Well ALL vaccines have the potential to be "harmful". Human vaccines, pet vaccines, all vaccines are is they are injecting a weakened strain of whatever virus into your system so your body can build up antibodies against the virus or whatever so if you DO come in contact with it again you will have a strenghtened immunity against it. BUT sometimes it backfires, and eventhough its a weakened strain, you can become infected by it and get sick. So that is always a risk you take, whether it be you, your children, or your pets.