Men and women tend to respond differently to many kinds of vaccines. That’s probably because of a mix of factors, including hormones, genes and the dosing of the shots.
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 Ms. Kendeffy, 44, a medical technician in State College, Pa.
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,” Ms. 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.”
“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.”
In general, women “have more reactions to a variety of vaccines,” said Julianne Gee, a medical officer in the C.D.C.’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 (M.M.R.) 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,” Dr. Klein said.
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Still, there’s no question that biology plays an important role. “The female immune response is distinct, in many ways, from the male immune response,” said Eleanor Fish, an immunologist at the University of Toronto.
Research has shown that, compared with their male counterparts, women and girls produce more — sometimes twice as many — infection-fighting antibodies in response to the vaccines for influenza, M.M.R., yellow fever, rabies, and hepatitis A and B. They often mount stronger responses from immune fighters called T cells, too, Ms. Gee noted. These differences are often most robust among younger adults, which “suggests a biological effect, possibly associated with reproductive hormones,” she said.
The size of a vaccine dose may also be important. Studies have shown that women absorb and metabolize drugs differently than men do, often needing lower doses for the same effect. But until the 1990s, drug and vaccine clinical trials largely excluded women. “The drug dosages that are recommended are historically based on clinical trials that involve male participants,” Dr. Morgan said.
Clinical trials today do include women. But in the trials for the new Covid vaccines, side effects were not sufficiently separated and analyzed by sex, Dr. Klein said. And they did not test whether lower doses might be just as effective for women but cause fewer side effects.
Until they do, Dr. Klein said, health care providers should talk to women about vaccine side effects so they are not scared by them. “I think that there is value to preparing women that they may experience more adverse reactions,” she said. “That is normal, and likely reflective of their immune system working.”
Times reporters answered questions from readers about getting the vaccine, what to expect and what happens next.
Source: New York Times