Antibody-Based Correlates of Protection against Cholera: Analysis of a Challenge Study of a Cholera-Naive Population

Immunologic correlates of protection can be used to infer vaccine efficacy for populations in which challenge trials or field studies are infeasible. In a recent cholera challenge trial (W. H. Chen et al., Clin Infect Dis 62:1329–1335, 2016, https://doi.org/10.1093/cid/ciw145), 134 North American cholera-naive volunteers were randomized to receive either the live, attenuated single-dose cholera vaccine CVD (Center for Vaccine Development) 103-HgR or placebo, and the titers of vibriocidal antibodies against the classical Inaba strain were assessed at 10 days after treatment. Subsequent to the immunologic evaluation, each subject ingested a fixed quantity of virulent Vibrio cholerae O1 El Tor Inaba. Data from this trial suggest that the vaccine-induced increase in the vibriocidal antibody titer prior to challenge is tightly linked with protection: 51/51 vaccinees with postvaccination vibriocidal antibody titers of ≥2,560 were protected against moderate/severe diarrhea, and 60/62 vaccinees who seroconverted or experienced a 4-fold or greater increase in vibriocidal antibody titer relative to prevaccination levels were similarly protected. Atypically high vibriocidal antibody titers were observed in some placebo subjects; protection was limited in these individuals and differed substantially from the level of protection experienced by vaccinees with the same postvaccination titers. Since only 1 of 66 placebo recipients experienced seroconversion, seroconversion was found to be uniquely associated with vaccination and insensitive to the effects of factors that can cause titers to be elevated but are weakly associated with protection. Thus, vibriocidal antibody seroconversion was found to be better than the vibriocidal antibody titer for inferring vaccine efficacy in cholera-naive populations for which studies based upon exposure to V. cholerae are impractical. (This study has been registered at ClinicalTrials.gov under registration no. NCT01895855.)