MMR Vaccine for Adults - What You Need to Know - Drugs.com The area where the vaccine was given may be red, tender, or swollen. You may get a fever, mild rash, or swollen glands in your cheeks or neck. Your joints may be painful and inflamed. You may have an allergic reaction to the vaccine. The MMR vaccine may cause a low platelet count, which may lead to internal bleeding. This can be life-threatening.