Small children and people in developing countries may take oral dosage forms of vaccines. Antigens passing through the GI tract will come into contact with Peyer's patches and be absorbed by microfold cells into lymphoid tissue, evoking an immune response.
Some of these vaccines are attenuated and can colonize in the small intestine, mimicking a natural infection. I am wondering why vomiting and diarrhea, which are common symptoms in many natural infections, aren't more prevalent.
To be even more specific: Serotonin is released in response to a cytokine release by immune cells. Serotonin activates 5-HT3 receptor to induce vomiting. Why is this response not provoked by an attenuated infection, which mimics an actual infection.