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A study of three million shows no relationship between Covid vaccines and menstrual cycle changes

The work observed a slight increase in bleeding in menopausal women, but the authors say it does not establish causality

Covid vaccines
Many women have been concerned about Covid vaccines and changes in menstruation.Halfpoint Images (Getty Images)

In just a few months, millions of women were vaccinated against Covid-19. Many of them reported changes in their menstruation shortly after receiving their doses, with irregular bleeding or heavier or lighter flows. Different drug safety systems in different countries, in charge of observing possible negative effects of approved medications, have noted these anomalies, which have also been registered in menstrual cycle tracking applications and mentioned on social media and in gynecologist appointments. Though menstrual cycles naturally vary, and such changes do not necessarily imply health problems, they can be a reason for concern amidst a massive and accelerated vaccination campaign in which the adverse effects may not be understood. The European Agency of medicines recommended in October that excessive menstrual flow be included as a possible side effect along with the vaccines.

To evaluate with greater precision the risk of menstrual disturbances after receiving the SARS-CoV-2 vaccine, a team led by the Swiss Agency for Therapeutic Products followed up with almost three million women between December 2020 and February 2022. The results, published today in the journal BMJ, indicate that there was no increase in hospital visits for menstruating women who received the vaccine. A slight increase in cases of bleeding among menopausal women was found with the third dose of the Pfizer and Moderna vaccines, but not with the AstraZeneca shot. The weak association, according to the authors, is not sufficient to establish a causal relationship between the vaccine and the bleeding.

Previous studies have been conducted to examine the effects of vaccines on the menstrual cycle, but no causal relationship has been established. Some works have found related disorders, although the results are inconclusive. A study published last year in Science Advances used a survey of around 40,000 people to estimate that 42% of women with regular menstrual cycles reported heavier bleeding after the Covid vaccine. The researchers recognized that the discoveries show a trend, but do not demonstrate that the changes were caused by the vaccines. The selection of the participants may have affected the results. “The people who participated probably had more probabilities of having experienced a menstrual change than the general population,” said Katharine Lee, professor of anthropology at Tulane University and the study’s co-author. The researcher also mentions that the changes reported, which have also been observed associated with Covid, tend to disappear within a short time.

Santiago Palacios, spokesperson of the Spanish Society of Gynecology and Obstetrics, who did not participate in the study, emphasizes the value of the results “because of the large number of vaccinated patients included.” He also highlights the irrelevance of the side effects registered: “As in the case of other vaccines, the authors talk about the emotional component of getting a vaccine, which can explain some changes in menstruation or bleeding.” Even so, Palacios says that “it can be criticized that they attribute those changes to emotional effects and not another reaction to the vaccine, related to the immune system.” The gynecologist recognizes that menstrual alterations after the vaccine are “something that patients mention frequently.” He observes, though, that when something unusual happens, “like being pregnant or receiving a vaccine, you are more aware of what happens to you, and you may notice changes that normally you would have ignored.”

Menstrual irregularities were not included among the factors monitored in the initial vaccine studies or additional follow-up. Studies like this one give more information about the risk of these side effects and how to interpret them. Along the same line, previous studies have discarded the possibility that the SARS-CoV-2 vaccines may produce fertility problems in men and women.

In recent years, debates about coronavirus vaccines have had two major consequences, according to a recent Unicef report for the London School of Hygiene and Tropical Medicine. The study found a decrease in immunizations in almost all the countries studied. In addition to distrust, the production and distribution difficulties caused by the pandemic are behind the global decrease in vaccination rates.

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