Coronavirus disease vaccine linked menstrual changes: mobile application study

Article information

Obstet Gynecol Sci. 2025;68(2):155-162

Publication date (electronic) : 2025 February 7

Sung Eun Kim, MD ¹, Joseph J. Noh, MD ², Yoo-Young Lee, MD, PhD ²

¹Department of Obstetrics and Gynecology, Samsung Medical Center, Sungkyunkwan University School of Medicine, Seoul, Korea

²Department of Obstetrics and Gynecology, Gynecologic Cancer Center, Samsung Medical Center, Sungkyunkwan University School of Medicine, Seoul, Korea

Corresponding author: Yoo-Young Lee, MD, PhD, Department of Obstetrics and Gynecology, Gynecologic Cancer Center, Samsung Medical Center, Sungkyunkwan University School of Medicine, 81 Irwon-ro, Gangnam-gu, Seoul 06351, Korea, E-mail: yooyoung.lee@samsung.com

Received 2024 April 11; Revised 2024 August 13; Accepted 2025 January 13.

Abstract

Objective

Since coronavirus disease 2019 (COVID-19) vaccination began, abnormal uterine bleeding (AUB) has occurred at a high rate. This study assessed the association between COVID-19 vaccination and AUB.

Methods

In this retrospective cohort study, we analyzed mobile application data on menstrual cycles to investigate differences in the prevalence, duration, and amount of intermenstrual bleeding (IMB) after COVID-19 vaccination. We also analyzed the duration of menstruation, menstrual cycle length, and associated symptoms after the COVID-19 vaccination. Additionally, we investigated the prevalence of IMB according to the vaccine type.

Results

After vaccination, IMB prevalence increased to 3.35% (odds ratio [OR], 1.61; 95% confidence interval [CI], 1.46–1.76; P<0.0001) and IMB duration increased by 0.43 days (95% CI, 0.25–0.60; P<0.0001). The proportion of respondents whose amount of IMB was heavier than regular menstruation increased (OR, 2.96; 95% CI, 1.47–5.93; P=0.002). Menstrual duration decreased by −0.01 days (95% CI, −0.023 to 0.003; P=0.114), and menstrual cycle length increased by 1.39 days (95% CI, 1.30–1.48; P<0.0001). The proportion of participants who answered that there was a difference in menstruation amount increased (OR, 1.52; 95% CI, 1.41–1.64; P<0.0001). The prevalence of IMB increased regardless of the vaccine type.

Conclusion

There were statistically significant differences in the prevalence, duration and amount of IMB, menstrual duration, menstrual cycle length, and menstrual amount after COVID-19 vaccination. However, these values were not clinically meaningful and could be regarded as within the normal menstruation range.

Keywords: Abnormal uterine bleeding; Intermenstrual bleeding; COVID-19; Vaccine; Vaccination

Introduction

The coronavirus disease 2019 (COVID-19) pandemic has led to the need for an urgent vaccine against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Due to concerns regarding the COVID-19 pandemic, a vaccine against SARS-CoV-2 was approved by the Food and Drug Administration’s Emergency Use Authorization Emergency Use Authorization without completing formal clinical trials [1]. Therefore, there is inevitably insufficient data on their long-term side effects [2]. Thus, even when vaccination began, there was great concern regarding new side effect [3,4]. After the initiation of COVID-19 vaccination, many patients complained of abnormal uterine bleeding (AUB) [5].

AUB includes problems with frequency, duration, regularity, and flow volume during a normal menstrual cycle, or intermenstrual bleeding (IMB). For example, in Google Trends, searches related to menstrual cycle abnormalities increased after COVID-19 vaccination [6]. Therefore, many studies have investigated the effects of COVID-19 vaccination on the menstruation cycle [7–17]. However, its exact mechanism of action has not yet been elucidated.

