antibody titre <1:10 were interpreted negative

Statistical Analysis The results were analyzed on the basis of age and number of vaccine dose received and they presented in percentages. Index of susceptibility (seronegativity) to poliomyelitis virus was taken as lacking demonstrated antibodies (Abs) at the lowest dilution tested (1:10 ) while seroprevalence (seropositivity) was taken as the number of children with Abs to at least one of the pv serotypes at the same dilution. Chi-square test was used for test of significance in percentages

RESULTS Neutralization Test: - 875 (80.5%) of the 1087 children had demostratable Abs to at least one of the PV serotypes ( Total seroprevalence). - 212 (19.5%) lacked antibodies to any of the serotype of PV (susceptible). - Seroprevalence rates of 65.5%, 71.4% and 66.7% were obtained for PV types 1, 2 and respectively. - 459 (42.2%) of the children had demostratable Abs to all three PV serotypes ( adequate immunity)

Analysis of Questionnaires: - 926 (85.2%) of the 1087 had a history of vaccination. - 161 (14.8%) had no history of vaccination (not immunize). -Of 161 with no history of vaccination, 77 (47.8%) had demonstrable abs to PV. -Amongst 926 children with a history of vaccination, 128(13.8%) had no demonstrable abs to PV. - Seroprevalence was highest (93.9%) in children who had received 5 doses of OPV and was however lowest for those who had received 6 doses (Table 1). - Seroprevalence was also highest for the three PV serotypes for children who had received 5 doses of OPV and lowest for those who received 2 doses (Fig. 2)

Vaccine Doses Type 1 Type 2 Type3 1 68.7 71.6 76.1 2 63.4 67.5 60.2 3 Table 1: : Percentage Distribution of Antibodies to Poliomyelitis in Children who Received one to six Doses OPV in Kaduna State-Nigeria Vaccine Doses Type 1 Type 2 Type3 1 68.7 71.6 76.1 2 63.4 67.5 60.2 3 77.2 82.5 4 72.8 77.8 72.1 5 81.6 89.8 91.8 6 65.2 73.9

Fig. 2: Seroprevalence of antibody to poliomyelitis virus amongst children that receive one or more doses of OPV in Kaduna State, Nigeria

Fig. 3: Percentage of Children by Number of Vaccine Doses Received who had Antibodies to one, two and all three Serotypes of Poliovirus in Kaduna State- Nigeria

Seroprevalence rate associated with zero dose was 47.8%. Of this rate, 23.6%, 34.2% and 34.2% had Abs to PV types 1,2 and 3 respectively. 72.7%, 15.6% and 11.7% showed demonstrable Abs to one, two and all three serotypes of PV (Fig.3). Children who had received 5 doses had the highest prevalence rates for the three PV serotypes at titre > 40 (fig. 4) The observed differences in seroprevalence rates by number of vaccine dose received wre significant at p< 0.05

fig 4: Percentage distribution of poliovirus antibodies at titre < 40 and > 40 amongst children by number of vaccine doses received in Kaduna State, Nigeria

Discussion Nigeria has been implementing the four strategies for achieving polio eradication and many children had received more than 5 doses. Yet Nigeria ranked second in the list of endemic countries. 34.5%, 28.6% and 33.3% of the children in the study were susceptible to PV types 1,2 and 3. This disturbing because persons are presumed to be protected against the disease caused by a particular type if thy develop type specific Abs This therefore maybe responsible for the increase in number of cases recorded in the country in 2002 The seroprevalence rate (47.8%) for children with no history of vaccination is assumed to represent secondary or sub-clinical infections with wild strain of PV

72.7% had Abs to only one type of PV possibly implying exposure to circulating wild virus caused by sub-clinical infection . 23.3% had Abs to two or all three serotypes of PV and may possibly be due to secondary spread of OPV circulating in the community. The poor correlation between serological results and history of vaccination is probably due to over and under reporting of vaccination history by caretakers. This poor correlation is in part responsible for the gap between reported coverage level and seroprevalence level reported in this study.

CONCLUSION The seroprevalence of Abs against all three polio serotypes in Kaduna State was below expectation and inadequate for herd immunity. Secondary spread of OPV currently play a modest role in increasing seroprevalence level in children in Kaduna state. The substantial level of immunity to poliovirus in children who had no history of immunization suggests under estimation of true coverage by parent verified OPV coverage. The coverage – immunity gap probably resulted from a gap in true and parent verified coverage.

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