Hi Dan,
It looks like you are putting together a taxonomy for the library of immunization resources - indeed, a taxonomy is very useful both for the development of a data model as well as ensuring findability & discoverability in the front-end user experience.
While I'm not an expert in the knowledge domain around immunization, the medical profession is generally pretty strong on the development of taxonomies, so you should be able to find something out there that you can borrow. One approach might be to consider if there is a "higher" topic level above immunization, and look for a taxonomy at that higher level, in the hopes that you can find a "branch" that is specific to immunization.
Managing large collections of digital resources will require metadata beyond the "topic" of the resources however - you'll likely need information around what format the resources are in, the source of the resource, whether the resource is current, what region does it apply to, usage rights, what other resources are related to it, etc. There are many different standards out there for this type of metadata, however the 15 basic elements of the Dublin Core standard are both easy to understand and widely used, so it may be a good place to start:
You also can read a post on a similar taxonomy project I did last year on my site, which may give you a few more ideas:
Also keep in mind that if you do use an existing taxonomy it will be worthwhile to do some work on synonyms - words that mean the same thing. You may find that your user base refers to a concept using a specific term, but the external standard uses a different term to describe the same concept, which will result in users having trouble finding the resources using the terms they know, effectively rendering the resources "lost" in the library.
I am an independent information architect in Geneva, I'd be more than happy to chat with you in person about other approaches you can take on the project, or things you can do to make sure the taxonomy you create is a usable & sustainable one.
Hope this helps & all the best!
Adam
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UNGSTAD information architects