In Korea, the COVID-19 vaccine safety research center investigated the relationship between AUB and vaccines on March 30, 2023 [18]. From February 26, 2021, when the first COVID-19 vaccination was initiated in Korea, 4,234 women reported AUB after vaccination until September 30, 2022. This is equivalent to 20 cases out of every 100,000 vaccinated women. Among these, 79% visited medical institutions and received AUB-related treatment. AUB was added as a suspected disease related to COVID-19 in August 2022 in Korea [19,20], with notable increase in AUB-related reports thereafter.

In October 2022, the Pharmacovigilance Risk Assessment Committee of the European Medicines Agency recommended adding heavy menstrual bleeding to the list of side effects on the package insert of nucleoside-modified messenger ribonucleic acid vaccines to prevent COVID-19.

It has been recognized that AUB, especially IMB, is related to the COVID-19 vaccine, and many studies have been published; however, no studies have targeted Korean women.

Materials and methods

This retrospective cohort study was designed to analyze the impact of COVID-19 vaccination on the menstrual cycles and AUB. The primary outcome was the change in IMB prevalence. The secondary outcomes included changes in IMB duration and amount, menstrual duration and amount, menstrual cycle length, and menstrual symptoms. We also analyzed the prevalence of IMB according to the type of vaccine administered.

1. Study population

Study participants were selected from Heymoon (Happy Moonday Inc., Seoul, Korea) users, a menstrual cycle application developed by Happy Moonday, based on the following criteria.

The inclusion criteria were as follows: 1) women aged 18 years or older; 2) at least one COVID-19 vaccination record from March 2021 to December 2022; 3) menstrual data available for three cycles before and after vaccination; and 4) regular menstrual cycles before vaccination (defined as cycles lasting 24–38 days) [21].

Exclusion criteria were as follows: 1) women using contraceptives; 2) incorrect records of menstrual cycle or vaccination; 3) irregular menstrual cycles within three cycles before vaccination (<24 days or ≥38 days); and 4) interval between the 1st and 2nd vaccine doses recorded as less than 21 days.

All application users were required to record their menstrual cycles and AUB. The beginning and end of the menstrual cycle were recorded and bleeding in the middle was marked as IMB. All data were reviewed by a reproductive endocrinologist (S.E.K) to determine whether the women recorded IMB as prolonged menstruation or as real IMB. Bleeding within 2 days of the end or beginning of menstruation was classified as normal menstruation.

The amount of menstrual bleeding was expressed as increased, similar, or decreased compared with prior cycles. The amount of IMB was also recorded to determine whether it was less than, similar to, or greater than during menstruation. Menstrual duration was defined as the period indicated by the user, and the interval between the start of menstruation and the start of the next menstruation was calculated as the menstrual cycle length. Symptoms accompanying menstruation included headaches and abdominal pain, and the users were allowed to choose whether they had the corresponding symptoms.

Age was adjusted based on the time of registration. For users who joined in 2019, 2 years were added to their joining age as of 2021, and for those who joined in 2020, 1 year was added.

All data were anonymized by Happy Moonday. This study was approved by the Institutional Review Board of Samsung Medical Center (IRB No. 2022-02-047).

2. Statistical method

Data on the responses before and after the first and second vaccinations were generated by combining the order of vaccination with the order and date of menstrual status responses for all participants.

Continuous response variables were expressed as means and standard deviations, and categorical variables were ex pressed as frequencies and rates.

The trends of the response variables over time were graphically examined and a generalized estimating equations analysis, which assumed an unstructured correlation structure, was applied to estimate the within-subject effects. ORs were calculated for categorical responses and mean differences were estimated for continuous responses with 95% confidence intervals and P-values. The significance level was set at P<0.05.

Results

A total of 11,545 of the 21,443 women were analyzed after excluding 9,582 women with irregular menstruation before vaccination and 316 women with suspected recording errors. Table 1 shows the baseline patient characteristics. The mean age of the women who used the application was 24.83 years, mean menstrual cycle length was 30.93 days, and mean menstrual period was 5.71 days.

