On the morning that Shelly Kendeffy received her second dose of the Moderna COVID-19 vaccine, she felt fine. By afternoon, she noticed a sore arm and body aches, and by evening, it felt like the flu.
“My teeth were chattering, but I was sweating — like soaked, but frozen,” said Kendeffy, 44, a medical technician in State College, Pennsylvania.
The next day, she went to work and surveyed her colleagues — eight men and seven women — about their vaccine experiences. Six of the women had body aches, chills and fatigue. The one woman who didn’t have flu symptoms was up much of the night vomiting.
The eight men gave drastically different reports. One had mild arm pain, a headache and body aches. Two described mild fatigue and a bit of achiness. 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.”
n a 2013 study, scientists with the CDC and other institutions found that four times as many women as men between ages 20 and 59 reported allergic reactions after receiving the 2009 pandemic flu vaccine, even though more men than women got those shots. Another study 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. That 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.
The news isn’t all bad for women, though. 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,” Klein said.