Development of Pathogenicity‐Driven Definitions of Outcomes for a Field Trial of a Killed Oral Vaccine against EnterotoxigenicEscherichia coliin Egypt: Application of an Evidence‐Based Method

Background. To design an efficacy trial of a killed oral vaccine against enterotoxigenic Escherichia coli ( ETEC) diarrhea in Egyptian children, we derived for ETEC diarrhea an empirical definition that increased the probability that diarrhea associated with excretion of ETEC was caused by the detected ETEC. Methods. We conducted a cohort study of 397 Egyptian children < 24 months old and monitored them until they were 3 years old. Vaccine-preventable (VP) ETEC was defined as ETEC expressing &GE;1 of the toxin- (heat-labile [LT] toxin) and colonization-factor antigens (CFA I, II, and IV) in the vaccine. Results. Although fecal excretion of VP-ETEC was highly associated with diarrhea, excretion of LT-ETEC per se was not related to diarrhea (adjusted odds ratio [ORA], 1.16 [95% confidence interval {CI}, 0.90-1.49]). The fecal excretion of antigenic types of VP-ETEC other than LT-ETEC (non-LT VP-ETEC) was highly associated with diarrheal symptoms (ORA, 3.91 [95% CI, 2.78-5.49]; P < .001), and this association was greater for nonbloody than for bloody diarrhea. Conclusions. Because the vaccine had been anticipated to protect primarily against symptomatic ETEC diarrhea, these results indicate that the primary-outcome definition of ETEC diarrhea for the trial should be restricted to nonbloody diarrheal episodes associated with fecal excretion of non-LT VP-ETEC.