Sequential use of inactivated poliovirus vaccine followed by oral poliovirus vaccine in Oman.

Seroprevalence and geometric mean titers (GMTs) were compared at 6 and 10 months after vaccination with monovalent type 1 oral poliovirus vaccine (OPV) at 6 months and trivalent OPV at 7 and 9 months. Group 1 had received 4 doses of OPV, group 2 OPV at birth and 3 doses of OPV and inactivated poliovirus vaccine (IPV), and group 3 placebo at birth and 3 doses of IPV. A total of 547 infants completed the study. At 10 months, seroprevalence to poliovirus type 1 was 98%, 99%, and 98% in groups 1, 2, and 3; 100%, 100%, and 98% to poliovirus type 2; and 80%, 96%, and 91% to poliovirus type 3. Differences in seroprevalence among the groups were significant for poliovirus type 3 (P < .001). Between 6 and 10 months, significant increases in seroprevalence and GMTs occurred for poliovirus type 1 but not for types 2 and 3. Two OPV doses following 3 IPV doses did not significantly increase seroprevalence or raise GMTs for poliovirus types 2 and 3; however, significant increases were found for poliovirus type 1, which may have benefitted from monovalent type 1 administration.

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 1988-1989 epidemic of type 1 poliomyelitis in Oman provided the stimulus to test the hypothesis that these observations were due to regional differences in the response of infants to trivalent oral poliovirus vaccine (OPV). Seroprevalence studies of 394 children born during the outbreak were conducted in six different regions of Oman and in two socioeconomic status (SES) groups in the capital city of Muscat; a seroconversion study was also carried out in 105 infants born after the outbreak. Seroprevalence rates by region after receipt of at least three doses of OPV ranged from 90% to 100% (median 94%) to poliovirus type 1, and from 86% to 100% (median 97%) to type 2, and from 47% to 79% (median 72%) to type 3, with the lowest rates observed in regions with the highest incidence of type 1 paralytic disease. In Muscat, seroprevalence rates were also significantly lower in low versus high SES groups (type 1: 84% versus 98%, respectively [P = 0.006]; type 3: 59% versus 86%, respectively [P = 0.001]). In the seroconversion study conducted after the outbreak, 89%, 100% and 50% of infants had detectable antibodies to types 1, 2, and 3, respectively, after four doses of OPV. Low responses to type 3 were also associated with the occurrence of sporadic cases of type 3 poliomyelitis in 1991, in spite of high rates of coverage with at least four doses of OPV (> 96%) throughout the country.(ABSTRACT TRUNCATED AT 250 WORDS)

Attributable risk of DTP (diphtheria and tetanus toxoids and pertussis vaccine) injection in provoking paralytic poliomyelitis during a large outbreak in Oman.

Although injections administered during the incubation period of wild poliovirus infection have been associated with an increased risk of paralytic poliomyelitis, quantitative estimates of the risk have not been established. During a poliomyelitis outbreak investigation in Oman, vaccination records were reviewed for 70 children aged 5-24 months with poliomyelitis and from 692 matched control children. A significantly higher proportion of cases received a DTP (diphtheria and tetanus toxoids and pertussis vaccine) injection within 30 days before paralysis onset than did controls (42.9% vs. 28.3%; odds ratio, 2.4; 95% confidence interval, 1.3-4.2). The proportion of poliomyelitis cases that may have been provoked by DTP injections was 35% for children 5-11 months old. This study confirms that injections are an important cause of provocative poliomyelitis. Although the benefits of DTP vaccination should outweigh the risk of subsequent paralysis, these data stress the importance of avoiding unnecessary injections during outbreaks of wild poliovirus infection.

Outbreak of paralytic poliomyelitis in Oman: evidence for widespread transmission among fully vaccinated children.

From January, 1988, to March, 1989, a widespread outbreak (118 cases) of poliomyelitis type 1 occurred in Oman. Incidence of paralytic disease was highest in children younger than 2 years (87/100,000) despite an immunisation programme that recently had raised coverage with 3 doses of oral poliovirus vaccine (OPV) among 12-month-old children from 67% to 87%. We did a case-control study (70 case-patients, 692 age-matched controls) to estimate the clinical efficacy of OPV, assessed the immunogenicity of OPV and extent of poliovirus spread by serology, retrospectively 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 neutralising 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 immunisation programme in reducing spread of endemic strains, suboptimum efficacy of OPV, and delay in completing the primary immunisation series until 7 months of age. Additionally, the estimated attack rate of infection among children aged 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.