Without vaccines, thousands of dogs would be at risk for serious and often fatal illnesses. The lump is your dog's natural way of reacting to the shot.
Vaccinating Your Dog
When you take your dog to the veterinary clinic to get his shots, most veterinarians will warn you that your dog may feel a little lethargic or sore at the injection site. Vaccines work by injecting a small amount of a virus or disease into your dog and then allowing his natural immune system to react to the threat to build his immunity to the disease.
Swelling at Injection Site
Regardless of how beneficial vaccines are to your dog, you need to remember that you are piercing his skin with a shot and injecting foreign matter into his body. The natural reaction for your dog's immune system is to react to that foreign matter and protect your dog from getting sick. The tissue at the injection site will swell as your dog reacts to the perceived injury, creating the lump that you are seeing. The more severe of a reaction your dog has to the vaccine, the larger the lump will be. Not all dogs will get a lump at the injection site and your dog may not get a lump every time or for every vaccine.
Treating Swelling at the Injection Site
You can apply a cold compress to the lump at the injection site to help reduce swelling. The lump should disappear on its own, without treatment, within a couple of days of the initial injection. If the lump appears to be growing or shows signs of infection, such as redness or pus, you need to take your dog back to the veterinarian for treatment.
Tumors are a relatively rare vaccine side effect but they do occur from time to time. If the lump on your dog's injection site does not disappear on its own or it appears days or even weeks after your dog was vaccinated, there is a chance the lump is actually a tumor. If you suspect your dog has developed a tumor as a result of a vaccination, take him to the veterinarian immediately for diagnosis and care.