Why are people reporting irregular menstruation after the COVID-19 shot? Experts explain.

As you’re about to head home from your COVID-19 vaccine appointment, your care provider might warn you of a sore arm or even a fever.

But what about an irregular period?

Hundreds of menstruating individuals have flooded social media to report changes in their cycles, which they say happened after taking the COVID-19 vaccine.

The most common reports? Heavy flow, spotting, lengthier cycles, or even bleeding during menopause.

“It’s worth a look for sure. I believe what women are saying,” said Dr. Jerilynn Prior, scientific director for the Centre for Menstrual Cycle and Ovulation Research (CemCOR), and professor of endocrinology and metabolism at the University of British Columbia.

Prior recently penned an article for CemCOR about these changes, citing what she says is a striking similarity between what menstruating individuals are experiencing after the COVID-19 vaccine, and after being tear-gassed at BLM protests.

At the start of April, an associate professor at the University of Illinois posted about her post-vaccine experience on Twitter. After receiving hundreds of replies from users who also reported similar menstrual changes, she started a survey that apparently garnered 22,000 responses.

“It suggests it’s a real phenomenon, and that we need to understand what’s occurring physiologically,” Prior said.

But just because these people’s experiences are real doesn’t mean they’re directly caused by the COVID-19 vaccine.

“The normal menstrual cycle is set up weeks before flow. … In other words, it’s a process. So how could something that happened a day, or two, or three ago, or five — how could it influence menstrual flow,” said Prior.

To prove whether these menstrual changes are directly caused by the COVID-19 vaccine, menstrual flow should be monitored during clinical trials of the drug.

But Dr. Tali Bogler says that usually never happens with any vaccine.

“We don’t have the answers because it wasn’t studied,“ said Bogler, who is the chair of family medicine obstetrics at St. Michael’s Hospital in Toronto, and co-founder of Pandemic Pregnancy Guide.

“It’s somewhat infuriating, I believe, when I feel like I can’t counsel my patients — women who are menstruating,  — on the full spectrum of potential side effects that could impact them. … I don’t want my patients surprised if they have spotting, or earlier bleeding, or heavier menses.”

But Bogler said this gap in research is not shocking. In fact, it consistently presents itself in numerous issues that affect women — like how pregnant individuals were left out of the COVID-19 initial vaccine trials.

As of May 9, 323 menstrual events associated with the COVID-19 vaccine have been reported on the United States’ Vaccine Adverse Event Reporting System (VAERS).

It’s important to note that most events on the website are voluntarily reported, which means the real figures might be higher. It’s also critical to highlight that an adverse event reported on VAERS does not mean that the vaccine has been established as the actual cause of that event.

Meanwhile, a search of the Canada Vigilance Adverse Reaction Online Database on May 9 produced zero results relating to menstrual changes.

Any individual menstrual changes can be triggered by many life events, including weight changes or stress, as well as the changes that occur as a part of a woman’s life,” the statement read.

Bogler and Prior have a few theories as to why this could be happening.

One possibility, according to Prior, is that these menstrual changes are happening to individuals who believe they ovulate regularly, but actually don’t.

This is called a silent ovulatory disturbance.

During a typical menstrual cycle, an egg makes its way from the ovary into the uterus every month, and the uterine lining begins to thicken as the body prepares for a baby.

If the egg isn’t fertilized, the uterine lining sheds, causing menstrual blood.

However, those experiencing silent ovulatory disturbances don’t release an egg into the uterus every month.  According to Prior, this means their lining is more fragile, and could be susceptible to irregular spotting or bleeding — like the one experienced after taking the vaccine.

But there’s another possibility.

Bogler said the uterus lining is part of the immune system, which means these side effects could be inflammation triggered by a female’s immune response to the vaccine.

Both experts maintain that the events are short-lived, do not impact fertility, and are non-life-threatening, but they should be reported to your doctor.

Yet even if these changes may not put you in immediate danger, Bogler says it’s still a frustrating and scary ordeal to many women and menstruating individuals, who don’t have science behind them to make them feel secure about their experiences.

As for whether you should be worried about the vaccine, Bogler says the safest option is to get it.

“Everyone should be receiving the vaccine as their first shield against COVID-19,” Bogler said.

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