Shelly Kendeffy felt fine on the morning she received her second dose of the Moderna Covid-19 vaccine. By afternoon, she noticed a sore arm and body aches, and by the evening, it felt like the flu, reported the New York Times.
“My teeth were chattering, but I was sweating — like soaked, but frozen,” said Kendeffy, 44, a medical technician in State College, Pennsylvania.
She went to work the next day and surveyed her colleagues about their vaccine experiences. Six out of 8 of them reported body aches, chills and fatigue. Among them, one woman who didn’t have flu symptoms was up much of the night vomiting.
However, the eight of them gave a radically different report in which one had mild arm pain, headache and body aches. Meanwhile, two reported mild fatigue and a bit of achiness and one got a headache. And four had no symptoms at all.
“I work with some very tough women,” Kendeffy said. But “clearly, us women suffered a severity of the side effects.” She felt better after 24 hours, and is thrilled she got the vaccine. “I wouldn’t change a thing, because it sure beats the alternative,” she said. “But I also didn’t know what to expect.”
The differences Kendeffy observed among her co-workers are playing out across the country. In a study published last month, researchers from the Centers for Disease Control and 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% came from women, even though only 61.2% of the vaccines had been administered to women.
Nearly all of the rare anaphylactic reactions to Covid-19 vaccines have occurred among women, too. CDC researchers reported 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.
“I am not at all surprised,” said Sabra Klein, a microbiologist and immunologist at the Johns Hopkins Bloomberg School of Public Health. “This sex difference is completely consistent with past reports of other vaccines.”
A study conducted by scientists with the CDC and other institutions in 2013 found that about four times as many women as men between ages of 20 and 59 reported allergic reactions after receiving the 2009 pandemic flu vaccine and that was even though more in men than women got those shots.
Other than that, in another study, it was found that between 1990 and 2016, women accounted for 80% of all adult anaphylactic reactions to vaccines.
In general, women “have more reactions to a variety of vaccines,” said Julianne Gee, a medical officer in the CDC’s Immunization Safety Office.
However, it includes influenza vaccines given to adults, as well as some given in infancy, such as the hepatitis B and measles, mumps and rubella (MMR) vaccines. Though, the news isn’t all bad for women.
Klein said, 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.”
Still, there’s no question that biology plays an important role. Eleanor Fish, an immunologist at the University of Toronto said, “The female immune response is distinct, in many ways, from the male immune response.”