International Notes Paralytic Poliomyelitis -- Senegal, 1986-1987: Update on the N-IPV Efficacy Study

In 1986, an outbreak of paralytic poliomyelitis, which was most likely caused by type 1 poliovirus, in the Kolda Region of Senegal provided an opportunity to conduct vaccine efficacy studies of the new, enhanced-potency inactivated poliovirus vaccine (N-IPV). N-IPV had been used in that area since 1980. Serologic studies of patients who received two doses of N-IPV given under field conditions have shown seroconversion rates of 90% to 100% for each of the three polio types included in the vaccine (1-4). However, preliminary results from Phase I of the matched case-control study conducted in the Kolda Region from October-November 1986 showed a discrepancy between the apparent clinical efficacy of N-IPV and the expected efficacy based on prior serological data (5).

In May 1987, Phase II of the Senegal study was conducted to improve the reliability of the estimate and to determine what factors might have had an impact on the efficacy of N-IPV. Methodology was the same for Phase I and Phase II. During Phase II, investigators conducted house-to-house searches for additional cases of poliomyelitis in 10 large villages, one city, and 1,841 (81%) of the 2,263 villages in the Kolda Region. Medical specialists who were unaware of the patients' vaccination status examined each patient and certified all cases identified during both phases.

The matched case-control study initiated in Phase I was continued, and up to five matched controls were selected for each case. Patients who had onset of paralytic disease since April 1, 1986, and who had been jointly certified by the medical specialists (a neurologist and an infectious disease specialist) to have residual paralysis after a standardized examination* were included in the study. Controls who had experienced no polio-like illness were matched to patients by age and village.

Investigators identified a total of 89 certified cases of paralytic poliomyelitis in the Kolda Region (crude attack rate = 16 certified cases per 100,000 persons). Onsets of paralysis occurred from April 1986 through February 1987, with a clear peak of activity during August 1986 (Figure 1). Eighty-five (96%) of the patients were under 5 years of age, and 63% were male. Thirty percent of the patients had had contact with the official health-care system.

Vaccination status was determined from vaccination cards for both patients and controls. Persons lacking cards were counted as unvaccinated. The vaccination histories of 87 of the 89 patients and their 364 controls were compared (Table 1). Eighteen percent of patients and 20% of controls had received one dose of N-IPV, and 6% of patients and 22% of controls had received two doses of N-IPV.

Analysis of vaccine efficacy was completed using logistic regression (6,7). The clinical efficacy of one dose of N-IPV (compared with zero doses) against residual paralysis was 36% (95% confidence interval (CI), 0% to 67%). For two doses (compared with zero doses) the point estimate of efficacy was 89% (95% CI, 62% to 97%).

Separate vaccine coverage surveys were completed in each of the three departments of the Kolda Region. A standard methodology using 30 randomly selected clusters was employed (8). For each survey, vaccination status was obtained for an estimated 210 children who were 12-35 months of age as of May 1986. Results indicated that 26% to 28% of the children in that age group had received two doses of N-IPV as of May 1986. Reported by: Direction de l'Hygiene a la Promotion Sanitaire, Ministry of Health, Senegal. Association pour la Promotion de la Medicine Preventive, Paris, France. Task Force for Child Survival, Atlanta, Georgia. Div of Immunization, Center for Prevention Svcs; Div of Field Svcs, Epidemiology Program Office; International Health Program Office, CDC. Editorial Note: The results obtained during Phase II of this study based on certified cases of poliomyelitis in the Kolda Region indicate that two doses of N-IPV were approximately 89% effective in preventing paralytic poliomyelitis with residual paralysis. This estimate is compatible with previous serological reports. One dose of N-IPV did not confer effective protection. Although none of the cases in the Kolda Region were confirmed by viral isolation, all were most probably due to type 1 poliovirus, which was documented as the overall cause of the outbreak in both Senegal and The Gambia. The higher efficacy obtained during Phase II as compared with Phase I probably reflects the more specific case definition used, especially the requirement for certification of the diagnosis by experts.

The low level of polio vaccine coverage in the Kolda Region failed to prevent this outbreak of paralytic poliomyelitis. References

1. Salk J, Stoeckel P, van Wezel AL, Lapinleimu K, van Steenis G. Antigen content of inactivated poliovirus vaccine for use in a one- or two-dose regimen. Ann Clin Res 1982;14:204-12.

2. Stoeckel P, Schlumberger M, Parent G, et al. Use of killed poliovirus vaccine in a routine immunization program in West Africa. Rev Infect Dis 1984;6(suppl 2):S463-6.

3. Drucker J, Soula G, Diallo O, Fabre P. Evaluation of a new combined inactivated DPT-polio vaccine. Dev Biol Stand 1986;65:145-51.

4. Simoes EAF, Padmini B, Steinhoff MC, Jadhav M, John TJ. Antibody response of infants to two doses of inactivated poliovirus vaccine of enhanced potency. Am J Dis Child 1985; 139:977-80.

5. Centers for Disease Control. Preliminary report: paralytic poliomyelitis--Senegal, 1986. MMWR 1987;36:387-90.

6. Breslow NE, Day NE. Statistical methods in cancer research. Vol.

1. The analysis of case-control studies. Lyon, France: International Agency for Research on Cancer, 1980. (IARC scientific publication no. 32).

7. Orenstein WA, Bernier RH, Dondero TJ, et al. Field evaluation of vaccine efficacy. Bull WHO 1985;63:1055-68.

8. Henderson RH, Sundaresan T. Cluster sampling to assess immunization coverage: a review of experience with a simplified sampling method. Bull WHO 1982;60:253-60. *In all cases, examination was performed greater than or equal to 60 days after onset of illness.

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