Immunizations are a deductible medical expenses. Although it is not specifically listed in IRS Publication 502, it meets the very basic definition of medical expenses in that publication. See the following from IRS Publication 502:
Medical expenses are the costs of diagnosis, cure, mitigation, treatment, or prevention of disease , and the costs for treatments affecting any part or function of the body. These expenses include payments for legal medical services rendered by physicians, surgeons, dentists, and other medical practitioners. They include the costs of equipment, supplies, and diagnostic devices needed for these purposes.
Medical care expenses must be primarily to alleviate or prevent a physical or mental defect or illness. They don't include expenses that are merely beneficial to general health, such as vitamins or a vacation.
Medical expenses include the premiums you pay for insurance that covers the expenses of medical care, and the amounts you pay for transportation to get medical care. Medical expenses also include amounts paid for qualified long-term care services and limited amounts paid for any qualified long-term care insurance contract.