Funding pre-MTC vaccination cost

Had a bishop pose a question today I couldn't find a specific reference for. He has a missionary preparing to leave for the MTC and needs help paying for the required vaccinations. He thought he had read "somewhere" that FO funds cannot be used for vaccinations. He attempted to use Ward Missionary funds but it wasn't an option to pay the expense. Rightly so as that is not an authorized use for Ward Missionary funds.

GH 22.5.2.1 addresses use of FO funds for medical, which medically necessary vaccinations could fall under. In that paragraph there is a link to “Use of Fast Offerings for Medical Expenses.” Unfortunately my calling as stake finance clerks does not allow access to review. Based on info I have and was able to review, it seems FO-Medical would be appropriate for this Church mandated medical expense considering the circumstances. The member does not yet fall under coverage for mission medical coverage so that's not an option. Personal medical insurance, for whatever reason, is not an option and the member nor their family are unable to cover the expense out of pocket.

Is anyone aware of any additional guidance I can point this bishop to or is FO-Medical the appropriate fund to use in this case?

The final decision to use FO-Medical is up to the bishop. He may counsel with the stake president for further guidance. More than 8 years ago my ward then used FO funds for this very similar situation.

I can confirm the link in GHB 22.5.2.1 goes to the Help Center page for "Use of Fast Offerings for Medical Expenses in the United States and Canada" (there's similar pages for all other regions in the world). Nothing in this page mention vaccinations. The general counsel in the first bullet point of this page might also be applicable to your scenario:
* Before providing fast-offering assistance for medical expenses, the bishop encourages members to seek assistance from extended family, insurance providers, government resources, and other available sources
So for this case, perhaps there is help out there to pay for the vaccinations, without having to directly use fast offering funds. Also, a Bishop may know a member who is willing to provide the financial support to help this missionary be vaccinated, and who could discreetly connect with this future missionary (many Stakes have members that would love to share their blessings in this way). This would likely create a wonderful bond between the two of them as well.