The morning Shelly Kendeffy received her second dose of Modner Covid-19 vaccine, she was feeling well. By noon she noticed a sore arm and body ache, and by evening it felt like flu.
“My teeth chattered, but I was sweating – like soaked but frozen,” she said. Kendeffy, 44, a medical technician at State College, Pa. Said.
The next day, she went to work and interviewed her colleagues – eight men and seven women – about their vaccination experiences. Six of the women had body aches, chills and fatigue. The one woman who did not have flu symptoms vomited much of the night.
The eight men delivered drastically different reports. One had mild arm pain, headache and body aches. Two describe mild fatigue and some pain. One got a headache. And four had no symptoms at all.
“I work with some very tough women,” she said. Kendeffy said. But it is clear that our women have had a serious side effect. She felt better after 24 hours and was delighted with the vaccine. “I will not change anything, because it will definitely be the alternative,” she said. “But I also did not know what to expect.”
The differences that me. Kendeffy observed among her co-workers, playing across the country. In a study published last month, researchers from the Centers for Disease Control & Prevention analyzed safety data from the first 13.7 million Covid-19 vaccine doses given to Americans. Among the side effects reported to the agency, 79.1 percent of women occur, although only 61.2 percent of the vaccines have been administered to women.
Almost all of the rare anaphylactic reactions to Covid-19 vaccines have also occurred in women. CDC researchers reported that all 19 individuals who experienced such a response to the Moderna vaccine were female and that 44 of the 47 women had anaphylactic reactions to the Pfizer vaccine.
“I’m not surprised at all,” says Sabra Klein, a microbiologist and immunologist at the Johns Hopkins Bloomberg School of Public Health. “This gender difference is consistent with reports of other vaccines in the past.”
In a 2013 study, scientists from the CDC and other institutions found that four times as many women as men between the ages of 20 and 59 reported allergic reactions after receiving the 2009 vaccine against pandemic flu, although more men than women got the shots. Another study found that between 1990 and 2016, women accounted for 80 percent of all anaphylactic reactions to adult vaccines.
In general, women have “more responses to a variety of vaccinations,” said Julianne Gee, a medical officer in the CDC’s immunization safety office. These include flu vaccines given to adults, as well as some given in infancy, such as the hepatitis B vaccines and measles, mumps and rubella (MMR).
However, the news is not bad for women. Side effects are usually mild and of short duration. And these physical reactions are a sign that a vaccine is working – that ‘you are getting a very strong immune response, and that you will probably be protected as a result,’ said Dr. Klein said.
But why do these gender differences happen? Part of the answer may be behavior. It is possible that women are more likely than men to report side effects, even if their symptoms are the same, said Rosemary Morgan, an international health researcher at the Johns Hopkins Bloomberg School of Public Health. There is no specific vaccine study to support this claim, but men are less likely than women to see doctors if they are ill, so they may also be less likely to report side effects, she said.
After all, there is no doubt that biology plays an important role. “The female immune response differs in many ways from the male immune response,” said Eleanor Fish, an immunologist at the University of Toronto.
Research has shown that women and girls, compared to their male counterparts, produce more – sometimes twice as much – antibodies against infection in response to vaccinations for influenza, MMR, jaundice, rabies and hepatitis A and B. stronger reactions of immune fighters called T-cells, it’s me. Give noticed. These differences are often strongest among younger adults, suggesting a biological effect possibly associated with reproductive hormones, ‘she said.
Sex hormones, including estrogen, progesterone and testosterone, can bind to the surface of immune cells and affect how they work. Exposure to estrogen causes immune cells to produce more antibodies in response to, for example, the flu vaccine.
And testosterone, the dr. Klein said, “is very pretty immunosuppressive.” The flu vaccine tends to be less protective in men with a lot of testosterone compared to men with less sex hormone. Testosterone, among other things, suppresses the body’s production of immune chemicals known as cytokines.
Genetic differences between men and women can also affect immunity. Many immune-related genes are on the X chromosome, of which women have two copies and men only one. Historically, immunologists believed that only one X chromosome was activated in women and that the other one was inactivated. But research now shows that 15 percent of the genes escape this inactivation and that it is more pronounced in women.
These robust immune responses help explain why 80 percent of autoimmune diseases plague women. “Women have a greater immunity, either to ourselves, whether it’s for a vaccine antigen, or whether it’s to a virus,” said Dr. Klein said.
The size of a vaccine dose can also be important. Studies have shown that women absorb and metabolize drugs differently than men, often requiring lower doses for the same effect. But until the 1990s, clinical trials with drug and vaccine largely excluded women. “The recommended doses are historically based on clinical trials involving male participants,” said Dr. Morgan said.
Clinical trials today do include women. But in trials for the new Covid vaccines, side effects were not sufficiently separated and analyzed according to sex, said Dr. Klein said. And they have not tested whether lower doses can be just as effective for women, but can cause fewer side effects.
Until they do, dr. Small said, healthcare providers should talk to women about side effects of vaccines so that they are not afraid of it. “I think it’s worthwhile to prepare women so that they can experience more adverse reactions,” she said. “This is normal and probably reflects the functioning of their immune system.”