A recently-released study found COVID-19 vaccination extended the menstrual cycles of women, but only if the vaccine was received within the two weeks prior to ovulation.Researchers in the UK, US and Mexico collaborated to write Timing of Coronavirus Disease 2019 (COVID-19) Vaccination and Effects on Menstrual Cycle Changes. The study was published February 27 in Obstetrics and Gynecology. The study found that women who received a COVID-19 shot in the first half of their menstrual cycle (the follicular phase) were more likely to experience changes to their menstrual cycle than women who received it in the second half. The follicular phase begins at menstruation and ends at ovulation.Researchers used data from 20,000 women who used a birth control smartphone app called Natural cycles which is approved by the US Food and Drug Administration. The app uses inputs to analyze the user’s menstrual cycle and determine the window of time pregnancy is possible.The researchers separated three different groups: women who received the vaccine during the first (follicular) phase, those who received the vaccine during the second (luteal) phase and a control group that never got the vaccine.About 80% of the women were under 35 years of age, with 28.6% from North America, 33.5% from continental Europe, and 31.7% from the UK Almost two-thirds of the total sample (63.8%) received an mRNA-based vaccine, such as Pfizer or Moderna.As a sign of the times, the study mentions “individuals” but never calls them “women.” Those in the follicular phase averaged a one-day longer menstrual cycle after one dose and 1.11 days after a second dose. Those in the luteal phase and those in the unvaccinated control group experienced “no change in cycle length.”“Those vaccinated during the follicular phase were also more likely to experience a clinically significant change in cycle length (8 days or more; first dose: 6.8%) than those vaccinated in the luteal phase or unvaccinated (3.3% and 5.0%, respectively),” the researchers found.Researchers theorized how vaccine could have been caused these results.“The underlying mechanism for a vaccine-related cycle length disturbance is still under investigation. The leading hypothesis is that these disturbances are due to the immune response that vaccines are designed to produce,” the authors explained.“The menstrual cycle is driven by hormonal signaling through the HPO [hypothalamic-pituitary-ovarian] axis and a variety of stressors are known to act on this axis, resulting in increased cycle length, particularly when the stressor acts in the follicular phase. Cytokines, produced as an early event in the vaccine response, can directly affect the HPO axis.”The study found broad demographic differences between those who vaccinated and those who did not.“Unvaccinated individuals were more likely to be younger than age 25 years (19.4% vs approximately 10% for both vaccinated groups), have less than a college degree (25.9% vs approximately 15%) and be from the US or Canada (42.7% vs approximately 24%),” the authors reported.The recent study was not the first to report post-vaccination changes in female bleeding. A study published in the British Medical Journal in May of 2023 found for the Pfizer vaccine the risk of postmenopausal bleeding in the long-term rose 14% for the second dose and between 23% and 41% for the third dose.For the Moderna vaccine, the first dose risk of postmenopausal bleeding rose 33%, the same increase as found in the long-term risk after the third dose. Increased risks of such bleeding following the AstraZeneca vaccine were less and ranged from 14% to 24%.Another study, this one funded by the National Institutes of Health and the University of Illinois in 2022, found evidence of the problem in a web-based survey of women who received the vaccine (though again, they were not called “women”). This study found “42% of people with regular menstrual cycles bled more heavily than usual, while 44% reported no change after being vaccinated. Among respondents who typically do not menstruate, 71% of people on long-acting reversible contraceptives, 39% of people on gender-affirming hormones and 66% of postmenopausal people reported breakthrough bleeding,” the researchers concluded.“Generally, changes to menstrual bleeding are not uncommon or dangerous, yet attention to these experiences is necessary to build trust in medicine.”
A recently-released study found COVID-19 vaccination extended the menstrual cycles of women, but only if the vaccine was received within the two weeks prior to ovulation.Researchers in the UK, US and Mexico collaborated to write Timing of Coronavirus Disease 2019 (COVID-19) Vaccination and Effects on Menstrual Cycle Changes. The study was published February 27 in Obstetrics and Gynecology. The study found that women who received a COVID-19 shot in the first half of their menstrual cycle (the follicular phase) were more likely to experience changes to their menstrual cycle than women who received it in the second half. The follicular phase begins at menstruation and ends at ovulation.Researchers used data from 20,000 women who used a birth control smartphone app called Natural cycles which is approved by the US Food and Drug Administration. The app uses inputs to analyze the user’s menstrual cycle and determine the window of time pregnancy is possible.The researchers separated three different groups: women who received the vaccine during the first (follicular) phase, those who received the vaccine during the second (luteal) phase and a control group that never got the vaccine.About 80% of the women were under 35 years of age, with 28.6% from North America, 33.5% from continental Europe, and 31.7% from the UK Almost two-thirds of the total sample (63.8%) received an mRNA-based vaccine, such as Pfizer or Moderna.As a sign of the times, the study mentions “individuals” but never calls them “women.” Those in the follicular phase averaged a one-day longer menstrual cycle after one dose and 1.11 days after a second dose. Those in the luteal phase and those in the unvaccinated control group experienced “no change in cycle length.”“Those vaccinated during the follicular phase were also more likely to experience a clinically significant change in cycle length (8 days or more; first dose: 6.8%) than those vaccinated in the luteal phase or unvaccinated (3.3% and 5.0%, respectively),” the researchers found.Researchers theorized how vaccine could have been caused these results.“The underlying mechanism for a vaccine-related cycle length disturbance is still under investigation. The leading hypothesis is that these disturbances are due to the immune response that vaccines are designed to produce,” the authors explained.“The menstrual cycle is driven by hormonal signaling through the HPO [hypothalamic-pituitary-ovarian] axis and a variety of stressors are known to act on this axis, resulting in increased cycle length, particularly when the stressor acts in the follicular phase. Cytokines, produced as an early event in the vaccine response, can directly affect the HPO axis.”The study found broad demographic differences between those who vaccinated and those who did not.“Unvaccinated individuals were more likely to be younger than age 25 years (19.4% vs approximately 10% for both vaccinated groups), have less than a college degree (25.9% vs approximately 15%) and be from the US or Canada (42.7% vs approximately 24%),” the authors reported.The recent study was not the first to report post-vaccination changes in female bleeding. A study published in the British Medical Journal in May of 2023 found for the Pfizer vaccine the risk of postmenopausal bleeding in the long-term rose 14% for the second dose and between 23% and 41% for the third dose.For the Moderna vaccine, the first dose risk of postmenopausal bleeding rose 33%, the same increase as found in the long-term risk after the third dose. Increased risks of such bleeding following the AstraZeneca vaccine were less and ranged from 14% to 24%.Another study, this one funded by the National Institutes of Health and the University of Illinois in 2022, found evidence of the problem in a web-based survey of women who received the vaccine (though again, they were not called “women”). This study found “42% of people with regular menstrual cycles bled more heavily than usual, while 44% reported no change after being vaccinated. Among respondents who typically do not menstruate, 71% of people on long-acting reversible contraceptives, 39% of people on gender-affirming hormones and 66% of postmenopausal people reported breakthrough bleeding,” the researchers concluded.“Generally, changes to menstrual bleeding are not uncommon or dangerous, yet attention to these experiences is necessary to build trust in medicine.”
