Natural infection with certain viruses can weaken the immune system. So when children are infected with one virus, they can’t fight off other viruses or bacteria as easily. This happens most notably during natural infection with chicken pox or measles. Children infected with chicken pox are susceptible to certain bacterial infections (MRSA or flesh-eating bacteria, for example). Children infected with measles are more susceptible to bacterial infections (resulting in sepsis) of the bloodstream. But vaccines are different.
The viruses in the measles and chicken pox vaccines (the so-called vaccine viruses) are very different from those that cause measles and chicken pox infections (the wild-type viruses).
Simply put, vaccine viruses cannot weaken the immune system.
You can read more about vaccine myths in my book God’s Design for the Highly Healthy Child.
Here are other blogs in this series you might find useful: