USPS Packaging Instruction 6C

Category B Infectious Substances

Infectious substance means a material known or reasonably expected to contain a pathogen. A pathogen is a microorganism that can cause disease in humans or animals. Examples of pathogens include bacteria, viruses, fungi, and other infectious agents. An infectious substance must be assigned to one of the following two packaging categories:

Proper Shipping Name and ID Number

Required Authorization

All vendors shipping COVID-19-related UN3373 Category B Infectious Substances kits to end-users must obtain an authorization from the Postal Service before mailing. The shipper has the responsibility to ensure awareness of, and compliance with, all other applicable requirements and regulations for mailing these materials. The shipper must be able to provide evidence of compliance before submitting a written request to the director of Product Classification, Postal Service Headquarters (see part 214 for address).

Under these provisions, only tests developed and performed by laboratories certified under the Clinical Laboratory Improvement Amendments (CLIA) or equivalent clinical oversight regulations, and commercial tests and home collection kits authorized by either the FDA or an Institutional Review Board, will be considered.


Required Packaging

Primary Receptacle (Container)
Cushioning and Absorbent Material
Secondary Container

Note: Only cold packs or dry ice may be used as a refrigerant and must be placed outside of the secondary packaging. Interior supports must be provided to secure the secondary packaging in the original position. If a cold pack is used, the packaging must be leak-proof. If dry ice is used, the container must permit the release of carbon dioxide gas and conform to 49 CFR 173.217. The primary receptacle and secondary packaging must maintain their integrity at the temperature of the refrigerant used, as well as the temperature and pressures of transport by aircraft they could be subjected to if refrigerant were lost, and sufficient absorbent material must be provided to absorb all liquids, including melted ice.

Outer Shipping Container

As required by 49 CFR 173.199:


Each vendor shipping COVID-19 kits must provide clear instructions to end-users regarding the procedures to be followed for preparing the samples and packaging used to transport an Infectious Substance Category B. Shippers must instruct end-users to adhere to all applicable mail-related preparation requirements before mailing to ensure the package is properly prepared for safe transportation.

International Mail

Note: Countries distinguish between infectious and noninfectious (nonregulated) biological substances and may prohibit one or the other or both. See “Prohibitions” in the Individual Country Listings.