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