I always spell out medication names so there is no confusion and the patient does not get the wrong one (I worked as a pharmacy tech and take it very seriously). Having said that, you need to know what immunization is an abbreviation and what is spelled that way by the manufacturer.
An example might be MMR (measles, mumps, rubella. A doctor might say "the child is scheduled for his 6-month MMR immunization." Versus "the child received his 6-month M-M-R II injection today."
Another example is mumps vax. "The child presented for his mumps vax (which you would transcribe as vaccination depending on your account specs)" versus "we gave the child a MUMPSVAX injection today."
Pediatrics can be especially challenging for an MT.