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MARK SAWYER, MD: You sometimes hear that maybe we give too many vaccines and that they actually overwhelm the immune system and there's no reason to think that that's really the case.

For me, the best evidence for that is that after we give all those vaccines, children aren't more susceptible to getting infection. When your immune system is overwhelmed, you then get other infections because you can't fight them off. That doesn't happen in children after they've been vaccinated. Your immune system can handle many, many more vaccines than we actually give at one time.

PAUL A. OFFIT, MD: When a child is in the womb, they're in a sterile environment. When they enter the birth canal and the world, they're not. And very quickly they have living on the surface of their body trillions and trillions of bacteria to which they make an immune response.

I mean, babies will make grams of immunoglobulins every day to try and meet the challenges in their environment. The food they eat isn't sterile; the dust they inhale isn't sterile.

Let me put it to you this way: if the 14 different vaccines that we gave to children in the first few years of life were overwhelming their immune system, the species wouldn't survive given that, frankly, those vaccines are a drop in the ocean, a literal drop in the ocean, of what children typically encounter and manage every day.