Table 1

Baseline characteristics of vaccinated women before coronavirus disease 2019 vaccination

The proportion of women who responded that they had undergone IMB before vaccination was 2.12%. The prevalence of IMB before vaccination was 2.03% in the 3rd cycle before vaccination, 2.13% in the 2nd cycle before vaccination, and 2.19% in the 1st cycle before vaccination. The prevalence rates were 3.04% during the vaccination cycle, 3.45% during the 1st cycle after vaccination, 3.47% during the 2nd cycle after vaccination, and 3.14% during the 3rd cycle after vaccination (Fig. 1). The overall prevalence of IMB after vaccination was 3.35%. Comparing the average prevalence of IMB in the three cycles before and after vaccination revealed a significantly increase in IMB prevalence after vaccination (OR, 1.61; 95% CI, 1.46–1.77; P<0.0001).

Fig. 1

Prevalence of intermenstrual bleeding before and after coronavirus disease 2019 vaccination. Before 3 refers to the 3rd cycle prior to COVID-19 vaccination. Similarly, after 3 refers to the 1st cycle after COVID-19 vaccination. COVID-19, coronavirus disease

The duration of IMB was prolonged by 0.43 days after vaccination compared to before vaccination (95% CI, 0.25–0.60; P<0.0001).

The amount of IMB was heavier after vaccination than before (Fig. 2). The proportion of participants who reported heavier bleeding compared to menstrual flow was 1.3% in 3rd cycle before vaccination, 2.4% in 2nd cycle before vaccination, and 0.8% in 1st cycle before vaccination. It increased to 3.5% in 1st cycle after vaccination, 4.5% in 2nd cycle after vaccination, and 3.9% in 3rd cycle after vaccination (OR, 2.96; 95% CI, 1.47–5.93; P=0.002). In contrast, the proportion of those who answered that they had lighter bleeding compared to menstrual flow decreased (OR, 0.77; 95% CI, 0.61–0.98; P=0.037). The rates were 82.1% in 3rd cycle before vaccination, 81.7% in 2nd cycle before vaccination, and 83% in 1st cycle before vaccination. After vaccination, there were significant decreases of 80.7% after the 1st cycle of vaccination, 77.3% after the 2nd cycle, and 78.0% after the 3rd cycle. There was no significant difference between the before- and after-time points in the proportion of women who responded that they had IMB amounts similar to their menstrual amounts (OR, 0.99; 95% CI, 0.77–1.27; P=0.915).

Fig. 2

Amount of intermenstrual bleeding before and after coronavirus disease 2019 vaccination. The proportion of people who answered that their intermenstrual bleeding (IMB) was heavier than menstruation increased (OR, 2.96; 95% CI, 1.47–5.93; P=0.002). The proportion of people who answered that their IMB was lighter than menstruation decreased (OR, 0.77; 95% CI, 0.61–0.98; P=0.037). No statistically significant change in the percentage of people who answered that their IMB was similar to their menstrual flow (OR, 0.99; 95% CI, 0.77–1.27; P=0.915). COVID-19, coronavirus disease 2019; OR, odds ratio; CI, confidence interval.

There was no difference in menstrual cycle duration before and after vaccination (−0.01 days; 95% CI, −0.023 to 0.003; P=0.114). However, in terms of menstrual cycle length, the cycle after vaccination was longer by 1.39 days than before vaccination (95% CI, 1.30–1.48; P<0.0001).

Regarding the menstrual cycle, it was judged whether it increased, was similar, or decreased compared with the previous cycle. The proportions of women who reported that their menstrual volume increased after vaccination were 44.2% in 3rd cycle before vaccination, 45.9% in 2nd cycle before vaccination, and 50.1% in 1st cycle before vaccination, respectively. After vaccination, there were significant increases of 58.1% after the 1st cycle of vaccination, 56.5% after the 2nd cycle, and 52.5% after the 3rd cycle. Similarly, the proportion of participants who reported that their menstrual volume decreased after vaccination also increased, from 0.4% in 3rd cycle before vaccination, 0.7% in 2nd cycle before vaccination, and 0.3% in 1st cycle before vaccination to 1.4% after the 1st cycle of vaccination, 1.8% after the 2nd cycle of vaccination, and 1.3% after the 3rd cycle of vaccination, respectively. To summarize, the proportion of participants who answered that there was a difference in the amount of menstruation increased after vaccination (OR, 1.52; 95% CI, 1.41–1.64; P<0.0001) (Fig. 3).

