Thus, a stronger antibody response (i.e. higher titres of more diverse antibody molecules) having improved affinity towards antigen is typically observed in the secondary immune response.
In the absence of serological data we can not rule out the possibility that vaccinated animals had poor antibody titres, which may have played a part in the outbreak.
We have another daughter whom we no longer vaccinate (she still has immunity to measles, mumps, chicken pox, etc--we know this because we run titres to make sure).