Search

The 1966-67 cholera vaccine field trials in East Pakistan tested 1- and 2-dose schedules of a commercial cholera vaccine in 40 000 children aged 3 months to 14 years. Randomsample serological surveys, made prior to the inoculations and 3 months and 6 months after the inoculations, demonstrated that there was a rise in the vibriocidal titres of the vaccinated children during the first 3 months after inoculation and a subsequent fall by the end of the second 3 months. The antibody response to 2 doses of cholera vaccine was better than the response to a single dose in children under 5 years of age. In children aged 5-14 years, the antibody response was similar for both inoculation schedules. Since the majority of the older children had vibriocidal antibodies before inoculation, the data suggest that the single dose acted as a booster, and this effect was not enhanced by a second inoculation.Serological studies of the hospitalized cholera patients indicated that the majority had low vibriocidal titres on admission to hospital. By comparing the distribution of admission titres of the hospitalized patients with the distribution of titres found in the population survey, it was possible to demonstrate a progressive reduction in the cholera case rate for the population with high levels of vibriocidal antibody.The sample surveys from the control population revealed a rise in titre following the peak of the cholera season, and a fall 3 months later. The data suggest that the rate of infection with V. cholerae for the 10 000 children in the control group during the cholera season may have been as high as 27%, while the clinical case rate was only 0.26%.

Controlled cholera vaccine field trials were held in Matlab Bazar in rural East Pakistan in 1963 and 1964. In July—September 1965, a serological survey for cholera antibodies was carried out on a random sample of the field-trial population. Results are given for the control group only, as representative of the pattern found in an endemic cholera area. Only 2% of the blood samples from children under 10 years of age were found to have detectable agglutinating antibody (titres of 1:20 or more), while the proportion in the age-group over 30 years was 27%-30%. Tests for vibriocidal antibody showed that none of the blood samples from children under 1 year of age contained detectable antibody, while 87%-90% of those from adults over 30 years did so. The rise in the geometric mean vibriocidal titre showed an almost linear correlation with a sharp fall in the cholera case rate with age. Since a similar survey in a non-endemic area (Czechoslovakia) did not reveal any rise in antibody titre with age, and since cholera epidemics in areas without recent experience of the disease are characterized by a higher incidence rate in adults than in children, it is suggested that the antibody titres give a measure of the level of immunity in a population.

Controlled field trials of a highly antigenic cholera vaccine were held in Matlab Bazar in rural East Pakistan in 1963 and again in 1964. In July—September 1965, a serological survey for cholera antibodies was carried out on a random sample of the field-trial population. This survey revealed that it was possible to demonstrate the effect of a single injection of the cholera vaccine per head on the proportion of the population with detectable vibriocidal and agglutinating antibody 10 and 22 months after injection. More significantly, the reduction in cholera case rate caused by the vaccine could be correlated with the rise in vibriocidal antibody after vaccination, suggesting that the serological response to vaccine in man may be a useful measure of vaccine potency. The survey also indicated that in this endemic cholera area, with a high level of immunity in adults, a single injection of cholera vaccine was in fact a booster dose for the majority of the population. Thus, the results of cholera vaccine field trials in endemic areas cannot be directly extrapolated to predict the effects of the same vaccine in non-endemic areas.

The relative effectiveness of daily rectal-swab cultures and cultures of purged stools in detecting inapparent infections of Vibrio cholerae were compared in family contacts of cholera patients. In one study of 187 contacts, a combination of purging by various methods plus 5 daily swabs detected 50 infections. Only 27 (54%) were detected by the purge alone, while 47 (94%) were detected by 5 daily rectal-swab cultures. In a second study of two groups of family contacts, purging on the first day did not appear to influence significantly the number of positive rectal-swab cultures obtained during the 4 subsequent days. The relative inefficiency of purging in detecting the inapparent infections among cholera contacts, as compared with its reported efficacy in the chronic convalescent carrier, probably reflects a basic difference in the pathogenesis and site of infection in these different conditions.