Outbreak of paralytic poliomyelitis in Oman: evidence for widespread transmission among fully vaccinated children.
Between January 1988-March 1989, a widespread outbreak (118 cases) of poliomyelitis type 1 occurred in Oman. Incidence of paralytic disease was highest in children under age 2 (87/100,000) despite an immunization program that recently had raised coverage with 3 doses of oral poliovirus vaccine (OPV) among 12-month old children from 67% to 87%. The authors undertook a case-control study (70 case patients, 692 age- matched controls) to estimate the clinical efficacy of OPV, assessed the immunogenicity of OPV and the extent of poliovirus spread of serology, retrospective evaluated the cold chain and vaccine potency, and sought the origin of the outbreak strain by genomic sequencing. 3 doses of OPV reduced the risk of paralysis by 91%; vaccine failures could not be explained by failures in the cold chain nor on suboptimum vaccine potency. Cases and controls had virtually identical type 1 neutralizing antibody profiles, suggesting that poliovirus type 1 circulation was widespread. Genomic sequencing indicated that the outbreak strain had been recently imported from South Asia and was distinguishable from isolates indigenous to the Middle East. Accumulation of enough children to sustain the outbreak seems to have been due to previous success of the immunization program in reducing spread to endemic strains, suboptimum efficacy of OPV, and delay in completing the primary immunization series until 7 months of age. In addition, the estimated attack rate of infection among children 9-23 months exceeded 25% in some regions, suggesting that a substantial proportion of fully vaccinated children had been involved in the chain of transmission. (author's)