Some women noticed changes in the timing and length of their menstrual periods after receiving the COVID-19 vaccine when it first became widely accessible.
Nearly two years later, a global investigation has verified that for some individuals, COVID-19 immunization can cause transitory alterations in cycle duration.
An FDA-approved birth control app called Natural Cycles was used by nearly 20,000 women from all over the world to self-report their menstrual cycles for the study, which was then published in the medical journal BMJ.
Following their first dosage of the COVID vaccine, study participants who received the injection reported, on average, a nearly one-day increase in the length of their menstrual cycle, and a half-day increase following their second dose.
A single menstrual cycle with both vaccine doses caused an almost four-day increase in period length.
The study concluded that variations in cycle length are merely transient and have no long-term impacts, which is in line with other studies’ findings.
A smaller study conducted earlier this year involving about 4,000 women had similar findings, noting that a normal menstrual cycle returned one to two months following vaccination.
The persistence of the women who spoke out on social media and shared details of their adverse effects in an online database set up by two researchers allowed the two studies to get off the ground.
In August 2021, a few months later, the National Institutes of Health declared it was committed
In an interview with ABC News earlier this year, Dr. Alison Edelman, professor of obstetrics and genecology at Oregon Health and Science University School of Medicines, said that the research is crucial because it can support women’s firsthand accounts and inform them of what to anticipate after receiving vaccinations.
According to Edelman, the principal author of the two studies that have been published so far, “On a human level, any visible alteration to a person’s cycle — whether it be related to vaccination or other external stressors — might actually feel substantial. “As a clinician, I can assist in informing patients about what to expect with vaccination, which may involve a little difference in their cycle length, and having them ready for this possibility so that they need not worry.”
Menstrual changes are controlled by a part of the brain called the hypothalamus, along with the ovaries, which use hormones as signals. These hormone signals can be disrupted when the body goes through changes that occur with an infection and even a vaccine.
Getting vaccinated produces a strong immune system response in the days following the shot, which may cause temporary changes to menstrual cycles. Studies have also documented temporary menstrual cycle changes among women who get COVID-19 infections.
Temporary changes to the menstrual cycle should not be a concern for women, experts say. Changes lasting “three months consecutively, or more” are when health care providers typically make investigation or treatment plans, Dr. Jessica Shepherd, OBGYN and chief medical officer at Very ell Health, told ABC News last year.