Analysis of one of the largest outbreaks of polio since the advent of the vaccine shows that it occurred in an area with supposedly excellent vaccine coverage.
Between January 1988 and March 1989, 118 cases of poliomyelitis were reported in Oman. The incidence of paralytic disease was among children less than 2 years old, despite the fact that 87 per cent of Oman children had received three doses of oral polio vaccine from 7 months of age.
Some 25 per cent of the attack rate occurred among children between 9-23 months, "suggesting that a substantial proportion of fully vaccinated children had been involved in the chain of transmission," wrote the authors, from the immunization divisions of the Centers for Disease Control in Atlanta.
"Among the most disturbing features of the outbreak was that it occurred in the face of a model immunization programme," they said. "Suboptimum performance of OPV was also suggested by our serological studies of case patients and controls, nearly 50 per cent of whom had no detectable antibody against poliovirus type 3."
The results have left the CDC scratching its head for new answers to the vaccine problem, including vaccinating children at birth.