Use cases for Patient and RelatedPerson in Immunization.performer.actor include self-administered vaccinations or parent-administered-vaccinations (e.g., https://www.clinicaltherapeutics.com/article/S0149-2918(16)30856-6/fulltext). ehaas also mentioned use cases in veterinary care.

Generally should consider all resource types listed in http://build.fhir.org/participant.html#Participant