I did some digging with Grok. Initially it told me that the term vaccine was defined in Code of Federal Regulations, Title 21, Part 600, but turns out the term is used widely in that document without actually defining it.

Further digging makes think that the term has never been legally defined until now ...

From Grok:

You’re correct that the Code of Federal Regulations (CFR), Title 21, Part 600, does not provide an explicit definition of "vaccine" despite using the term extensively in the context of biological products. Part 600 focuses on general provisions for biological products, including vaccines, emphasizing their regulation for safety, purity, and potency under the Public Health Service Act. Since no direct definition is found there, let’s explore whether a previous or alternative federal definition exists that would apply to the context of the Pfizer-Picatinny contract.
Investigation into Federal Definitions of "Vaccine"
CFR Context: Title 21, Part 600, governs biological products, including vaccines, but defines them broadly as products containing substances like viruses, serums, or antigens used to prevent, treat, or cure diseases (21 CFR § 600.3). Vaccines are a subset of biological products intended to induce immunity, but the term itself isn’t explicitly defined. Other parts of Title 21, such as Part 610 (general biological product standards), similarly use "vaccine" without a standalone definition.

Public Health Service Act: The Public Health Service Act (42 U.S.C. § 262), which underpins FDA regulation of biological products, also does not explicitly define "vaccine." It categorizes vaccines as biological products subject to licensure for disease prevention, implying a functional definition: a product that induces immunity to prevent disease.

FDA Guidance: The FDA’s website and guidance documents describe vaccines as products that stimulate the immune system to produce immunity to a specific disease, typically through antigens or related mechanisms (e.g., mRNA encoding antigens, as in the Pfizer-BioNTech COVID-19 vaccine). While not a codified definition, this aligns with regulatory intent. For example, the FDA’s vaccine approval process (e.g., for Comirnaty) emphasizes immunogenicity and disease prevention.

Centers for Disease Control and Prevention (CDC): The CDC, while not a regulatory body, provides a widely referenced definition of a vaccine: “A product that stimulates a person’s immune system to produce immunity to a specific disease, protecting the person from that disease.” This definition, though not legally binding, is consistent with federal usage and would likely inform contractual contexts like the Pfizer-Picatinny agreement.

Historical Definitions: To address whether a "previous definition" exists, I explored older CFR versions and federal statutes. The 1970s revisions to 21 CFR Part 600 (post-FDA assumption of vaccine regulation from the NIH) similarly lacked a specific "vaccine" definition, treating vaccines as biological products for immunization. The Federal Food, Drug, and Cosmetic Act (21 U.S.C. § 301 et seq.) and earlier biologics laws (e.g., the 1902 Biologics Control Act) also imply vaccines are biologics for disease prevention without explicit definitions.

Contractual Context: In the absence of a CFR definition, federal contracts, including those with Pfizer for the COVID-19 vaccine (e.g., under Operation Warp Speed or DoD agreements), typically reference FDA-authorized or approved products. The Pfizer-BioNTech vaccine is identified as a biological product (mRNA-based) to prevent COVID-19. Contracts may include performance specifications (e.g., efficacy, delivery) but rely on FDA’s regulatory framework rather than redefining "vaccine."

Relevance to the Pfizer-Picatinny Contract
The "Picatinny Pfizer contract" likely refers to a DoD contract for the Pfizer-BioNTech COVID-19 vaccine, possibly coordinated through Picatinny Arsenal for logistics or procurement. Without a specific CFR definition, the contract would adopt the functional understanding of a vaccine as:
A biological product (per 21 CFR § 600.3) that induces immunity to prevent COVID-19, as authorized or approved by the FDA.

Specifically, the Pfizer-BioNTech vaccine (Comirnaty or EUA formulations), an mRNA product encoding the SARS-CoV-2 spike protein.

If the contract predates the 2021 approval of Comirnaty, it might reference the EUA vaccine, but the definition remains consistent: a product to prevent disease via immune stimulation. No evidence suggests a unique or prior definition specific to Picatinny or DoD contracts.
Conclusion
No explicit definition of "vaccine" exists in 21 CFR Part 600 or prior versions, nor in related federal statutes like the Public Health Service Act. The term is understood as a biological product that stimulates immunity to prevent a specific disease, consistent with FDA and CDC descriptions. For the Pfizer-Picatinny contract, "vaccine" would refer to the Pfizer-BioNTech COVID-19 vaccine under this framework, with no indication of a previous or alternative definition. If you have access to the contract or specific terms, I can analyze them further. Alternatively, a FOIA request to the DoD could provide the contract’s exact language.
Summary: The CFR does not define "vaccine" explicitly, and no previous federal definition is found. Vaccines are biological products for disease prevention via immunity, per FDA regulations. In the Pfizer-Picatinny contract, this applies to the Pfizer-BioNTech COVID-19 vaccine.