Poliomyelitis is endemic in Senegal, with 100-200 cases reported each year. However, in 1986, an outbreak of paralytic poliomyelitis occurred throughout the country. A total of 618 cases of poliomyelitis with onsets of paralysis from May through November 1986 were reported (crude attack rate = 9.6 reported cases per 100,000 persons) (Figure 1). Patients with onsets during July, August, and September accounted for 71% of reported cases. Seventy-two percent of patients were less than 3 years of age, and 84 were less than 5 years. Data were collected by active and passive surveillance.

Oral polio vaccine (OPV) and inactivated polio vaccine (IPV) have been used in different regions of the country. A new, more potent IPV (N-IPV) combined with diphtheria and tetanus toxoids and pertussis vaccine (DTPP) has been used since 1980 in Kolda and since 1982 in Sedhiou, two departments of the Kolda Region. Through 1981, the N-IPV vaccine had 40-4-16 D-antigen units against polio types 1, 2, and 3, respectively; thereafter, a vaccine with 40-8-32 D-antigen units was used. In rural areas, mobile teams used jet injectors to administer the vaccine at 5- to 6-month intervals during the 7-month dry season (October-April). Vaccine was given by needle and syringe year-round in three urban (fixed) sites. Ideally, children received their first and second doses of DTPP vaccine 6 months apart. Children aged 3-23 months were eligible for the first dose of polio vaccine.

In order to calculate the efficacy of one or two doses of N-IPV, a case-control study was conducted in Kolda and Sedhiou departments. Persons who had had acute onsets of paralytic disease since May 1, 1986, and who had been diagnosed as having poliomyelitis by a physician or a senior medical student after a standardized examination were included in the study. Surveillance included house-to-house searches in two cities (5,573 houses were visited) and visits to alternate villages along passable roads in rural areas (492 villages were visited).

A total of 60 cases were found in the Kolda study area (crude attack rate = 16/100,000). Onsets of paralysis for all 60 patients occurred from May through November. There was no clear peak of activity. Of the 60 patients, 55% were less than 2 years of age, and 95% were less than 4 years of age; 55 were male. Only 33% of the patients had had contact with the official health-care system.

Up to five matched controls were selected for each case. Controls had the following characteristics: 1) they had no history of previous paralytic illness, 2) each had been a resident of the same village (but not the same compound) as the matching patient for at least 1 month before onset of illness in the patient, and 3) each was within 6 months of the age of the matching patient.

Vaccination status was determined from vaccination cards for both patients and controls. Those lacking cards were counted as unvaccinated. Only vaccinations received at least 30 days before the patient's onset of illness were counted. Four patients were excluded; one because of receiving OPV vaccine, and three because they had each received three doses of N-IPV. The vaccination histories of 56 patients and their 217 matched controls were compared (Table 1). Twenty-two percent of patients and 18% of controls had received one dose of IPV, and 12% of cases and 24% of controls had received two doses of IPV.

Vaccine efficacy analysis was completed using a logistic regression program for variable, matched analysis with more than one control per patient ( 1,2 ). The clinical efficacy of one dose of N-IPV (compared with zero doses) was 5% (95% confidence interval {CI} = 0%, 57%) and for two doses (compared with zero doses) was 76% (95% CI =28%, 92%).

Thirty-four of the 56 patients resided in Kolda Department, and 22 resided in Sedhiou Department. All seven of the patients who had received two doses of vaccine resided in Kolda Department. Because no patients from Sedhiou Department had received two doses of N-IPV, the apparent two-dose vaccine efficacy in Sedhiou is 100% (95% confidence intervals cannot be computed). Using only patients and controls from Kolda Department, two-dose vaccine efficacy was 55% (95% CI = 0%, 87%).

A cluster survey of vaccine coverage was completed in the Kolda study area during the first week of December 1986 for each of three age groups consisting of children who were 12-23 months, 24-35 months, or 36-47 months of age as of May 1, 1986 (3). As of that date, 53% of children in all of these age groups combined had had one or more doses, and 34% had had two or more doses (Table 2).

Reported by: DHPS, Ministry of Health, Senegal; Association Pour la Promotion de la Medicine Preventive (APMP), Dakar, Senegal. Task Force for Child Survival, Atlanta, Georgia. Association Pour la Promotion de la Medicine Preventive (APMP), Paris, France. Div of Immunization, Center for Prevention Svcs; Epidemiology Program Office; International Health Program Office, CDC.

Editorial Note

Editorial Note: Serologic studies of N-IPV under field conditions, including one done in Kolda, have shown seroconversion rates of 95%-100% after two doses (4-8). However, clinical efficacy of this vaccine in developing countries has not been published previously. Preliminary results of the study conducted in the Kolda Region of Senegal suggest that a single dose of N-IPV provided little or no protection and that two doses were approximately 75% effective in preventing paralytic poliomyelitis. These results, particularly the estimate of two-dose efficacy, are lower than expected based on either earlier serological studies or the known clinical efficacy of the older, less potent IPV in several other countries ( 9,10 ).

The reasons for the marked discrepancy between the observed clinical efficacy in this study and the expected efficacy based on serological data for N-IPV are not presently known. Possible explanations include: 1) operational factors, such as inadequate supervision of field personnel, deficiencies in the cold chain, or falsification of vaccination records; 2) vaccine-related factors, such as hitherto unrecognized heat lability; 3) immunologic factors, such as the possibility that low levels of circulating antibodies may not necessarily indicate protection in the face of exposure to large inocula of wild poliovirus. In addition, true vaccine efficacy might lie at the upper limit of the 95% confidence interval rather than at the point estimate.

Because all patients in the study who had received two doses of IPV were from Kolda Department alone, it is possible that there were operational differences between Kolda and Sedhiou departments. Further study is underway to determine the potential role of this and any other factors. In addition, active surveillance has been extended to include villages in Kolda and Sedhiou departments that were not visited during the initial investigation. Additional cases of paralytic poliomyelitis will be included in the case-control study. A follow-up report will be published when these studies are completed.

Senegal began an Acceleration of the Expanded Program on Immunization (EPI) on November 17, 1986. Three national immunization weeks were held from January 5-10, February 16-21, and March 23-28, 1987. Both N-IPV and OPV were administered. Vaccines have also been made available on a daily basis at fixed sites nationwide. The goal of the Accelerated EPI is to fully immunize 75% of Senegalese children less than or equal to 2 years of age with polio (N-IPV or OPV), measles, DTP, BCG, and yellow fever vaccines by April 6, 1987. Preliminary data concerning the number of doses delivered suggest that this goal was achieved.

References

1. 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).

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

3. 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.

4. 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):S463-6.

5. Bernier RH. Improved inactivated poliovirus vaccine: an update. Pediatr Infect Dis 1986;5:289-92.

6. Krishnan R, Jadhav M, John TJ. Efficacy of inactivated poliovirus vaccine in India. Bull WHO 1983;61:689-92.

7. Schatzmayr HG, Maurice Y, Fujita M, De Fillipis AMB. Serological evaluation of poliomyelitis oral and inactivated vaccines in an urban low-income population at Rio de Janeiro, Brazil. Vaccine 1986;4:111-3.

8. McBean AM, Thoms ML, Johnson RH, et al. A comparison of the serologic responses to oral and injectable trivalent poliovirus vaccines. Rev Infect Dis 1984;6(suppl):S552-61.

9. Bijkerk H. Surveillance and control of poliomyelitis in the Netherlands. Rev Infect Dis 1984;6(suppl):S451-6.

10. Lapinleimu K. Elimination of poliomyelitis in Finland. Rev Infect Dis 1984;6(suppl):S457-60.

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