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.

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    $\begingroup$ It should be noted that only some vaccines are recommended to be delivered orally, while others require an injection (typically intra-muscular) for optimal efficacy. $\endgroup$ – MattDMo Dec 19 '16 at 15:17

First, ongoing inflammatory process/immune reaction in the gastrointestinal tract is not necessary associated with vomiting or diarrhoea. As an example we can consider infection with Helicobacter Pyroli, which may last for many years unnoticed.

Second, exposition to foreign antigens obtained form food or microflora and subsequent antigen presentation to the immune cells (which is indeed partially driven by locally released cytokines) occur in the GI tract all the time and again, is not usually manifested by vomiting or diarrhoea.

But moving to vaccines, attenuated oral vaccines (like Polio Sabin or Rotavirus) are developed in order to diminish the risk of pathological symptoms. Attenuation weaks the pathogen that is usually no longer able to proliferate and invade, but still poses antigens that may be presented to the immune cells and induce long-term immunological memory. Moreover, immune response (antigen presentation, production of the local IgA, B-cells affinity maturation) triggered by attenuated vaccines is the closest to the one induced by full infection (in comparison to inactivated/toxoid vaccines), but significantly weaker and localised. This we mean by "mimicking a natural infection".

Also, see: https://www.sciencedirect.com/science/article/pii/S0042682215001725

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