In context of https://jira.hl7.org/browse/FHIR-28375 we propose that the Data Type list be limited to dateTime while also indicating that string is an optional choice.

Considering current systems in place, it is unreasonable to require systems that already provide structured data support for dateTime to also now have to step back and support a string instead.  In fact, doing so would violate immunization reporting standards to public health that already require structured datetime format  In those use cases, fuzzy dates where only part of the datetime is known are already addressed  by the dateTime data type String should be optional for systems that need it, but not a hard requirement to support if unnecessary.

As the FHIR US Core profile is used for accessing and exchanging data based on what the source system has, this should not require the data source system to add functionality to enable Period as part of Immunization.occurrence[x]. Additionally, C-CDA does not impose this requirement either.

Links to pages:

http://hl7.org/fhir/us/core/STU3.1/StructureDefinition-us-core-immunization.html

https://build.fhir.org/ig/HL7/US-Core-R4/StructureDefinition-us-core-immunization.html