A one-and-a-half year old baby may have a fever with his second dose of the MMR vaccine. MMR refers to the MMR vaccine, which protects against three infectious diseases: measles, mumps and rubella. The MMR vaccine is a combined live attenuated vaccine, and like ordinary vaccines, some adverse reactions may occur after vaccination, the more common ones are swollen lymph nodes or fever, etc. The main reason is that the vaccine will stimulate the body’s immune system to produce specific antibodies, and the interaction between the antigen and the antibody will cause fever. If the temperature does not exceed 38.5℃ and the baby’s mental state is relatively good, no special treatment is needed after the second dose of MMR vaccination. If the temperature exceeds 38.5℃ and the baby’s mental state is poor, you should consult a doctor promptly. Don’t treat the fever symptoms after vaccination casually, you should consult a doctor in time, and then deal with it.