Vaccines are intended to stimulate the immune system. In effect, they induce the response the immune system should have in the face of a real infection. They are like a fire drill for the immune system. As a result, our pets' bodies can have appropriate vaccine response symptoms as well as unexpected adverse vaccine reactions.

Normal or Expected Vaccine Response Symptoms:

1. Soreness at the site of vaccination / itchiness. Small gauge needles are used to administer vaccines in the subcutaneous space between the skin and underlying muscle. There is no penetration into the muscle, but the local immune response causes inflammation and often soreness at the site. In most pets this soreness can persist for a few hours up to 2-3 days.
2. Swelling at the site of vaccination. In some cases with dogs, inflammation results in immune cells crowding the injection site, causing a lump or firm swelling at the site of vaccination. In most pets this will go away over a few weeks or months. Your veterinarian can distinguish between a vaccine reaction and other types of tumors by sampling the cells in the lump.
3. Decreased appetite/tiredness / fever. As the vaccine is stimulating the immune system your pet can experience symptoms similar to those of a person who receives the flu vaccine. A reduction in appetite, increase in sleeping, or a lack of energy are normal for 2-3 days after vaccination.

If your pet experiences any of these symptoms it is important to notify your veterinarian so it can be documented in your pet’s medical record. However, none of these reactions are a reason to stop vaccinating your pet against infectious diseases.

Abnormal, Unexpected, and Possibly Dangerous Vaccine Reactions:

1. Repeated vomiting and/ or diarrhea. If your pet begins vomiting or having diarrhea after a vaccination it is important to notify your veterinarian and have your pet examined. Often these symptoms do not lead to dangerous reactions, but your veterinarian can prescribe medications to alleviate the symptoms and may elect to modify your vaccine schedule.
2. Facial swelling / hives. Immediately return to your veterinarian if your pet experiences these symptoms. Swelling can be an early warning sign of a severe reaction and your pet needs to be medicated and monitored. Again, this may not be a reason not to vaccinate.
3. Loss of consciousness / disorientation / seizures. If your pet is unconscious, stumbling, or has a seizure -- go immediately to your veterinarian or the nearest emergency hospital
4. Difficulty breathing. If you see your pet struggling to breathe or he is unable to breathe (the gums and tongue may appear dark in color or purple), go immediately to your veterinarian or the nearest emergency hospital.

If your pet experiences a vaccine reaction, from mild to severe, it is important that you notify your veterinarian and any future veterinarians so that appropriate measures can be taken to ensure your pet's health.

Source:

Journal of American Veterinary Medical Association 2005 Oct 1;227(7):1102-8, Adverse events diagnosed within three days of vaccine administration in dogs