Polio drops are not given to children suffering from diarrhea.

The reason oral polio vaccine is given is because, the vaccine virus is expected to multiply in the intestine and cause local immunity. This prevents subsequent infection in even in the intestine by wild polio virus. Moreover, the amplified number of vaccine virus is excreted by stool and hopefully overwhelms and replaces the wild polio virus gradually.

However, if the patient is suffering of diarrhea, the transit time in intestine becomes less and the opportunity for multiplication is reduced. This leads to reduced efficacy of the polio vaccine.

In diarrhea gut motility is increased. Whatever is ingested comes out quicker. Polio drops contain attenuated polio virus. Once given, a large proportion of this viral dose stays in intestine and proliferates to show its effect. If given during diarrhea, most of it will be thrown out resulting in to lesser efficacy. For this very reason we wait till diarrhea is over.

Thus the vaccine is avoided in diarrhea. As such though there is not much harm if it is given in diarrhea, other than reduced efficacy. This may lead to a false complacency of being vaccinated.