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Women Report Worse Side Effects After COVID Vaccines

Editor’s note: Find the latest COVID-19 news and guidance in Medscape’s Coronavirus Resource Center.

Women are reporting more severe side effects after receiving COVID-19 vaccines, which could stem from a mix of factors, according to  The New York Times.

Side effects tend to differ for men and women across a broad variety of vaccines, often due to hormones, genes and the vaccine doses.

“I am not at all surprised,” Sabra Klein, an immunologist at the Johns Hopkins Bloomberg School of Public Health, told the newspaper.

“The sex difference is completely consistent with past reports of other vaccines,” she said.

Last month, CDC researchers published a report about the safety data from the first 13.7 million COVID-19 vaccine doses administered in the U.S. Among the 7,000 people who reported side effects, about 79% were women, although about 61% of the vaccines were given to women.

In another report about rare anaphylactic reactions, CDC researchers found that all 19 people who had anaphylaxis after the Moderna shot were female. With the Pfizer vaccine, 44 of the 47 people who had anaphylaxis were women.

In previous CDC studies, researchers have found that four times as many women had allergic reactions after the 2009 pandemic flu vaccine, The New York Times reported. Between 1990-2016, women reported 80% of the anaphylactic reactions to vaccines. Overall, women tend to have more reactions to vaccines for the flu and hepatitis B, as well as the combined measles, mumps and rubella vaccine.

The differences happen for many reasons. Women tend to have a more robust immune system that can produce more antibodies in response to vaccines, the newspaper reported, which may be related to reproductive hormones such as estrogen, progesterone and testosterone. Estrogen, for instance, can cause immune cells to produce more antibodies, and testosterone can suppress the production of immune chemicals in the body.

In addition, immune-related genes are on the X chromosome, which women have two copies of; men have one. Scientists believe that partially explains why more women have autoimmune diseases.

“Women have greater immunity, whether it’s to ourselves, whether it’s to a vaccine antigen, whether it’s to a virus,” Klein told the newspaper.

Vaccine doses can make a difference, too. Women and men absorb drugs differently, and women often need lower doses for the same response. COVID-19 vaccines, which provide the same dosage to everyone, could create different responses and side effects in people, the Times reported.

In good news, Klein said, the COVID-19 vaccine side effects reported by women tend to be mild and short. The reactions also show that the vaccine is working, she said. As the vaccine rollout continues, health care providers and patients should discuss potential side effects.

“I think that there is value to preparing women that they may experience more adverse reactions,” Klein told the newspaper. “That is normal, and likely reflective of their immune system working.”


New York Times, “Women Report Worse Side Effects After a Covid Vaccine.”
CDC, “First Month of COVID-19 Vaccine Safety Monitoring — United States, December 14, 2020–January 13, 2021.”
CDC, “Reports of Anaphylaxis After Receipt of mRNA COVID-19 Vaccines in the US—December 14, 2020-January 18, 2021.”

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  • Posted on March 10, 2021