Fig. 3

Flow change of menstruation before and after coronavirus disease 2019 vaccination. The proportion of people who answered that there was a difference in the amount of menstruation, such as an increase or decrease, after vaccination increased (OR, 1.52; 95% CI, 1.41–1.64; P<0.0001). OR, odds ratio; CI, confidence interval.

Symptoms of discomfort that occurred during the menstrual cycle were classified as headache, abdominal pain, back pain, skin trouble, constipation, diarrhea, dyspepsia, nausea, abdominal discomfort, and breast discomfort. Each symptom increased after vaccination compared to before vaccination (Supplementary Table 1).

We also analyzed whether there were differences in IMB according to the vaccine type. Only 30 women received the Janssen vaccination; therefore, analysis could not be performed. The AstraZeneca vaccine, which was initially administered in Korea, was administered only to medical staff working at the hospital. Therefore, statistical significance was not achieved because the total number of women vaccinated with Astra-Zeneca was as small as 60, and only 1–2 women complained of IMB per cycle (P=0.452). Pfizer and Moderna showed significant IMB prevalence changes regardless of the vaccine type (P<0.001) (Supplementary Table 2).

Discussion

To the best of our knowledge, this is the first study to demonstrate an association between COVID-19 vaccination and AUB in South Korea. In this study, we found an increased prevalence, duration, and amount of IMB after COVID-19 vaccination. The menstrual cycle increased, and there was a change in the amount of menstrual volume; however, there was no difference in menstrual duration. There was no difference in the prevalence of IMB according to the vaccine type.

Regular menstruation is an important indicator of woman’s health and fertility, and it is considered normal for the menstrual cycle to vary from to 7–9 days [21]. Menstruation may become temporarily irregular, and one of the causes of such irregularities is the temporary disturbance of the hypothalamic-pituitary-ovarian (HPO) axis due to psychological or physiological stress [22–25]. Although further research is needed, it has been reported that mRNA vaccination may temporarily disrupt the normal functioning of the HPO axis during the generation of a strong immune response in the host [26,27]. This is because cytokines produced in response to vaccination affect the HPO axis. Additionally, as it affects macrophages and natural killer cells, it is presumed to affect the uterus and alter menstrual flow [28].

Many studies have shown that the COVID-19 vaccination causes AUB or affects the menstrual cycle. These results are consistent with previous studies [9,15,29]. The COVID-19 vaccination has increased the prevalence, duration, and number of IMB.

The prevalence of AUB after vaccination varies widely between studies. An Israeli study reported that 61.1% of women experienced AUB after vaccination [9], while another reported an AUB frequency of 0.6% after the first dose [30]. A study involving 79,000 people also found no association between the COVID-19 vaccine and incidence of AUB [31]. Because this large difference may vary depending on the definition of AUB, we analyzed IMB separately. Although AUB is a comprehensive concept that includes menstrual abnormalities, IMB is considered a narrower concept because it only refers to bleeding that occurs between cycles.

In our study, the IMB before vaccination was 2.03–2.19%, which increased to 3.04–3.47% after vaccination. A study in Saudi Arabia reported IMB at 13.5% after the first dose [32]; as the prevalence of IMB varies depending on the self-perception of AUB [33], this may have resulted in a difference in IMB prevalence.

Other studies have only analyzed the duration and amount of AUB, making it difficult to compare them with our study. When IMB was analyzed separately, the duration of IMB increased by 0.43 days compared with that before vaccination; however, this change may not be clinical significant. The proportion of respondents who answered that the amount of IMB increased after the vaccination also increased, whereas the proportion who answered that it decreased after the vaccination decreased. Although the amount of IMB appears to increase after vaccination, other studies have shown that AUB resolves within 6 months [34].

