Can the COVID vaccine delay your period?

Experts address the rumors swirling social media: Can the COVID vaccine delay your period?

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After a year fraught with grief and global pandemonium, Americans finally have full access to vaccines that will immunize them from the coronavirus. This news brought peace of mind to many, and concerns for others.

For travelers, there are questions over the necessity of the vaccine passport. And for those who menstruate, there's a physical worry going around: Can the COVID vaccine delay your period or mess up your menstruation cycle? Let's dive into it.

Can the COVID vaccine delay your period?

In short, yes. There have been connections made between the COVID-19 vaccine and people's menstrual cycles, especially surrounding the stress that vaccination can put on the body and the period irregularities that could occur as a result. However, as research on the topic is happening literally as we speak, most of these connections have anecdotal roots and, frankly, more science is needed.

As of this writing, more than 132 million adults have received at least one dose of the COVID-19 vaccine, according to the Centers for Disease Control and Prevention (CDC). And while most of us can expect a sore arm and some fatigue after getting the jab, a slew of new reports suggests there could be more side effects for people with periods.

Dr. Kathryn Clancy, Period Podcast host and a professor studying the menstrual cycle at the University of Illinois, posted on Twitter about the changes in her own period, saying it not only arrived early but was significantly heavier than normal after receiving her first dose of the Moderna vaccine. Discussion with a colleague who experienced similar effects led Clancy to crowd-source on social media. 

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A flurry of replies came rolling in: women with both accelerated and delayed cycles, others with heavier flows and exacerbated menstrual cramps, some even reporting hot flashes and various hormonal responses. This mountain of anecdotal evidence, though scientifically inconclusive, could not be ignored.

With little research to link vaccination with changes in menstruation, Dr. Clancy—along with Dr. Katharine Lee, a research fellow in the Division of Public Health Sciences at Washington University—decided to bridge the gap. Together they developed a formal study to collect and analyze data surrounding this menstrual phenomenon after inoculation.

“It really sucks to be surprised by your period,” Dr. Lee told The Verge. “It’s nice to know that it could happen, in the same way that it is knowing you might have a fever and headache.”

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Abnormal periods are not a side effect that the COVID vaccine clinical trials focused on, nor is it listed on the CDC’s vaccine health platform, the V-Safe After Vaccination Health Checker. Dr. Lee credits this to bias: women weren’t regularly included in clinical trials until the 1990s, according to Healthline. However, just one week after launching their menstrual survey, Lee said more than 19,000 participants have shared their personal accounts.

A study published in January in the journal of Reproductive BioMedicine Online found that 28-percent of patients with confirmed coronavirus cases had changes to their menstrual cycle, 19-percent with prolonged periods and 25-percent with changes in the volume of their flows.

Stress is a known cause of irregular periods. Vaccines can put stress on the immune system, which, in turn, can trigger slight changes to your cycle. The endometrium (the lining of the uterus) is part of the immune system and helps to protect newly implanted embryos from pathogens. “It’s not inconceivable that there could be a connection,” Jen Gunter, an obstetrician-gynecologist and women’s health advocate, told Mother Jones.

Gunter also outlines two more major factors that could be at play: the COVID vaccine could impact the chemical messaging from brain to ovaries and/or the chemical messaging from ovaries to uterus. “These effects could be from the vaccine itself, the immune system response to the vaccine, or potentially related to fever or feeling unwell from the vaccine or stress related to vaccination,” she notes.

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  • Bye-bye tampons—menstrual cups are kinder to your body and the planet

After getting a shot, your body is assigned a pretty big task: produce antibodies and build a protective wall against possible future COVID infection. “Vaccines make the immune system work harder, which will have an effect on menstruation,” says Dr. Sandra El Hajj, a global health policy expert and member of the American Preventative Health Organization.

However, menstrual cycles are impacted by a multitude of elements. “Every month can have its own affecting factors,” the doc says. “Each woman witnesses changes continuously at a personal, lifestyle, and dietary level. All these can have a big impact on the woman's hormones, which in turn will affect menstruation.”

While we can’t directly connect vaccines and periods just yet, a February CDC report found that women received 61.2-percent of vaccine doses but reported 78.7-percent of adverse side effects. More questions were raised regarding how the vaccine affects women differently than men after seven women developed a rare blood clotting disorder post-inoculation with the Johnson & Johnson vaccine (which has since been suspended).

Timing, of course, is everything. “Vaccines are made of very small nanoparticles that may cause changes in the bleeding patterns, a temporary platelet-killing effect,” Dr. El Hajj explains. “These platelets are normally involved in blood clotting and can regenerate very fast, but if someone gets a shot right before a period, it may lead to heavier bleeding.”

Does the COVID vaccine affect future pregnancy?

In terms of fertility and pregnancy, “there are no known safety concerns with the vaccine,” according to Dr. Sigal Klipstein, a reproductive endocrinologist and member of the American Society for Reproductive Medicine COVID-19 Task Force.

However, according to the American Society for Reproductive Medicine, fertility patients should avoid getting the vaccine three days before and three days after any fertility procedures like egg retrieval and embryo transfer. The precaution is to avoid confusion with any post-procedure side effects.

Can you have an abnormal mammogram after COVID vaccine?

The COVID vaccine has also been found to enlarge lymph nodes. While this is a normal immune system reaction, it can light up a routine mammogram and cause concern for radiologists looking to flag signs of cancer. This typically occurs on the same side you received the shot, and typically only lasts for a few weeks, Dr. Geeta Swamy—a maternal-fetal medicine specialist and a member of the American College of Obstetricians and Gynecologists' COVID vaccine group—told The New York Times.

To avoid confusion and alarm, the Society of Breast Imaging recommends scheduling mammograms before your first dose or at least one month after your second.

If you’re wondering, 'Can the COVID vaccine delay your period?' or if you've experienced abrupt changes to your menstrual cycle, contact your doctor. You can also use period tracker apps to monitor symptoms, flow, and ovulation.

Rheanna O'Neil Bellomo
Rheanna O'Neil Bellomo

Rheanna is a multimedia journalist whose culinary, travel, and wellness work has appeared in Esquire, Cosmopolitan, Us Weekly, Eating Well, Brides, and more. She was previously the resident bar critic and features writer at Time Out New York, and the news and video editor for Delish. She has also helped produce documentaries. Outside the office, you can find her shooting 35mm film and adventuring with her rescue mutt, Brody.