Nearly one-third of adults in the United States have received their first dose of COVID-19 vaccine. That number is not only inspiring and hope-inducing, but also high enough to identify patterns in the real world use of the vaccine. What we’ve found so far (besides that the vaccines are highly effective in real life!) is that women are experiencing worse vaccine side effects than men. The reasons for that are varied and worth looking into.
Side Effects For Everyone Are Generally Mild And Not A Reason To Worry
Before getting into the emerging patterns, it’s important to keep in mind that side effects for everyone are generally mild and temporary.
Common side effects include sore arm, pain at the injection site, tiredness, headache, muscle aches, fever, chills, and tiredness. In most cases, the side effects go away on their own in a day or two.
And side effects, while unpleasant, aren’t all bad. They are proof 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. (The absence of side effects is not something to be alarmed about though—that’s common, too!)
Initial COVID Vaccine Study Confirmed Women Were More Likely To Experience Side Effects
In a study published last month, the CDC found that 79.1 percent of reported side effects were experienced by women, even though women received only 61.2 percent of the doses.
Likewise, almost all of the very rare anaphylactic reactions to the vaccines have occurred in women. According to the data, the 19 individuals who experienced an anaphylactic reaction to the Moderna vaccine were women. Similarly, 44 of the 47 individuals who reported an anaphylactic reaction to the Pfizer vaccine were women.
The Difference In Response Is Not Limited To The COVID Vaccines
According to Klein, “This sex difference is completely consistent with past reports of other vaccines.”
Women and girls produce more antibodies in response to vaccines for the flu, M.M.R. yellow fever, rabies, and hepatitis A and B, according to previous research. Sometimes, women and girls can even produce twice as many antibodies. Additionally, their T calls often generate a stronger immune response.
Dr. Larry Schlesinger, president and chief executive officer of the Texas Biomedical Research Institute in San Antonio, confirmed this in an interview with Healthline. He noted, “In women, there is an exuberant and stronger response [to many vaccines].”
Hormones Play A Role In Immune Responses
Hormones like estrogen, progesterone, and testosterone can also influence how vaccines work. Estrogen, which is found in higher amounts in women, helps activate the body’s immune response to illness, and also to vaccines. Conversely, testosterone, found in greater amounts in men, has been shown to slow done the immune response, according to Healthline.
“Simply put, women in general have a stronger response to vaccines because their bodies are quicker and stronger when it comes to activating what the vaccine introduces in the body,” Schlesinger told Healthline.
The Difference Between Responses Might Be Written In Our DNA
Women have two X chromosomes, whereas men have one. This chromosomal difference may also factor in to vaccine response.
The “X chromosome is enriched for immune response genes–genes that really play fundamental roles in mediating the immune response,” Klein said in an interview with MedPage Today. Which means: that extra X chromosome may give women an advantage in vaccine response. A greater immune response could explain a higher chance of experiencing side effects.
In the past, immunologists believed that only one X chromosome was turned on. The other was inactivated. However, new research shows that’s not always the case. (And this may explain why autoimmune diseases more commonly afflict women than men.)
Reporting Bias And Dosage
Social norms could partly explain why women seem to experience worse side effects. While there’s no specific research on the matter, it’s possible women accounted for a higher percentage of those who experienced side effects because they were more likely to report than men.
“Women do often report pain and feelings of pain more often than their male counterparts,” Klein told MedPage Today.
It’s also possible that women experienced more side effects because of the size of the vaccine dose. According to studies, women often need lower doses of a drug to achieve the same result as men because they metabolize drugs differently. The COVID-19 vaccine trials did not test whether a lower dose would be as protective, but with less side effects, in women, Klein told The New York Times.
Ultimately, why women experience worse side effects than men is probably a little bit of all the reasons suggested above. And while there’s value in understanding what side effects you might experience and why yours might be worse than the man getting vaccinated next to you, the most important thing to keep in mind is that the side effects are usually mild and short lived. They are nothing compared to actual infection by COVID-19.
“COVID-19 disease can cause serious complications and even death, and vaccination is an important prevention tool to prevent disease and complications,” Julianne Gee, MPH, a lead author of the study and a medical officer in the CDC’s Immunization Safety Office, told Healthline. “COVID-19 vaccines will help society return to normal.”
Information about COVID-19 is rapidly changing, and Scary Mommy is committed to providing the most recent data in our coverage. With news being updated so frequently, some of the information in this story may have changed after publication. For this reason, we are encouraging readers to use online resources from local public health departments, the Centers for Disease Control, and the World Health Organization to remain as informed as possible.