A study reveals a high frequency of menstrual irregularities after vaccination and COVID-19 infection

In 2020, the COVID-19 pandemic generated a global health crisis, triggering an unprecedented research effort focused on preventing and treating the disease. However, the effects of SARS-CoV-2 infection and vaccination on women's menstrual health received little attention. Thus, “when medical consultations and social media began to fill with accounts of menstrual changes after vaccination, researchers around the world took an important scientific step by launching studies to understand the magnitude and causes of this phenomenon. This is the context in which the research carried out in the doctoral thesis is situated,” explains gynecologist Miriam Al Adib Mendiri .

Miriam Al Adib Mendiri's doctoral thesis, entitled "Study of the Impact of the COVID-19 Pandemic on the Menstrual Cycle," was supervised by researchers Cristina Carrasco Romero and Ana B. Rodríguez Moratinos , from the Department of Physiology at the University of Extremadura. The research team worked for several years with a large database of over 17,500 women who responded, in just 15 days, to an online survey designed to assess the impact of infection and vaccination on the menstrual cycle. The study included women with and without active menstruation, as well as the clinical and sociodemographic factors associated with potential alterations.

The results of this retrospective observational study show a high frequency of menstrual changes after vaccination against SARS-CoV-2, both in non-menstruating women and in those with active cycles, although most of these alterations were transient.

Associated factors

The research identifies several factors associated with menstrual cycle irregularities, which vary according to a woman's immunoendocrine profile. Estrogens activate the humoral immune response; women with higher estrogen activity showed greater reactogenicity, thus exhibiting not only a greater predisposition to cycle irregularities but also other associated adverse effects. Approximately half of the menstruating women reported menstrual changes, especially after the second dose. Factors that increase the risk of menstrual irregularities after vaccination include: age, endometriosis, having experienced other side effects from the vaccine, certain pre-existing conditions (including endometriosis), use of oral contraceptives, and having experienced menstrual irregularities with the first dose.

In women with amenorrhea (absence of menstruation), bleeding-related abnormalities were also observed after vaccination. Associated factors included being overweight, perimenopause, certain illnesses, the occurrence of other vaccine-related adverse effects, and having experienced adverse reactions after the first dose, among others.

In both cases (menstruating and non-menstruating women) the effects were greater after the second dose.

Finally, statistical analysis reveals that approximately 40% of non-menstruating women infected with SARS-CoV-2 experienced menstrual irregularities, primarily unexpected vaginal bleeding or spotting. Within this subgroup, perimenopause and a history of menorrhagia were identified as associated factors.

Although in most cases these observed alterations were temporary, the study identifies a subgroup of women in whom the alterations persisted for a longer period, highlighting the need for further investigation of this phenomenon through prospective observational studies. One of the most relevant aspects of the thesis is that it analyzes these changes independently of reproductive status, including women with different types of amenorrhea, a group that has been underrepresented in previous research on menstrual health during the pandemic.

Women's health in biomedical research

Beyond its clinical impact, the research reopens a fundamental debate: the historically scant attention given to women's health in biomedical research. “Female sex hormones are highly interconnected with the rest of the endocrine system, as well as with the nervous and immune systems. This explains the close neuroimmunoendocrine coordination that occurs after menstruation, which is why menstruation can be considered a vital sign for women, given that menstrual irregularities can indicate problems in other bodily systems. Therefore, clinical trials cannot be based exclusively on a male model, as our systems do not function in the same way. Furthermore, some diseases manifest differently in women than in men, such as myocardial infarction, for example,” emphasizes Miriam Al Adib. The researcher hopes that the evidence provided by this and other studies will be key to advancing our understanding of the neuroimmunoendocrine mechanisms involved.

These findings expand our understanding of the effects of COVID-19 and immunization strategies on menstrual health, reinforcing the need to systematically consider this aspect in clinical studies. Furthermore, they underscore the importance of integrating this knowledge into clinical practice to avoid minimizing unexpected adverse effects, promoting informed decision-making by patients, and fostering more equitable, evidence-based clinical care.

Bibliographic reference

González M, Al-Adib M, Rodríguez AB and Carrasco C (2024) COVID-19 and menstrual-related disturbances: a Spanish retrospective observational study in formerly menstruating women. Front. Glob. Womens Health 5:1393765. https://doi.org/10.3389/fgwh.2024.1393765

González, M., Al-Adib, M., Rodríguez, AB, & Carrasco, C. (2025). Factors associated with menstrual-related disturbances following SARS-CoV-2 vaccination: a Spanish retrospective observational study in formerly menstruating women. Women & Health , 65 (2), 167–181. https://doi.org/10.1080/03630242.2025.2451360

Al-Adib M, Rodríguez AB, Carrasco C. Characteristics and health factors influencing menstrual changes after COVID-19 vaccination: A Spanish retrospective observational study in currently menstruating women. Women's Health . 2025;21. doi: 10.1177/17455057251406958

Source: Scientific Culture Dissemination Service of the UEx