Women Are Reporting Worse Side Effects From the COVID-19 Vaccine—Here’s Why Experts Think That’s Happening

FYI: It's not necessarily a bad (or new) thing.

You probably know that there's a risk of developing minor side effects after getting any of the COVID-19 vaccines authorized for use. But what you likely don't know is that the data clearly show that more women than men are reporting side effects from the vaccines.

A study published by Centers for Disease Control and Prevention (CDC) in late February crunched data from the first month of administering the COVID-19 vaccine in the US, and found a clear sex divide. People who were given the 13.7 million doses of the vaccine were encouraged to report any side effects they might experience through V-safe, the CDC's after-vaccine health checker. Of the 6,994 people that actually reported side effects, 79.1% of them were women. And, it's definitely worth pointing out, just 61.2% of those who were vaccinated were women.

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This doesn't seem to be a one-off. Another study from CDC researchers that was published in JAMA in February found that all 19 people who experienced the severe and potentially life-threatening allergic reaction anaphylaxis after receiving the Moderna vaccine were women. So were 44 of the 47 people who experienced the reaction after receiving the Pfizer-BioNTech vaccine.

Anecdotally speaking, the evidence is there too: In a recent article for The New York Times, Shelly Kendeffy, a 44-year-old medical technician in State College, Pennsylvania, shared that she and her colleagues—eight men and seven women—were all given their second dose of the Moderna COVID-19 vaccine and had drastically different reactions. Six of the women reportedly had body aches, chills, and fatigue; one had vomiting. Of the men, however, only four had very mild symptoms, and four had no symptoms at all.

Though Kendeffy, who also experienced flu-like symptoms from the dose, felt better within 24 hours and "wouldn't change a thing," she also says she "didn't know what to expect." Here's what you need to know about why women might experience some worse side effects from the COVID-19 vaccine.

First, here are the possible COVID-19 vaccine side effects

While you can get the COVID-19 vaccine and have no side effects, there's a chance you could experience something. If it happens to you, don't freak out—the CDC says that side effects are normal signs that your body is building protection against the virus, should you ever come into contact with it. Possible side effects include:

  • Pain in the arm where you got the shot
  • Redness around the area where you were vaccinated
  • Swelling in the arm where you got the shot
  • Tiredness
  • Headache
  • Muscle pain
  • Chills
  • Fever
  • Nausea

The CDC study published in late February found that the most commonly reported symptoms were headache (22.4%), fatigue (16.5%), and dizziness (16.5%).

So, why might more women than men report side effects from the vaccine?

It's tough to say at this point. While research has found more women reporting side effects from the vaccine, nothing at this point has determined why that might be the case.

"I don't think there's enough information to be able to draw any strong conclusions about why this may be occurring more in females than males," infectious disease expert Amesh A. Adalja, MD, senior scholar at the Johns Hopkins Center for Health Security, tells Health. It's entirely possible that women are just more likely than men to report their symptoms, he says.

But, while women are more likely to report symptoms, that's not the only thing that's going on, William Schaffner, MD, an infectious disease specialist and professor at the Vanderbilt University School of Medicine, tells Health. Women have more autoimmune diseases than men and "it seems to be because their immune system is more reactive," he says. While Dr. Schaffner says the exact reason for this is unclear, he says hormonal differences may be at play. Estrogen "jazzes up" the immune system, while testosterone seems to dampen it a little, he says.

"There is a real difference between immune reactions between men and women," Dr. Schaffner says. "We in the medical community haven't studied that as thoroughly as we ought to."

Men and women also metabolize drugs differently, women's health expert Jennifer Wider, MD, tells Health. "Clinical trials often do not take into account this difference and the vaccine dose may be on the higher side for female recipients," she says.

Antibody responses to vaccines can also be slightly higher in women than men, Dr. Wider says, with research showing more of a difference with age. As a result, she says, the side effect response "may be more robust."

There's also this to consider: The phenomena seems to happen with the flu vaccine, too. A 2013 study published in the journal Vaccine analyzed reports of side effects after the 2009 H1N1 vaccine and found that women between the ages of 20 and 59 reported much higher rates of allergic reactions than men, even though more men than women received that vaccine. (The rates were equal in other age groups.)

And a 2019 study published in the Journal of Allergy and Clinical Immunology that analyzed cases of anaphylaxis after vaccination from 1990 and 2016 found that women made up 80% of all anaphylactic reactions to vaccines in adults.

Overall, the link is "something that needs to be studied in more detail to be able to be fully unraveled," Dr. Adalja says. But it's also not anything terribly new. "This has been happening for years," Rajeev Fernando, M.D., an infectious disease expert working in field hospitals around the world, tells Health. "I would just tell women to be aware of it."

What to do if you have side effects from the COVID-19 vaccine

First, know that it's actually a good thing to have side effects from the vaccine. "This means the vaccine is working," Jamie Alan, PhD, associate professor of pharmacology at Michigan State University, tells Health.

Dr. Fernando also stresses that women—and everyone—should still absolutely get the vaccine when they can, regardless of potential side effects. "It's very straightforward: Vaccination saves lives," he says. "The benefit of getting the vaccine and not dying from COVID-19 far outweighs the risks."

The CDC says it's OK to take OTC medication like ibuprofen, acetaminophen, aspirin, or antihistamines, for any pain and discomfort you may have after getting vaccinated. But the CDC recommends against taking any of those medications before you get vaccinated, noting that it's not known at this point how they may impact how well the vaccines work.

For arm soreness, the CDC suggests putting a clean, cool, wet washcloth over the area where you received your shot or trying to move your arm more. And, if you have a fever, the CDC says you should drink plenty of fluids and dress lightly.

If you develop severe side effects or you're not getting relief from the above methods, Dr. Adalja recommends calling your doctor about next steps.

The information in this story is accurate as of press time. However, as the situation surrounding COVID-19 continues to evolve, it's possible that some data have changed since publication. While Health is trying to keep our stories as up-to-date as possible, we also encourage readers to stay informed on news and recommendations for their own communities by using the CDC, WHO, and their local public health department as resources.

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