After receiving a vaccine(s) intramuscularly, some horses experience local muscular swelling and soreness or transient, self-limiting signs including fever, anorexia, and lethargy. Severe reactions at sites of injection can be particularly troublesome, requiring prolonged treatment and convalescence. Systemic adverse reactions (such as urticaria, purpura hemorrhagica, colic, or anaphylaxis) can also occur. Other systemic adverse reactions have been anecdotally reported. It is important to differentiate between an immune response (e.g., transient muscle soreness, low grade fever) which would be an expected and acceptable response to stimulating the immune system, and a severe, life-threatening vaccine reaction requiring medical intervention.

There is potential risk in vaccinating animals whose immune status may be compromised due to disease (e.g., primary  immunodeficiency, hyperadrenocorticism), physiologic states (e.g., pregnancy), or immunosuppressive medications (e.g., corticosteroids). Veterinarians are advised to consider the benefit versus risk of vaccination in these individuals. 
 
Veterinarians should report all adverse reactions to the vaccine’s manufacturer. Adverse events may also be reported to the USDA Center for Veterinary Biologics at (1-800-752-6255) or through the Veterinary Biologics agency for Adverse Event Reporting. 

Vaccine lot and serial numbers should be noted in horses’ vaccination records. The ability to provide this information when reporting an adverse reaction will facilitate an investigation.

Adverse reactions are not always predictable and are an inherent risk of vaccination. 

Vaccination with an IN vaccine should be carefully administered when giving other injectable vaccinations at the same time or administered at a different time than injectable products to minimize the risk of local abscessation at injection sites.

As the possibility of adverse reactions (including anaphylaxis) exists with the administration of any vaccine, vaccines should ideally be administered by, or under the direct supervision of a veterinarian. Rabies vaccination, in particular, is required to be administered by, or under the supervision of a veterinarian in most states. 

For more information regarding vaccinating horses with a history of severe adverse events, please see AAEP Guidelines for Serology in Horses with Adverse Events from Vaccination.
 
Injection site selection 

Considerations should include safety of the administrator, minimizing potential for increased adverse reactions, and targeting an area large enough to allow the needle to be buried to the hub without danger of hitting vital structures such as bone, nerves, blood vessels or ligaments.

A large muscle mass that is actively used by the horse is ideal to decrease potential swelling and pain at the injection site and to promote drug absorption into the bloodstream. Ideal locations for vaccine injections include:

Base of the neck: The cervical musculature at the base of the horse's neck is an injection site that is both easy to find and relatively safe for the injector.  The site is a triangle bordered by the nuchal ligament (along the crest of the neck), the cervical vertebrae (running along the bottom half of the neck from the poll to the point of the shoulder), and the scapula (shoulder blade).  

Hamstrings: The semitendinosus (hamstring) is a large muscle mass below the point of the buttocks that makes a good injection site.   To find this injection site, locate the bony protrusion that makes up the point of the buttocks (i.e., tuber ischii), and drop about 1 inch down.  Inject anywhere in this large muscle mass along the back of the leg. 

Injection in the gluteal muscles/hip region is not recommended, as gravitational drainage along fascial planes cannot be easily achieved. Should an abscess develop, considerable tissue damage can occur and result in eruptions in undesirable locations with lesions that require prolonged time to heal.

Vaccination Interval:

The time prior to a scheduled event or a predictable risk of exposure should be sufficient for:

• Generation of a protective immune response to vaccination.

• Recovery from unexpected adverse vaccination reactions that might otherwise interfere with the horse’s performance or health prior to or during shipment.

It should be recognized that:

• Safety and efficacy data are not available regarding the concurrent use of multiple vaccines.

Reviewed by the AAEP Infectious Disease Committee in 2021.