Adverse Consequences of Vaccination

Modern, commercially produced, government-approved vaccines are generally very safe. Nevertheless, they are not always innocuous. The more common risks associated with vaccines include residual virulence and toxicity, which may cause injection-site reactions, allergic responses, incomplete inactivation, disease in immunodeficient hosts (modified-live vaccines), neurologic complications, and, rarely, contamination with other live agents. For example, lesions of mucosal disease may be seen in calves vaccinated against bovine viral diarrhea. Vaccines that contain killed gram-negative organisms may also contain endotoxins, which stimulate release of interleukin-1 and can cause stress with fever and leukopenia.  Although such a reaction is usually only a temporary inconvenience to males, it may be sufficient to induce abortion in females. In general, it is prudent to avoid vaccinating pregnant animals unless the risks of not vaccinating are greater. Certain modified-live virus Bluetongue vaccines have been reported to cause congenital anomalies when given to pregnant ewes. The stress from a vaccination reaction may be sufficient to activate latent infections. For example, activation of equine herpesvirus has been demonstrated after vaccination against African horse sickness. Another adverse reaction is the “sting” that occurs when some vaccines are administered. This can cause problems for the vaccinator if the vaccinated animal objects strenuously.  Some vaccines cause mild immunosuppression.

In addition to potential virulence or toxicity, vaccines, like any antigen, may provoke hypersensitivity reactions. For example, type I hypersensitivity may be seen in response to any of the antigens found in vaccines, including those from eggs or tissue-culture cells. All forms of hypersensitivity are more commonly associated with multiple injections of antigen; therefore, they tend to be associated with use of inactivated products. Type III hypersensitivity reactions are also potential hazards of vaccination. These may cause an intense local inflammatory reaction or a generalized vascular disturbance such as purpura. An example of a type III reaction is clouding of the cornea in dogs vaccinated against infectious canine hepatitis. Type IV hypersensitivity reactions, expressed as granuloma formation, may be seen at the site of inoculation in response to the use of depot adjuvants. Some adjuvanted feline vaccines are associated with the subsequent development of a fibrosarcoma at the site of injection.