Question

Why vaccine is given to a healthy person

Answer 1

$\bf\huge\underline\red{Vaccination:-}$

Vaccination is the most effective method of preventing infectious diseases; widespread immunity due to vaccination is largely responsible for the worldwide eradication of smallpox and the restriction of diseases such as polio, measles, and tetanus from much of the world.

Answer 2

To make him or her immune to the virus of disease for which the vaccine is made. Or let's say precisely,that the vaccines prevent the virus or disease to enter the body if the virus or disease has entered the vaccine would kill it.