The red rash after MMR vaccination may be a normal reaction, or it may be caused by an allergic reaction. 1. Normal reaction: After receiving MMR vaccine, if the red rash is small in size and small in number, this is a normal reaction to vaccination, try to avoid scratching and contacting irritating objects, and this situation usually does not need special treatment, and will subside in 2 to 3 days after vaccination. 2. Allergic reaction: After receiving the MMR vaccine, if there is a large area of red rash with obvious itching, it may be an allergic reaction caused by the vaccination, and it is recommended to go to the hospital in time for treatment, and in severe cases, there will be symptoms such as nausea, vomiting, or respiratory difficulties. After the MMR vaccine should be observed for half an hour at the vaccination site, if there is a large red rash, it is recommended that the patient go to the hospital in time, under the guidance of the doctor for treatment.