The Common Vaccine Side Effect That No One Is Talking About, Experts Say

This side effect was left out of clinical trials, but you should know about it before your vaccine.

Whether it's a fever, chills, headache, fatigue, or muscle soreness, there are a handful of side effects that you expect to experience in the hours following your COVID vaccine. As of April 12, the Centers for Disease Control and Prevention (CDC) says 74 million are fully vaccinated—and with more people in the U.S. vaccinated, some lesser reported side effects have cropped up that may be more common than previously believed. Recently, experts have been alerting people of one side effect that many people are experiencing even though it wasn't mentioned in the vaccines' original clinical trials. To make sure you know what to expect before your shot, read on, and to see what you should expect if you get this one vaccine in particular, check out Pfizer Caused This Reaction in Half of Recipients, New Study Says.

People are reporting heavier or off-cycle periods after the COVID vaccine.

Woman with PMS cramps

People are reporting that their periods are earlier or heavier than usual after they receive the COVID vaccine. After hearing these reports, Kate Clancy, PhD, an associate professor at the University of Illinois, tweeted, "A colleague told me she has heard from others that their periods were heavy post-vax. I'm curious whether other menstruators have noticed changes too? I'm a week and a half out from dose one of Moderna, got my period maybe a day or so early, and am gushing like I'm in my 20s again." Clancy received a deluge of responses from people who had similar experiences, so she and a colleague, Katherine Lee, PhD, set out to do the first formal study on this vaccine side effect. She's already received over 700 responses regarding people's periods and the COVID vaccine, according to The Verge.

And for an odd effect that's cropped up, check out The Strange New COVID Vaccine Side Effect That's Confusing Even Doctors.

The side effect was left out of clinical trials, likely due to oversight.

A doctor wearing gloves prepares to give a young woman a COVID-19 vaccine injection.

While the side effect was left out of clinical trials, the tracking system for side effects from the CDC, called the Vaccine Adverse Event Reporting System (VAERS), has received at least 32 reports of altered periods, which is a small percentage of the about 56,000 participants who responded. Of course, some people may not have made the connection.

Lee, a research fellow in the Division of Public Health Sciences at Washington University, told The Verge that she wasn't surprised that this notable side effect wasn't considered during clinical trials for any of the vaccines. She pointed out that women weren't included in any clinical trials until the 1990s and noted that some of those lingering biases still exist. "These are just not things some people think about. It's not part of their daily lived experience," said Lee. "I think a lot of it is related to that history and the bias around who gets to decide what's important to consider as a side effect." To see what the majority of Moderna patients experienced after their shot, check out Moderna Caused This Reaction in 82 Percent of People, New Study Says.

Stress could be part of the problem.

Woman stressed on the couch

There is very little research on how vaccines in general affect menstruation, but they are known to stress the immune system, and the menstrual cycle can sometimes respond to those kinds of changes. "The menstrual cycle is really dynamic, and it responds to tons of things," Lee told The Verge.

Gloria A. Bachmann, MD, associate dean for women's health at the Rutgers Robert Wood Johnson Medical School, told Health, "Menstrual cycles can be altered or influenced by many factors, including stress, poor sleep, exercise, and some medications. Therefore, it wouldn't be that unusual for some women to notice, after receiving the vaccination, changes in their period, such as it coming on earlier, or having a heavier flow, or noticing more cramping than they usually have." For more COVID vaccine news delivered straight to your inbox, sign up for our daily newsletter.

It could also be related to the vaccine changing bleeding patterns.

Woman with cramps or stomachache

Clancy told The Verge that there could be a relationship between the nanoparticles in the vaccine and changes in bleeding patterns, resulting in heavier or earlier periods. The nanoparticles can create a temporary immune reaction in some people that kills off platelets, which is what helps with clotting, explained Clancy. While these cells regenerate frequently, the phenomenon can be noticeable during a bleeding event, such as your period. To make sure you don't feel badly after your vaccine, check out Doing This After Your Vaccine Can Make Side Effects Worse, Doctors Say.

This doesn't meant the vaccine affects fertility.

Couple consulting a fertility doctor

If Clancy and Lee's study finds that there is a connection between the vaccine and the menstrual cycle, that doesn't mean there's anything wrong with the vaccine; it's just another side effect. Lee also said that it probably wouldn't have any effect on fertility, but it's helpful to know that this side effect could be coming your way.

Clancy noted that heavy periods are often something that causes concern. If a formal study confirms it's a side effect of the vaccine, that can relieve some worry for people who experience it following their shot. "It's nice to know that it could happen, in the same way that it is nice knowing that you might have a fever and headache," said Lee. To see what you should be ready for after your shot, check out Be Prepared for This the Night You Get Your COVID Vaccine, Doctors Warn.

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