Why the Potential Effects of COVID-19 Vaccines on the Menstrual Cycle Are Not Concerning

There have been growing reports of women experiencing changes in their menstrual cycle after receiving a shot of a COVID-19 vaccine.

Some women have said they got their periods early following vaccination, or that their periods were unusually heavy and/or painful. There have also been reports of post-menopausal women and trans men experiencing bleeding or spotting after getting the shot. This had led some to wonder whether period changes may be a potential side effect of the vaccines.

Currently, there is no scientific evidence to suggest that the vaccines induce any hormonal changes that would lead to irregular periods.

Most physicians say there is nothing to worry about and that the changes are temporary. In fact, there is evidence that the influenza and HPV vaccines can affect the menstrual cycle temporarily; however, there are no long-term side effects.

Until any kind of scientific evidence shows otherwise, this should serve as reassurance to people concerned about period changes after vaccination, and help thwart conspiracy theories about the effect of the vaccines on menstruation and fertility.

Related: Moderna’s COVID-19 Vaccine Effective Against New Variants

Nevertheless, given the growing accounts of the post-vaccination period phenomenon, some researchers have started looking into the issue.

After sharing on Twitter that she had an unusually heavy period after getting the Moderna COVID-19 vaccine, Dr. Kate Clancy, a medical anthropologist, received dozens of replies from women with similar experiences. This prompted her to launch a survey with former colleague Dr. Katherine Lee to document people’s experiences to interrogate the matter.

It remains unknown whether the vaccines are causing the menstrual changes as it hasn’t been studied. Some physicians say it may be that some women may have become more aware of changes in their body after vaccination, especially after hearing about others’ experiences. The changes could also be due to stress over getting the vaccines or other things.

Immune Links

Scientifically, although not shown yet, there may be a plausible link. The uterus is lined with immune cells which play a role in building, maintaining and breaking down the lining of the uterus, which follows a monthly cycle culminating in shedding of the lining through the period.

Given that the uterus is a part of the immune system in this way, vaccination may trigger chemical immune signals that may cause circulating immune cells to induce shedding of the uterine lining, leading to earlier periods or spotting.

No Link to Miscarriage or Infertility

However, this does not suggest that there is a link to miscarriage. This is because the processes that maintain the uterine lining, including the placenta, during pregnancy are different than those that regulate the normal menstrual cycle.

There is now ample evidence that shows vaccinated women do not have a higher risk of pregnancy loss. This includes a recent study published in the NEJM that reported similar incidences of adverse pregnancy and neonatal outcomes in women vaccinated against COVID-19 who had a completed pregnancy compared with unvaccinated women (from data collected before the pandemic).

Nor have any links between infertility and the COVID-19 vaccines been shown. The CDC says that while “clinical trials that study the safety of COVID-19 vaccines and how well they work in pregnant people are underway or planned … based on how these vaccines work in the body, experts believe they are unlikely to pose a risk for people who are pregnant.”

Conspiracy theories citing similarities between a placental protein called syncytin-1 and the spike protein of the coronavirus started doing the rounds early, claiming that vaccines based on spike could lead the immune system to attack the placenta to cause pregnancy issues. However, the similarity between the two proteins is not great enough for anti-spike antibodies to cross-react with placental syncytin-1. Moreover, a study has shown that anti-spike antibodies from convalescent sera do not react with syncytin-1.

However, what we do know is that infections, including COVID-19, are linked to miscarriage, and that having COVID-19 during pregnancy has been linked to pre-term delivery, Dr. Alexandra Alvergne at the University of Oxford told the BBC.

She said there was a plausible link between the vaccine and menstrual changes, as inflammation can affect the timing of ovulation.

Inflammation could be another possible link between vaccination and the observed menstrual changes. Vaccines could cause an inflammatory response as part of the overall immune response. There is evidence that people with signs of inflammation from infection had more painful periods.

Overall, experts agree that any menstrual changes after COVID-19 vaccination are more than likely to be temporary, and hence are not a cause for concern. As we continue to accrue data on the COVID-19 vaccines and learn more about COVID-19 itself, it remains important to look to credible information and not get taken by misleading headlines, misinformation and dangerous conspiracy theories.