VACCINE RESOURCE COMMITTEE
of the Edmonton Zone Medical Staff Association
View Full List of Vaccine Questions Here
COVID-19 Vaccine Question #7:
Does the vaccine change a woman’s menstrual cycle?
The current research suggests the vaccine is associated with a small (approximately one day) increase in menstrual cycle length and only for the first menstrual cycle.
Interest grew in this question following the publication of four different studies. Two examined the relationship between the vaccines and menses. The other two looked at the relationship between the vaccines and the entire menstrual cycle.
As the vaccines were rolled out, a number of small studies suggested that women may have a change - usually a longer period- in their cycle after receiving COVID vaccination. The estimates of the number effected ranged from 13% to 42%, with the higher number coming from a web based survey - the problem with voluntary surveys is that people who have a concern of any sort are much more likely to take the time to fill out a survey. Then, in June 2023, the National Institutes of Health (NIH) released the results of a fifth study. The data came from a broader project: Boston University’s Pregnancy Study Online, also known as PRESTO. In this study, the women followed were trying to conceive and they were being followed in a larger study already when they were vaccinated, making this data much more reliable. The PRESTO study analysis found no significant relationship between COVID-19 vaccination and heavy menstrual flow or pain. However, the PRESTO trial and two studies published in 2022 also showed the same result (links are here) a study NIH-funded international study
Put together, the current, all observational research suggests that COVID-19 vaccination was associated with a small (about one day) increase in menstrual cycle length for one cycle after vaccination.
Interestingly, a more recent study of users of a period tracking app showed that COVID-19 infection also was associated with a longer cycle (similar to vaccination) - (Alvergne, Alexandra PhD; Boniface, Emily MPH; Darney, Blair PhD, MPH; Shea, Amanda PhD; Weber, Kirsten PhD; Ventola, Cécile PhD; Vitzthum, Virginia J. PhD; Edelman, Alison MD, MPH. Associations Among Menstrual Cycle Length, Coronavirus Disease 2019 (COVID-19), and Vaccination. Obstetrics & Gynecology 143(1):p 83-91, January 2024. | DOI: 10.1097/AOG.0000000000005343) and that this change was similar to the change seen with receiving COVID-19 vaccine - and only affect the cycle immediately after.
The reason is likely related to immune responses at the level of the uterus, and also the physical response to stress or challenges. There tends to be immune activation (link- immune system) as a person is about to ovulate, meant to prevent anything from interfering with fertilization and implantation of the egg. Afterwards, the immune response decreases in order to allow the body to accept a pregnancy. Also, emotional, physical and immune challenges can affect the hypothalamus and how it controls hormones, so cycles may change with significant body events of any kind. Changes to menses are a sign of the immune system responding, which is, of course, the goal of vaccination and is also expected in the case of infection.
It is important to note that these changes were only for the cycle immediately after infection or vaccination, and there is no evidence that COVID vaccination affects fertility, and that the vaccines are safe and recommended in pregnancy.
Sources:
Nature reviews immunology: Are COVID-19 vaccines safe in pregnancy?
Government of Canada: Benefits of COVID-19 vaccination during pregnancy and breastfeeding
Cleveland Clinic-Will a COVID-19 Vaccine Throw Your Period Off?
Obstetrics & Genecology: Associations Among Menstrual Cycle Length, Coronavirus Disease 2019 (COVID-19), and Vaccination