The guidance in the FAR is straight forward regarding issues related to contract tax questions. It is provided in FAR Part 29.101 and 29.201, provided below.  Recommend you request assistance from your agency legal counsel (FAR 29.101). Note the law concerning taxble vaccines is found at 26 U.S. Code Subchapter C—Certain Vaccines.

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29.101 Resolving tax problems.

       (a) Contract tax problems are essentially legal in nature and vary widely. Specific tax questions must be resolved by reference to the applicable contract terms and to the pertinent tax laws and regulations. Therefore, when tax questions arise, contracting officers should request assistance from the agency-designated legal counsel.

29.201 General.

       (a) Federal excise taxes are levied on the sale or use of particular supplies or services. Subtitle D of the Internal Revenue Code of 1954, Miscellaneous Excise Taxes, 26 U.S.C.4041, etseq., and its implementing regulations, 26 CFR parts 40 through 299, cover miscellaneous federal excise tax requirements. Questions arising in this area should be directed to the agency-designated counsel...