Safety and Immunogenicity of an Adjuvanted Clostridioides difficile Vaccine Candidate in Healthy Adults: A Randomized Placebo-Controlled Phase 1 Study

Bacterial infection with Clostridioides difficile can be severe or life threatening. A vaccine candidate to prevent C. difficile infection is being developed, containing a protein (F2 antigen) that stimulates the immune system to neutralize 2 C. difficile toxins. This is the first study investigating this vaccine candidate in people.Healthy people aged 18-45 years were given 2 doses of placebo or F2 antigen. People aged 50-70 years were given 2 doses of placebo, F2 antigen, or F2 antigen with an additional component (adjuvant) that helps stimulate the immune response. Some also received a third dose of F2 antigen or F2 antigen with adjuvant.The vaccine candidate was well tolerated with an acceptable safety profile. Injection site pain, tiredness, and headache were the most common side effects. The effects were mostly mild to moderate and transient, and were more frequently reported in the adjuvanted group, as expected. No serious safety events were considered related to the vaccine candidate.The vaccine candidate induced neutralization activity against both toxins, especially when given with adjuvant. In people with no measurable neutralization activity at the study start, a third dose increased the response. These findings offer potentially valuable insights for C. difficile vaccine development.