Just that increases in risk are usually approximated pretty well by a linear change with dose. Radiation, poison, allergens, etc. Drugs too – the ones where the toxic dose is close to the therapeutic dose are generally pretty nasty even when used as intended. It's possible for there to be a transition between "very safe" and "noticably unsafe" as the quantity of something increases twofold, but a priori unlikely. For this to happen not only in vaccines, but through an unknown mechanism that operates precisely in this particular range, would be truly rotten luck. It's also contradicted (a posteriori) by the evidence I referred to (overdoses and animal models).