The good news about women and COVID is very, very good. We’re less likely to get seriously ill or die from the virus. But here’s the slightly bad news. It’s the kind of bad that we think you’ll be happy to take, given the context: Women are more likely to feel bad after getting the vaccine.

The reason behind these somewhat contradictory findings is the same, strangely enough. Women mount faster and stronger immune responses, maybe because our bodies are prepped to fight pathogens that threaten unborn or newborn children. A more robust immune response means that women’s bodies often go all out when a vaccine (really any vaccine) is introduced, which leads to the COVID-vaccine symptoms that cause some women to want to hibernate under their duvets for a while.

While men our age are up to twice as likely to become severely sick and to die from coronavirus as we are, a recent study from the Centers for Disease Control and Prevention found that 79.1 percent of reported vaccine side effects came from women, even though women only received 61.2 percent of the vaccines. This is out of the first 13.7 million COVID vaccine doses given to Americans.

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The COVID Vaccine Side Effects

Among the most common side effects reported to the agency are headache (22 percent of those receiving the vaccine), fatigue (17 percent), and dizziness (17 percent). These are from either the Pfizer or the Moderna vaccine.

Anaphylaxis is a serious side effect that thankfully is rare. However, nearly all of the anaphylactic reactions to Covid-19 vaccines have occurred among women. The New York Times reports that all 19 of the individuals who had experienced such a reaction to the Moderna vaccine have been female, and that women made up 44 of the 47 who have had anaphylactic reactions to the Pfizer vaccine.

Fortunately, side effects are usually mild and short-lived. And these physical reactions are a sign that a vaccine is working — that “you are mounting a very robust immune response, and you will likely be protected as a result,” Sabra Klein, a microbiologist and immunologist at the Johns Hopkins Bloomberg School of Public Health, told the New York Times.

Why We React Differently

Across all vaccines, including the flu vaccine, women generally have worse reactions than men. So scientists weren’t surprised by the COVID vaccine discrepancy.

“Genetic factors responsible for the female immune response begin with the X chromosome, of which women have two. The X chromosome contains many genes…that regulate immune and cellular function,” Patricia Robin McCartney wrote in an article about sex-based vaccine response published in a National Center for Biotechnology journal.

Researchers also think estrogen plays a part in the differing responses to vaccines. Exposure to estrogen causes immune cells to produce more antibodies in response to the flu vaccine, for example.

Estrogen “jazzes up” the immune system, while testosterone seems to dampen it a little, Dr. William Schnaffer, an infectious disease specialist at Vanderbilt University School of Medicine told Health magazine.

This is all good, in general. It means your body is taking the vaccine very seriously, but it’s nice to be prepared.

Dr. Klein, of Johns Hopkins, recommends that health care providers talk to women about vaccine side effects so they are not scared by them. “I think that there is value to preparing women that they may experience more adverse reactions,” she said. “That is normal, and likely reflective of their immune system working.”

And in case no one tells you this before you get your jab, we’re doing it now. Prepare to feel shitty for a short time, and if you don’t, then happy day to you!

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