It is well known now. Receiving the COVID vaccine may result in a set of clearly defined side effects including fever, chills, pain at the injection site, digestive problems, and headaches. We also know that these side effects can be particularly strong in people under age 55. This is an expected factor of stimulating the immune system to create immunity. More robust immunity leads to a more robust immune system response.
However, people are reporting other side effects. Since the early days of the COVID pandemic, women have reported changes to their menstrual cycles from the virus. The reports include changes such as more frequent, heavier flow, and more menstrual pain. Some women experience skipped periods.
In the same way, women who have received the COVID-19 vaccine also have reported similar changes like heavier periods, delayed periods, and more than one period during the first month after receiving the vaccine.
Since we know the vaccines affect immunity, and immune responses are also related to our menstrual cycles, it’s not a far leap to suspect that COVID vaccination could affect the menstrual cycle. Hypotheses suggest a variety of possible immune-mediated mechanisms are triggered by vaccination. These mechanisms may also impact menstruation and include: increased inflammation, nitric oxide production, mast cell activation, or toll-like receptor (TLR) signaling. These mechanisms can trigger changes in the uterine arteries and uterine lining, leading to increased uterine cramping and bleeding.
But, do COVID-19 vaccines cause changes in a woman’s menstrual cycle? A new study has provided data that suggests some good news.
COVID vaccination and menstrual cycles
A recently published study in Obstetrics and Gynecology found a statistically significant change following vaccination against the coronavirus. They partnered with the makers of the Natural Cycles app. However, the study also found other interesting issues.
This study looked retrospectively at the menstrual cycles as tracked by 3,959 women, 2,403 of whom were vaccinated against COVID (Pfizer-BioNTech vaccine 55%, Moderna 35%, Johnson & Johnson/Janssen 7%), and 1,556 unvaccinated. Cycle data was collected from October 2020 to September 2021, with initial COVID-19 vaccine doses administered between December 2020 and July 2021.
The study participants were:
U.S. residents
- Between the ages of 18–45 years
- Had at least three menstrual cycles post-pregnancy or post use of hormonal contraception
- Had a normal pre-vaccination menstrual cycle length (average 24–38 days)
The study compared menstrual cycle changes among vaccinated individuals for three consecutive menstrual cycles before and three consecutive cycles following vaccination, with data from six consecutive menstrual cycles in unvaccinated women.
The researchers found that women who were vaccinated experienced slightly longer menstrual cycles after receiving the vaccine than those who were not vaccinated.
However, the menstrual cycles were delayed by less than 1 day on average. A woman with a 28-day menstrual cycle might instead experience a 29-day menstrual cycle. Moreover, this 1-day change persisted for only the first menstrual cycle or two after being vaccinated. In comparison, the unvaccinated group demonstrated no significant changes over the six months of the study.
The proportion of women who experienced a clinically significant change in cycle length of 8 days was 4.3% for unvaccinated and 5.2% for vaccinated which is not considered statistically significant, and again, this shift was resolved after about two menstrual cycles.
The most significant changes were found in a subgroup analysis focusing on women who received two vaccinations within one menstrual cycle. In this subgroup, there was an average 2-day increase in the length of their next cycle, and approximately 10% of these women experienced an 8-day or more increase in that next menstrual cycle compared to 4.3% who were unvaccinated. However, in most of these women, menstrual cycle length returned to baseline within two cycles.
When women received their second vaccination in a subsequent cycle, there were no significant differences in menstrual cycle length. Cycle length returned to baseline by the second menstrual cycle post-vaccination.
There were no differences related to vaccine brand.
What does this mean for overall menstrual health?
This study, while finding changes did occur, also showed that COVID-19 vaccinations did not result in significant or lasting menstrual cycle changes. It also debunked the myths that standing near someone who received COVID-19 vaccination could affect your menstrual cycle, fertility, or cause a miscarriage.
This study also showed that something occurs when women receive two doses in one menstrual cycle, though it remains unclear what exactly and whether this is clinically significant. One theory is that the robust immune response or stressor of the mRNA vaccines could temporarily affect the hypothalamic-pituitary-ovarian axis if timed a certain way during the menstrual cycle, particularly if the first dose is received in the early follicular phase.
One problem with the study noted by the authors is that the population using the Natural Cycles application is not fully representative of the general population. The preponderance of users was white, college-age, educated women, not using hormonal contraception, and are thinner than the average US population. Also, no data was collected on the receipt of booster shots. The researchers analyzed data from individuals with consistently normal cycle lengths and many individuals do not fit into this “normal” category of 24-32 day menstrual cycles, and the findings could be different had a different cohort been analyzed.
Overall, this well-conducted study verifies the anecdotal reports that vaccination causes changes in menstrual cycle length. It also provides reassuring evidence suggesting that COVID-19 vaccination does not result in clinically significant or long-term changes in menstrual cycles. Vaccine protection outweighs the small possibility that your cycle might be a bit delayed post-vaccination. Conversely, an acute or severe illness, like COVID-19 can dramatically impact your hypothalamic-pituitary-ovarian axis function, leading to profoundly altered menstrual cycles.
One positive outcome of this study is that it affirms the importance for medicine and science to listen more carefully to women and to consider menstrual health as a topic for scientific studies. Clinical trials of the current COVID-19 vaccines did not collect menstrual cycle outcomes and the Vaccine Adverse Event Reporting System (VAERS) does not proactively collect information regarding menstrual cycles.
Researchers need to acknowledge women’s unique health concerns and address them in all clinical trials. This includes the potential impact on hormonal and reproductive health, medication metabolism, and physiology. Hopefully, this study is just the first of many more on COVID-19 and women’s immune, reproductive, and hormonal health.