PARALYTIC POLIOMYELITIS IN OMAN - ASSOCIATION BETWEEN REGIONAL DIFFERENCES IN ATTACK RATE AND VARIATIONS IN ANTIBODY-RESPONSES TO ORAL POLIOVIRUS VACCINE

Variation in attack rates of paralytic disease by region during the 19 88-1989 epidemic of type 1 poliomyelitis in Oman provided the stimulus to test the hypothesis that these observations were due to regional d ifferences in the response of infants to trivalent oral poliovirus vac cine (OPV). Seroprevalence studies of 394 children born during the out break were conducted in six different regions of Oman and in two socio economic status (SES) groups in the capital city of Muscat; a seroconv ersion study was also carried out in 105 infants born after the outbre ak. Seroprevalence rates by region after receipt of at least three dos es of OPV ranged from 90% to 100% (median 94%) to poliovirus type 1, a nd from 86% to 100% (median 97%) to type 2, and from 47% to 79% (media n 72%) to type 3, with the lowest rates observed in regions with the h ighest incidence of type 1 paralytic disease. In Muscat, seroprevalenc e rates were also significantly lower in low versus high SES groups (t ype 1: 84% versus 98%, respectively [P = 0.006]; type 3: 59% versus 86 %, respectively [P = 0.001]). In the seroconversion study conducted af ter the outbreak, 89%, 100% and 50% of infants had detectable antibodi es to types 1, 2, and 3, respectively, after four doses of OPV. Low re sponses to type 3 were also associated with the occurrence of sporadic cases of type 3 poliomyelitis in 1991, in spite of high rates of cove rage with at least four doses of OPV (>96%) throughout the country. Th ese findings demonstrate that antibody responses to OPV may vary widel y within individual countries, and that a uniform strategy to deliver at least three doses in routine programmes may be insufficient to achi eve elimination of wild poliovirus infection.