Question

why do polio vaccine given as oral drops why not to give as injection

Answer 1

Becz at that age the baby is not developed properly

Answer 2

First polio vaccine invented was injection. But soon found that it's hard to give injection to small children. So they started to work out on polio vaccine which can be given through polio drops. It was easy to give small children polio drops and even many children could be given and it was easy too.
As it's polio drops children don't find any pain so they don't hesitate to take polio vaccine but on the other hand if it's injection they make a lot of fuss. And I the volunteer is not experienced his injection can go wrong causing many problems.

The main reason for it is to reduce the chances of getting affected with polio.
There's a virus called as Wild virus found in the intestines of small children under 5 years. As it's found inside their intestines it's difficult to kill it with vaccine so polio drops were invented. By giving oral drops they are safe the chances for getting affected is really less.

Hope this helps : )