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Effect of COVID-19 vaccination on menstrual periods in a retrospectively recruited cohort

Our take —

This preprint, which has yet to undergo peer-review, examined changes in menstrual period timing and flow after COVID-19 vaccination in 1,273 adults in the United Kingdom. About half of the surveyed individuals reported changes in period timing (earlier or later than expected) and flow (lighter or heavier than expected) in their subsequent period after COVID-19 vaccination, though the study population is self-selected and the author notes that the frequency of menstrual changes after COVID-19 vaccination is likely overreported and does not represent the prevalence of menstrual changes after vaccination. Vaccine type did not impact menstrual changes, but participants on hormonal contraceptives were more likely to report changes in menstrual flow. Prospective studies with control groups are needed to estimate the frequency of menstrual changes following COVID-19 vaccination compared to baseline changes over time, and future vaccine trials should incorporate menstrual changes as a reportable side effect.

Study design

Retrospective Cohort

Study population and setting

This study described menstrual changes in a self-selected sample of adults (18+ years) in the United Kingdom (UK) who had received vaccination against SARS-CoV-2. A total of 2,241 participants reported their age, length of their normal menstrual cycle, contraceptive use, breastfeeding status, history of gynecologic conditions, vaccine type, vaccine date during their menstrual cycle, and the timing and flow of their subsequent period after each vaccination dose (doses were administered approximately 8 weeks apart in the UK regardless of vaccine type) on an online form. Among these respondents 1,273 (57%) provided complete data and were included in the analysis. The study assessed the timing and flow of each participant’s next period after vaccination based on the following variables: vaccine brand (Pfizer, Moderna, AstraZeneca); use of hormonal contraceptives; and vaccination timing during each participant’s menstrual period using Chi squared tests. It also assessed how subsequent period changes after the first vaccine dose related to period changes after the second vaccine dose. In a post-hoc analysis, it assessed differences in period timing and flow in participants with several gynecologic conditions. All analyses were unadjusted for potential confounders.

Summary of Main Findings

The participants’ median age was 33 years old (interquartile range [IQR] 29-39), the median cycle length was 28 days (IQR: 27-30), and the majority (1,117, 87.6%) were not using hormonal contraception. Among the participants who were vaccinated with Pfizer (n = 778, 61%), AstraZeneca (346, 27.1%), or Moderna (n = 136, 10.7%) there were no significant differences between subsequent period timing (late 29-35%, on time 40-47%, early 24-27%) or flow (heavier 31-35%, the same 49-55%, lighter 13-17%). While there were no statistically significant changes in subsequent period timing by hormonal contraceptive use (23-28% early, 43-45% on time, 29-32% heavier), participants on hormonal contraceptives were more likely to report lighter (19% vs. 14%) or heavier (42% vs. 32%) flow compared to participants not on hormonal contraceptives (p-value = 0.001). Vaccination timing during the menstrual cycle (relative to predicted ovulation dates among participants not on hormonal contraception) did not have a clear effect on subsequent period timing or flow. Among the subset of 813 respondents who had completed both vaccine doses, menstrual changes following vaccine dose one were very similar to changes following the second vaccine dose. Finally, among the small subset of individuals with endometriosis (n = 60, 4.7%) and polycystic ovarian syndrome (n = 87, 6.8%), vaccination was statistically significantly associated with earlier (38% vs. 23%) and later (41% vs. 31%) periods respectively, compared to participants with no history of gynecologic conditions.

Study Strengths

This study provided thoughtful explanations for who was included (and why they were included) in each analysis. It also provided a link to a pre-specified analysis plan and a de-identified dataset.


This study notes that it likely overestimates menstrual changes after COVID-19 vaccination given that people who experienced changes were more likely to respond to the survey. It therefore cannot be used to estimate the prevalence of menstrual changes following COVID-19 vaccination. This bias may also influence why participants using hormonal contraception reported more menstrual changes than non-users. There is also insufficient detail about participant recruitment to determine the temporal period of data collection or how representative the study population is of people who are menstruating. For example, 87.6% were not using hormonal contraception, which may reflect that this study population is actively attempting pregnancy. Despite providing reasoned explanations of which characteristics qualified participants for each analysis, this study does not provide information on how many participants were included in each analysis. Finally, each factor was evaluated without adjusting for factors which may confound their association with menstrual cycle timing and flow.

Value added

This study addresses an underreported side effect of COVID-19 vaccination, temporary changes in menstrual timing and flow.

This review was posted on: 15 December 2021