Practitioners must always walk a fine line when deciding how to tailor vaccine protocols for their patients, but a certain activity has drawn the attention of the federal government and veterinary associations recently.
The Iowa VMA, acting on a member's concern, posted a precaution in its March 14 Communique regarding the off-label use of rabies vaccine as a diluent to reconstitute either canine or feline distemper combination vaccines. The state association warned that this is not an approved use of the rabies vaccine unless it is specifically directed by the manufacturer, and that practitioners should seek advice from the technical services department of their vaccine manufacturer before using rabies vaccine as a diluent for any other vaccine.
Word of the notice reached the Center for Veterinary Biologics of the Department of Agriculture's Animal and Plant Health Inspection Service, which then issued its own statement in late April to further clarify the issue.
Although canine and feline vaccines were specifically cited in the IVMA post, the CVB says it is concerned with any off-label use of vaccines, but specifically rabies vaccines, as diluents to reconstitute desiccated live-virus vaccines. The center's statement said rabies vaccines used as a diluent can substantially reduce the efficacy of the live viruses contained in veterinary distemper vaccines.
"Inactivated liquid biological products, such as rabies vaccines, may contain components that are viricidal, such as preservatives, adjuvants, or residual amounts of inactivating agents. In effect, reconstituting a live viral vaccine with such a diluent can inactivate one or more live viral fractions in a combination product, such as distemper vaccines," which could reduce or eliminate the effectiveness of the product, according to the statement.
To ensure the efficacy of live viral vaccines licensed by the CVB, each serial of inactivated liquid vaccine to be used as a diluent is extensively tested for the detection of viricidal activity.
Using rabies vaccines as an unapproved diluent also raises concerns about the risk to public health, according to the CVB statement. That's because the efficacy of rabies vaccines when used as an unapproved diluent has not been evaluated. In addition, the expected prevalence of adverse vaccine reactions associated with the practice of off-label use of rabies vaccine to reconstitute distemper vaccines is unknown—for both live-virus and killed-virus vaccines.
The AVMA Council on Biologic and Therapeutic Agents and Clinical Practitioners Advisory Committee also note that other possible results from vaccinating with improperly reconstituted or mixed vaccines include potential toxemia from inappropriate mixtures of products, and changes in withdrawal interval times following vaccination of food animals.
As a result, inappropriate reconstitution or mixing of animal vaccines could be determined to be a breach of the standard of care. This could create a liability risk for veterinarians and a health risk for their patients, not to mention that it undermines the USDA's licensure process.