Local reactions can range from local inflammation to infection. Good technique will minimize reactions. Taking the symptoms in approximate order of severity and from symptoms of transient inflammation to abscessation:

• Symptoms of transient inflammation usually occurring 24 to 72 hours post vaccination. One or all of these symptoms may be present, usually developing in the order given:
  • Mild soreness at the site of injection
  • Stiffness walking. If given in the neck it may appear as a generalized stiffness and inability to lower the head and if given in the hip or leg lameness may result. The stiffness and soreness following vaccination can be remarkable accompanied by a short shuffling gait resembling founder.
  • Fever, depression, inappetence that responds to NSAID's (see treatment below).
  • Mild painful swelling that responds to NSAID's (see treatment below).
• Symptoms of abscess formation, usually following a week of symptoms of inflammation:
  • persistent and worsening remarkable swelling, soreness, stiffness, depression, fever that does not respond to NSAID's.

Vaccines differ in their ability to cause local irritation. Most of the common vaccines for life threatening diseases (tetanus, the viral encephalitides, and rabies) are efficacious and have a low local reaction rate. It is probable that all horses receiving these vaccines have some reaction: have you ever had a tetanus shot that did not cause some soreness? But when given properly most of these reactions are subclinical and do not cause overt soreness or swelling, the local reaction rate of these is on the order of 5%. On the other hand influenza and herpes vaccines have a higher reaction rate that approach 10 to 20%. Worse of the commonly used injectable vaccines is the Strangles vaccine, with a local reaction rate of around 30 to 60%, with the stiffness worse than for the other vaccines.

The abscess can be either sterile or infected. Sterile abscesses form when the reaction is severe enough to cause local necrosis and white blood cells flood the area in an attempt to clean up the necrosis. Infections maybe introduced by the injection but may also be activated bacteria that were lying dormant in the muscle tissue. Serious clostridial myositis can occur this way.

Systemic reactions can develop within minutes to hours after administration of the vaccine.

• Hives: may be a precursor to developing anaphylaxis
• Colic: cause unknown, perhaps a drop in blood pressure?
• Anaphylaxis: allergic shock, usually first noted as respiratory distress in horses.
• Purpura: immune mediated vasculitis, bilateral hot swelling in the legs, may take weeks to develop

Whether you get a local reactions or systemic reaction depends on which part of the immune system is over stimulated.