There's no question about it: Getting vaccinated for COVID can be followed by a set of clearly defined side effects including pain at the injection site, fever, chills, headaches, and digestive distress. We also know these responses may be particularly strong in those under 55. This is an expected part of the immune system stimulation that helps to create immunity – more robust immunity leads to a more robust immune system response and this is not considered to be a problem. One side-effect, hair loss, does seem to be more common in healthy young women.
Since the beginning of the pandemic, women have reported changes in their menstrual cycles – from more frequent to heavier to more painful to skipped periods. Similarly, there have been anecdotal reports of a variety of menstrual cycle changes post-COVID-19 vaccination including delayed periods, heavier periods, and having more than one period in the first month after a vaccination.
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. Theories, including my own, have suggested a variety of possible immune-mediated mechanisms that are known to be triggered by vaccination and that also happen to influence normal and abnormal menstruation including: increased inflammation, nitric oxide production, mast cell activation, or toll-like receptor (TLR) signaling (a theory I'm fond of since TLRs were first ‘discovered' at my medical school alma mater). These mechanisms are known to variously trigger changes in the uterine arteries and uterine lining, leading to increased uterine cramping and bleeding.
But are menstrual cycle changes a side-effect of COVID-19 vaccines? A new study has provided data that suggests we can put some of our concerns to rest.
COVID Vaccination and Menstrual Cycles
A recently published study in Obstetrics and Gynecology (ACOG's “Green Journal”), did indeed show a statistically significant change following vaccination against the coronavirus. But don't stop reading there – because there's much more to the story.
This study, conducted by the Oregon Health & Science University in conjunction with the makers of the menstrual cycle app Natural Cycles, looked retrospectively at the menstrual cycles as tracked by 3,959 individuals, 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.
To be included in the study one had to:
- Be a U.S. resident
- Be between the ages of 18–45 years
- Have at least three menstrual cycles post-pregnancy or post use of hormonal contraception
- Have 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 authors found that women who were vaccinated did, in fact, have slightly longer menstrual cycles after receiving the vaccine than those who were not vaccinated.
Here's the important caveat: According to this study, while the difference is considered statistically significant, menstrual cycles were delayed by less than 1 day on average. That is, a woman with a 28-day menstrual cycle might instead experience a 29-day menstrual cycle. Further, this 1-day change this 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 vaccination brand.
What Does this Mean for My Menstrual Health?
Overall this study is incredibly important in my opinion for two reasons: it validates women's individual observations of changes in their menstrual cycles in relationship to COVID-19 vaccination, and it provides reassuring data that receiving COVID vaccinations does not lead to significant or lasting menstrual cycle changes. As rumors have gone so far as to suggest that standing near someone who received COVID-19 vaccination could affect your menstrual cycle, fertility, or cause a miscarriage (this is simply fake science), I find this reassurance helpful.
This study does show us that something is happening 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. Clearly more research is needed here, however it is reassuring that even in this subgroup, menstrual cycles returned to baseline within 2 months of receipt of two doses in a month,
I do want to acknowledge that for many women, a 1-day menstrual cycle change can feel emotionally impactful. If you do have very regular cycles, a delay may be worrisome. If you are trying to conceive and experience even a 1-day late period, it might raise false hopes only to disappoint. It's important to keep in mind that most women's menstrual cycles do not occur, month after month, simply like clockwork. There's quite a bit of natural variation, so much so that it's considered normal to have menstrual cycles that vary up to 7 days in length from month-to-month. As a quick reminder, over 5% of unvaccinated women in this study had menstrual cycle length variations of 8 days or more each cycle. At this point it is not recommended to avoid getting two vaccines in one cycle to avoid the small risk of this effect, but if you prefer to schedule vaccination so that you avoid this, that of course is a personal choice.
One self-critique of this study by the authors is that the population using the Natural Cycles application is not fully representative of the general population. The preponderance of users are white, college age, educated women, not using hormonal contraception, and are thinner than the average US population. Also, no data was collected on receipt of booster shots. Another is that they only 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 does validate 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.
More Emphasis on Women's Health and Cycles is Imperative
One upshot (pun intended) of this study is that it highlights how important it is for science and medicine to listen more closely to women – and to include menstrual health parameters as part of 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. Until this study there has been an overall lack of attention to both women's concerns about COVID-19 vaccination and our health, and a lack of attention to the fact that women are not just small men.
Women's unique health concerns and needs should not be marginalized; rather, they should be included as priorities in all medical trials and research, including the impact of various lifecycles on our physiology, medication metabolism, and the impact of medical conditions on our hormonal and reproductive health. Perhaps this study will open the door to more research on COVID-19 and women's reproductive, immune, and hormonal health.
Edelman, A. et al. Association Between Menstrual Cycle Length and Coronavirus Disease 2019 (COVID-19) Vaccination, Obstetrics & Gynecology: January 5, 2022 – Volume – Issue – 10.1097/AOG.0000000000004695