In theory, a vaccine could affect a woman’s period. A vaccine is meant to induce an immune response in the body, and this immune response could have an impact on the menstrual cycle. The menstrual cycle is primarily under the control of a complex interplay of hormones released by the brain and acting on the ovaries and in turn, on the uterus. In the first half of the cycle, which is dependent on the female sex hormone oestrogen, the endometrial lining is starting to build up in the uterus and the follicles (eggs and their surrounding tissue) are maturing in the ovary. In the middle of the cycle, a surge in a hormone called luteinising hormone acts on the ovary to release an oocyte (egg) from the most mature of the follicles, or ovulation. In the second half of the cycle, which is dependent on another sex hormone called progesterone, the endometrial lining thickens significantly in preparation for a fertilised egg to implant. If pregnancy doesn’t occur, then progesterone falls quickly, leading to a shedding of the lining of the uterus, or menstruation. The cycle is also mediated in part by the immune system. For example, certain immune cells, such as macrophages, mast cells and neutrophils, are found in the endometrial lining, and involved in the shedding of the lining of the uterus during the menstrual cycle, and rebuilding it for the next cycle. So it’s possible receiving a vaccine and having the expected immune response could affect the complex interplay between immune cells and signals in the uterus, and lead to the next period being heavier, more painful or longer. Need to be further explored