In recent weeks, people who oppose Covid vaccinations have spread a claim that is not only false but defies the rules of biology: that being near someone who has received a vaccine can disrupt a woman’s menstrual cycle or cause a miscarriage.

The idea, promoted on social media by accounts with hundreds of thousands of followers, is that vaccinated people might shed vaccine material, affecting people around them as though it were secondhand smoke. This month, a private school in Florida told employees that if they got vaccinated, they could not interact with students because “we have at least three women with menstrual cycles impacted after having spent time with a vaccinated person.”

In reality, it is impossible to experience any effects from being near a vaccinated person, because none of the vaccine ingredients are capable of leaving the body they were injected into.

The vaccines currently authorized for use in the United States instruct your cells to make a version of the spike protein found on the coronavirus, so your immune system can learn to recognize it. Different vaccines use different vehicles to deliver the instructions — for Moderna and Pfizer, messenger RNA, or mRNA; for Johnson & Johnson, an adenovirus genetically modified to be inactive and harmless — but the instructions are similar.

“It’s not like it’s a piece of the virus or it does things that the virus does — it’s just a protein that’s the same shape,” said Emily Martin, an infectious disease epidemiologist at the University of Michigan School of Public Health. “Transferring anything from the vaccine from one person to another is not possible. It’s just not biologically possible.”

Microorganisms spread from person to person by replicating. The vaccine ingredients and the protein can’t replicate, which means they can’t spread. They don’t even spread through your own body, much less to anybody else’s.

“They’re injected into your arm, and that’s where they stay,” Jennifer Nuzzo, an epidemiologist at Johns Hopkins, said of the vaccines. “mRNA is taken up by your muscle cells near the site of injection, the cells use it to make that protein, the immune system learns about the spike protein and gets rid of those cells. It’s not something that circulates.”

It’s also not something that sticks around. Messenger RNA is extremely fragile, which is one reason we’ve never had an mRNA-based vaccine before: It took a long time for scientists to figure out how to keep it intact for even the brief period needed to deliver its instructions. It disintegrates within a couple days of vaccination.

Vaccinated people can’t shed anything because “there’s nothing to be shedding,” said Dr. Céline Gounder, an infectious disease specialist at Bellevue Hospital Center and a member of President Biden’s transition advisory team on the coronavirus. “The people who shed virus are people who have Covid. So if you want to prevent yourself or others from shedding virus, the best way to do that is to get vaccinated so you don’t get Covid.”

This brings us to the reports of women having abnormal periods after being near vaccinated people. Because one person’s vaccine can’t affect anybody else, it is impossible for these two events to be connected. Many things, like stress and infections, can disrupt menstrual cycles.

The shedding claims are “a conspiracy that has been created to weaken trust in a series of vaccines that have been demonstrated in clinical trials to be safe and effective,” Dr. Christopher M. Zahn, vice president of practice activities at the American College of Obstetricians and Gynecologists, said in a statement. “Such conspiracies and false narratives are dangerous and have nothing to do with science.”

Some women have expressed a related concern that getting vaccinated themselves could affect their menstrual cycles. Unlike secondhand effects, this is theoretically possible, and research is ongoing — but anecdotal reports could be explained by other factors, and no study has found a connection between the vaccine and menstrual changes.

“There’s no evidence that the vaccine affects your menstrual cycle in any way,” Dr. Gounder said. “That’s like saying just because I got vaccinated today, we’re going to have a full moon tonight.”

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