A large study on immunological response to a whole-cell killed oral cholera vaccine reveals that there are significant geographical differences in response and that O blood group individuals do not elicit a higher response.

The ABO blood group system has been implicated in susceptibility to cholera or in explaining variability in the immune response to a cholera vaccine. O blood group individuals were found to be more susceptible to cholera and elicited lower vibriocidal antibody response to cholera toxin B subunit-killed oral vaccine. Based on the observations that O blood group individuals were more susceptible to cholera and that high mortality was associated with cholera, an evolutionary explanation was provided for the extremely low prevalence of the O blood group in the Gangetic Delta (West Bengal, India, and Bangladesh). However, conflicting results were reported from a later study conducted in Indonesia using a live attenuated oral cholera vaccine; O blood group individuals showed a higher vibriocidal antibody response. In a study conducted in a region of India where cholera is endemic (Kolkata, West Bengal) that comprised 992 individuals vaccinated by a killed whole-cell oral cholera vaccine, we found no statistically significant difference between O and non-O individuals either in the frequency distributions of the fold increase or in the postvaccination increase in geometric mean titer compared to the baseline. Further, in contrast to the earlier observation that the O allele frequency is extremely low in the Gangetic Delta, we have noted that the O allele frequency exceeds 0.5 in the vast majority of ethnic groups of this region. In addition, we have found large differences in response to the vaccine among residents of an area where cholera is not endemic compared to an area where cholera is endemic to The percentages of vaccinees who seroconverted in an area where cholera is not endemic (Son La province of Vietnam) was >90% compared to approximately 50% in Kolkata, India, an area where cholera is endemic.