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Sera were obtained from 119 Nigerian villagers two years after immunization with group A and group C meningococcal polysaccharide vaccines. Measurement of meningococcal antibody levels in individual serum samples by the haemagglutination technique showed that two years after vaccination the mean group A meningococcal antibody level was no higher in immunized subjects than in 140 age-matched, non-immunized controls. Over-all the mean group C meningococcal haemagglutinating antibody level was significantly higher in immunized subjects than it was in controls but this difference was not found in those under the age of 15 years at the time of vaccination. However, when pooled serum samples from the same subjects were tested for meningococcal antibodies by radioimmunoassay, immunized subjects were found to have significantly higher mean levels of both group A and group C meningococcal antibodies than the controls. The possible reasons for this discrepancy are considered and the significance of these findings in relation to plans for mass immunization with meningococcal vaccines in tropical Africa are discussed.

The antibody response to group C meningococcal polysaccharide vaccine was studied in a Nigerian village. Household clustering of poor responders to immunization was detected. Age had a marked effect on antibody response, maximal titres being obtained only in those over the age of 10 years. Children with malaria parasitaemia had a lower antibody response than those without parasitaemia and subjects with the genotype AA had a lower antibody response than those with the genotype AS. The antibody response to the vaccine was not influenced by mild degrees of malnutrition but children with clinical marasmus or kwashiorkor were excluded from the study.