The vaccine does not usually affect the menstrual cycle. If menstruation is delayed after the third vaccination, pregnancy, endocrine disorders, gynecological diseases or medications should be considered as possible causes. 1. Pregnancy: Married or sexually active women can take a pregnancy test or blood HCG test to determine if pregnancy is the cause of delayed menstruation; if it is an early pregnancy, regular pregnancy checkups under the guidance of a doctor will suffice. 2. Endocrine disorders: Before and after the vaccination, the endocrine disorders may be caused by excessive stress and anxiety, or due to irregular diet, overwork, late night, cold and other factors, thus affecting the level of sex hormones and leading to delayed menstruation. It is recommended to relax, relieve stress, adjust lifestyle habits, and provide psychological counseling when necessary. 3. Gynecological diseases: The presence of certain gynecological diseases can lead to delayed menstruation, such as endometriosis, polycystic ovary syndrome or menopausal syndrome, etc. One should pay attention to whether it is accompanied by acne, hairiness, lower abdominal pain, hot flashes (a burst of fever) and night sweats, etc. It is recommended to consult a doctor in time, and after ultrasound, sex hormone test and other related examinations, follow the doctor’s instructions for treatment with medication. 4. Drugs: Some drugs may affect the menstrual cycle, such as birth control pills, domperidone, etc. If you take the drug during the injection of the new vaccine, it may cause delayed menstruation, and usually can be recovered after stopping the drug. If menstruation does not come for a long time, it is recommended to go to the hospital and conduct relevant examinations to clarify the cause of the disease and then treat the symptoms.