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Clinical efficacy of trivalent oral polio vaccine tested: polio outbreak in Honduras.


 In response to a polio outbreak in Honduras in 1984 largely affecting children under 5 years of age, a case-control study was conducted to evaluate the clinical effectiveness of trivalent oral polio vaccine (TOPV). The case definition utilized was any child with acute flaccid paralysis compatible with polio who was reported to the Ministry of Health between January 1-November 26, 1984. 59 of the 76 such cases were included, and 5 age-matched controls were selected for each case. 2 methods were used to calculate the odds ratio and vaccine efficacy. In the 1st method, a case-control matched analysis with variable matching ratio was utilized; in the 2nd method, only the first 2 controls with zero or 3 or more TOPV doses were studied. When the 1st method was applied, vaccine efficacy was 50% among all cases and 51% among the 27 comfirmed cases. When the 2nd method was used, vaccine efficacy was 35% overall and 43% among comfirmed cases. Of the 59 cases investigated, 31 (53%) had received 3 or more doses of TOPV 1 month or more before onset of disease. In addition, 51 (86%) had residual findings compatible with polio 2 months or more after onset of symptoms. 8 (14%) of the 59 cases died. Data on associated risk factors are currently being analyzed. A theoretical problem with this study is the effect of subclinical disease on vaccine efficacy. Because of the small number of cases, the 95% confidence limits were wide and the results should be interpreted with caution; however, the highest upper confidence limit was only 82%, suggesting that the receipt of 3 doses of TOPV has not provided in Honduras the expected clinical efficacy of over 90%. Vaccine management problems at the central level may account for this finding. Problems in the cold chain are being addressed, and routine monitoring of the potency of TOPV used in Honduras will be conducted.
