I think you have to consider several things: 1) dosing of a vaccine may not be valid (due to many reasons) and therefore just because something is labeled as OPT3 it might not be the third valid dose. That is why we use sequence number separately from the concept. 2) Whether something is considered a booster or not depends on formulation of the vaccine, so I am not sure that it matters what the reason is for the vaccine, but rather only when it was given, and its content. 3) Vaccines frequently have different components and whether a child has completed a particular series may depend on analyzing several different vaccine types administered. Hence the requirement that vaccines be coded with CVX codes. Having a free text entry is really not a good idea. 4) If you give someone a place to record something in free text, and they can’t bother to figure out which of the coded options is the correct one, they will use the free text version.