Herd Protection and Herd Amplification in Cholera

Recent analysis of results of the 1985 vaccine trial in Bangladesh showed that a killed oral cholera vaccine could provide herd protection (1), and this finding sheds new light on the potential utility of this vaccine and other oral cholera vaccines. Although there may be several mechanisms for herd protection, this finding of herd protection was somewhat unexpected. The nature of the herd protection with cholera vaccine is unlike that with live oral polio vaccine which can be excreted and can infect others, thereby immunizing per-sons who did not receive vaccine directly. By contrast, the cholera vaccine used in this study was inactivated, making it impossible for non-immunized persons to be immunized inadvertently. Another type of herd protection is seen with vaccine for Haemophilus influenzae type b in which the vaccination reduces respiratory carriage of the pathogen, thereby eradicating the reservoir and reducing transmission. Another example is that of measles vaccination which essentially stops transmission when the density of susceptible subjects is reduced below that needed to sustain transmission. Since cholera is transmitted directly from contaminated food or water, the finding of herd immunity seemed not entirely expected. This editorial reviews the evidence for herd protection and introduces new findings from the environmental studies on cholera to suggest a more complete understanding of the mechanisms for herd protection with cholera vaccine. Hopefully, by combining the observations of 'herd protection' with some newer concepts of 'herd amplification'coming from recent environmental studies, we may develop a better understanding of the most efficient ways to control cholera