Many of the vaccinations you get as a child are IM or Intra-Muscular. That means the injection is supposed to go into the muscle. With PFFD the markers that nurses use to find the muscle in the leg are not the same. With our child (unilateral PFFD) we opted to have all the IM shots in the unaffected leg. So intead of two shots in one leg and two in the other, we'd have three in one leg and one in the other. She cried a bit more during the shots, but suffered no ill effects afterwards.