The appearance of a mosquito-bite like bag after vaccination may be a normal phenomenon after vaccination, or it may be an allergic reaction caused by the vaccine, so there is no need to worry too much first, and it depends on the specific situation for treatment. Some of the vaccines will react with the tissue at the injection site after the injection, making the injection site appear hard nodules, redness and swelling similar to mosquito bites, but there is no other abnormal reaction in the body, which is normal and need not be worried too much. If you care about the redness, swelling or hard knots at the injection site, you can use hot towels to promote blood circulation at the red and swollen area, so that the redness or hard knots will slowly subside. If the allergic reaction is serious and there are manifestations such as redness and swelling of the whole body, body aches and pains, high fever, and even difficulty in breathing and shock, you should go to the hospital immediately for emergency treatment. Therefore, it is recommended to observe the body condition in time after vaccination, and if abnormal reactions such as dizziness, weakness, skin rash and difficulty in breathing occur, you need to seek medical treatment in time to avoid delaying treatment.