Source attribution studies report that consumption of contaminated poultry is the primary source for acquiring human campylobacteriosis. Oral administration of an engineered Escherichia coli strain expressing the Campylobacter jejuni N-glycan reduces bacterial colonization in specific-pathogen-free leghorn chickens, but only a fraction of birds respond to vaccination. Optimizing the vaccine for commercial broiler chickens has great potential to prevent pathogen entry into the food chain. Here, we tested the same vaccination approach in broilers and observed similar efficacy in pathogen load reduction, stimulation of host IgY response, lack of C. jejuni resistance development, uniformity in microbial gut composition, and bimodal response to treatment. Gut microbiota analysis of leghorn and broiler vaccine responders identified one member of the Clostridiales XIVa cluster, Anaerosporobacter mobilis, significantly more abundant in responder birds. In broilers, co-administration of the live vaccine with A. mobilis or Lactobacillus reuteri, a commonly used probiotic, resulted in increased vaccine efficacy, antibody response, and weight gain. To investigate whether the responder/non-responder effect was due to selection of a C. jejuni 'super colonizer mutant' with altered phase-variable genes, we analysed all polyG-containing loci of the input strain compared to non-responder colony isolates and found no evidence of phase state selection. However, untargeted NMR-based metabolomics identified a potential biomarker negatively correlated with C. jejuni colonization levels possibly linked to the increased microbial diversity in this subgroup. The comprehensive methods used to examine the vaccine response bimodality provide several opportunities to improve the C. jejuni vaccine and the efficacy of any vaccination strategy.ImportanceCampylobacter jejuni is a common cause of human diarrheal disease worldwide and listed by the World Health Organization as a high priority pathogen. C. jejuni infection is typically through the ingestion of contaminated chicken meat, so many efforts are targeted to reduce C. jejuni levels at the source. We previously developed a vaccine that reduces C. jejuni levels in egg-laying chickens. In this study, we improved vaccine performance in meat birds by supplementing with probiotics. In addition, we demonstrated that C. jejuni colonization levels in chickens are negatively correlated with Clostridial abundance, another group of common gut microbes. We describe new methods for vaccine optimization that will assist in improving the C. jejuni vaccine and other vaccines under development.