Study Links COVID Vaccine to Temporary Change in Menstrual Cycle

According to a recent study published in BMJ Medicine, some people who have received the COVID-19 vaccine have experienced a brief alteration in their menstrual periods.

With both the first and second vaccine doses, vaccinated individuals experienced an adjusted increase in menstrual cycle length of less than 1 day. There was no difference in the number of menstrual or bleeding days in relation to the increase. Regardless of the COVID-19 vaccination type, the modifications seemed to be the same.

The head of the National Institute of Child Health and Human Development, Diana Bianchi, MD, stated in a statement that these findings “offer further information for counselling women on what to expect after vaccination.”

According to her, post-vaccination changes “appear to be minor, within the usual range of variation, and transient.” Bianchi was not a part of the study.

The study, which was part of a $1.67 million award given to five institutions to investigate the potential links between COVID-19 vaccination and menstrual changes, was funded by the National Institute of Child Health and Human Development and the National Institutes of Health’s Office of Research on Women’s Health.

Between October 2020 and November 2021, researchers at universities in Oregon, Massachusetts, London, and Edinburgh examined data for users worldwide of the menstrual cycle tracking app Natural Cycles. (Users could have agreed to have their de-identified data used for study.)

The study team analysed data from over 20,000 persons, or more than 250,000 cycles, in the United States, Canada, the United Kingdom, and Europe between the ages of 18 and 45. In the nearly 15,000 persons who had received the vaccine, they included data for at least three cycles before and one cycle after, and for at least four cycles during a comparable time span in the nearly 5,000 people who had not received the vaccine.

The majority of the participants—about 80%—were under 35. The vaccines given to 2/3 of people were produced by Pfizer/BioNTech, 17% by Moderna, 9% by Oxford AstraZeneca, and 2% by Johnson & Johnson.

The average increase in cycle length for those who received vaccinations was 0.71 days after receiving the first dose and 0.56 days after receiving the second dose. Even though there was no change in the number of days of bleeding, those who received two dosages in one cycle experienced an increase of 3.7 days.

The modifications were transient. For those who received one dose of the vaccine in a cycle, the cycle length was 0.02 days longer in the cycle following immunisation than it was before. Those who received two dosages in one period still experienced an increase of 0.85 days in the following cycle.

The number of people whose cycle length increased by more than 8 days also increased significantly, according to the study’s findings. A total of 1,342 participants — 6.2% of those who had received vaccinations and 5% of those who hadn’t — reported a rise of more than 8 days. People were more likely to report the rise if they were younger and had a longer cycle duration prior to vaccination.

The NIH stated that a change in cycle length of fewer than eight days is regarded as being within the normal range of fluctuation. “Although tiny menstrual changes might not be significant to health care providers and researchers, perceived alterations in a bodily function connected to fertility may be frightening to persons experiencing them and may help explain vaccine reluctance,” according to a study.

Since the app focuses on natural cycles, people who use hormonal birth control were not participants, one of the limitations mentioned by the researchers. The number of post-vaccination cycles that the researchers could estimate during that time was also constrained, and they were unable to take into account the possibility of COVID-19 infection, which could also cause cycles to prolong.

The study’s results are consistent with those of an earlier investigation conducted by the same team of researchers in January, which was centred on American women and discovered that the average menstrual cycle duration rose by around a day following immunisation.

Future research on additional elements of menstrual cycle abnormalities associated with vaccinations, such as unexpected bleeding, menstrual flow, and discomfort, was requested by the researchers. They also stressed the significance of figuring out the physical causes of these alterations.

The researchers said that their study was not intended to identify potential causes for these alterations. More research is required, although it seems likely that these modifications result from a transient immunological response triggered by vaccination.

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