In our study, the menstrual cycle length became longer by 1.39 days after vaccination. In a study using “Natural Cycles”, a difference in cycle length was found; however, similar to our study, the cycle was prolonged by 0.71 days [7,8]. Another study found that the cycle was lengthened by about 0.5 days [35]. According to the nurses’ health study, these changes occurred within the first 6 months after vaccination [34]. However, this 1.39 days difference may not be clinically significant and is still likely associated with regular menstruation.

There are various results regarding the menstrual amount; while heavy bleeding increased in some studies [9,36,37], there was no difference before and after vaccination in other studies [38,39]. In our study, as for the results regarding menstrual amount, the proportion of women responded that their menstrual amount increased or decreased both increased. This indicates that there was a change in the menstrual cycle.

Previous studies have shown that menstrual cycle abnormalities are more common in patients with irregular menstrual cycles [34]. However, because our study excluded patients who had previously had irregular menstrual cycles, comparisons could not be performed. Another study reported that women with background hematologic or gynecologic diseases frequently experienced irregular bleeding or menstrual changes [17]. However, the data in our study had the disadvantage of not being able to properly obtain medical histories. Therefore, it is necessary to analyze these aspects in future research.

Among the menstruation-related symptoms after vaccination, the symptom that showed the greatest difference compared to before was nausea (OR, 3.2; 95% CI, 2.41–4.25; P<0.001). The most frequently reported symptom was abdominal pain; 7.57% of the women complained of symptoms in the 2nd cycle after vaccination. This is a very small response compared to the prevalence of dysmenorrhea (16–91%) [40]. This is because the data source was voluntarily recorded in the application; therefore, it is likely to have been underestimated. In addition, overall uncomfortable symptoms such as headache, back pain, and diarrhea increased. However, because the evaluation was performed by clicking only on the symptoms provided by the application, it had the disadvantage of being unable to evaluate new symptoms.

Vaccine type was unrelated to the increase in IMB. In other studies, there was no correlation between AUB and vaccine type [37], and the menstrual cycle and vaccine type were not affected [8,15,34]. This may be because of the influence of AUB on the immune system rather than the mechanism of the vaccine component itself.

This study had the advantage of being less biased because it was based on data recorded on objective changes in menstruation in women who used the application before vaccination. Second, IMB was classified and analyzed separately. A reproductive endocrinologist reviewed the data recorded as AUB and classified IMB separately. Therefore, it was possible to distinguish between side effects related to menstruation and those related to bleeding that may occur between periods.

However, because this application was used to record menstrual cycles before COVID-19, information on body mass index, parity, education, and ethnicity could not be obtained. Moreover, because it is a record of only some women using the application, there is a possibility that it may not represent the entire population. However, since this application is a popular and ranks within the top 3 when searching for “menstrual cycle” or “period app” on Android, it can compensate for the disadvantage of selection bias. Additionally, because the data used an application that checks the menstrual cycle, it was not possible to analyze the AUB in postmenopausal women. As our study only targeted three menstrual cycles, additional long-term studies targeting larger populations are needed.

This study demonstrated that COVID-19 vaccination may cause temporary changes in the menstrual cycle. However, these changes were within the normal range of variation. Therefore, there is no cause for concern, and these findings should reassure women that vaccination is safe and should not be delayed due to concerns regarding menstrual changes.

Supplementary Information

ogs-24104-Supplementary-Table-1.pdf

ogs-24104-Supplementary-Table-2.pdf

Notes

Conflict of interest

The authors have no potential conflicts of interest related to this article.

Ethical approval

This study was approved by the Institutional Review Board of the Samsung Medical Center (No. 2022-02-047).

Patient consent

Informed consent was exempted because patient identifiers were removed from the data for anonymization.

Funding information

None.

Acknowledgments

We thank Joonghyun Ahn, the Biomedical Statistics Center, the Data Science Research Institute, and the Samsung Medical Center. We also thank Happy Moonday Inc. for their support in this study.

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Articles published in Obstet Gynecol Sci are open-access, distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

	Before vaccination
Age (yr)	24.83±5.27
Menstrual cycle length (days)	30.93±5.02
Menstrual period (days)	5.71±1.25