Reply #8: The degree of immune response is the significant thing here.

[pnwmom]

In susceptible people, a vaccine, especially with an adjuvant designed to increase an immune response, or multiple vaccines at the same time, could -- as the researcher says -- put the immune system into "overdrive" (as compared to exposure to an ordinary virus.)

The same thing happens with allergies. Someone might not react to cat fur or to red wine -- but expose them to both at the same time and they have an allergic reaction. The body can handle only so much exposure to an allergen before it reacts.