After seven women developed a serious type of blood clot after receiving one of the Covid-19 vaccines, many people are questioning whether the vaccines affect women differently than men—and what women should consider as they schedule their vaccinations.

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Here are answers to the top five questions about how women are affected by the vaccines.

1. Do blood clots affect women more than men?

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Two vaccines—one developed by AstraZeneca and the University of Oxford and another developed by Johnson & Johnson (J&J)—have come under scrutiny in recent weeks amid reports that, in very rare instances, patients who've received either of those vaccines developed a serious type of blood clot called cerebral venous sinus thrombosis (CVST). Regulators in the United States and Europe have paused the use of J&J's vaccine and AstraZeneca's vaccine, respectively, to investigate these cases more closely.

And because a number of these rare cases involve female patients, there are questions about whether women are more vulnerable to the clotting issue, the New York Times reports. However, regulators say they are not sure whether women are at a higher risk of developing CVST following vaccination.

Experts say the risk of blood clots remains exceedingly rare, accounting—in J&J's case, for instance—for fewer than 10 cases out of nearly seven million shots administered. But experts recommend that anyone who develops a severe headache, abdominal pain, difficultly breathing, or pain in their legs following the J&J vaccine call their health care provider for next steps.

2. Does vaccination affect mammogram results?

According to experts, Covid-19 vaccinations may in some cases cause enlarged lymph nodes in the armpit, which can show up as "white blobs on mammograms," the Times reports. Often, the swollen lymph nodes appear on the same side as the arm where the vaccine was administered.

While this swelling is completely normal—and should dissipate within a few weeks—it can be a cause for concern among radiologists, said Geeta Swamy, a maternal-fetal medicine specialist and member of the American College of Obstetricians and Gynecologist's Covid-19 vaccination group, because "if someone has breast cancer, we might seen enlarged lymph nodes as well."

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As a result, the Society of Breast Imaging currently recommends women try to schedule their routine mammograms either before they receive the vaccine or at least one month after the last dose. However, Swamy noted that women who are getting a diagnostic mammogram should "not delay" treatment because of the vaccine. She advised such individuals to inform their radiologist of when they were vaccinated.

3. Does the vaccine affect fertility treatments?

According to the American Society for Reproductive Medicine, patients who are scheduled for surgical fertility treatments including egg retrieval, embryo transfer, or intrauterine insemination should, if possible, avoid getting the Covid-19 vaccine within three days of their procedures.

Experts recommend the delay because patients who are undergoing surgical procedures could develop vaccine-related side effects, such as fever or chills, that could make it challenging for doctors to know if a post-surgical infection is present, the Times reports. Further, many providers likely won't permit someone who is experiencing Covid-like symptoms in their facility, even if those symptoms are likely just the result of a vaccine.

But women who are pregnant or seeking fertility treatment shouldn't avoid getting vaccinated, experts said. According to Sigal Klipstein, a reproductive endocrinologist who is a member of the American Society for Reproductive Medicine Covid-19 Task Force, "there are no known safety concerns with the vaccine."  

Moreover, Klipstein said women undergoing fertility treatment should not be concerned if their vaccinations are scheduled during the time between ovulation and their expected period, when an embryo would implant in the uterus, even if they develop side effects. "Fever should not interfere with implantation," Klipstein said.

According to the Times, experts are currently advising pregnant and postpartum women interested in getting vaccinated to receive either the Pfizer/BioNTech or Moderna shots.

4. Does the vaccine affect your menstrual cycle?

Some women have reported that their period changes in terms of flow or timing after they have been vaccinated, the Times reports. Although experts say this is unlikely to be a side effect of the vaccine, there is little research on the topic.

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"It's unlikely that the Covid vaccine would affect menstrual cycles, and there's no plausible biological mechanism by which this would occur," Klipstein said. "However, there is little data on this topic."

To learn more about the issue, Kathryn Clancy, an associate professor of anthropology at the University of Illinois, and Katharine Lee, a postdoctoral research scholar in public health at Washington University in St. Louis, are collaborating on a survey asking women about the short-term effects of the vaccine on their menstrual cycle.

"The menstrual cycle is a really flexible and dynamic process and it responds to a lot of different things in life like stress, physical or mental or immune changes," Lee said. "The menstrual cycle is supposed to respond and adapt."

5. Do women experience more vaccination side effects than men?

According to the Times, research published by CDC in February found that while women at the time accounted for just 61% of vaccines administered, they accounted for 79% of side effects reported to the agency—prompting questions about whether they were more vulnerable than men to side effects.

However, according to Sabra Klein, a professor of molecular microbiology and immunology at Johns Hopkins Bloomberg School of Public Health, experts are "not sure" if women are experiencing side effects to a greater degree than men, or if they are simply more likely than men to report the side effects they do experience.

That said, research indicates that if women are experiencing more side effects than men, the phenomenon could have a biological explanation. Women can generate up to twice as many antibodies in response to flu shots and certain vaccines, probably because of their reproductive hormones, genetic differences, and other factors. Moreover, research indicates that women not only account for 80% of all adult allergic reactions to vaccines, they are also more likely to be among the few people who have experienced an anaphylactic reaction to a Covid-19 vaccine.

Experts said mild side effects should not be a cause for concern, and in fact indicate that a person's immune system is ramping up. However, they also noted that a lack of side effects doesn't mean the vaccine isn't working (Caron, New York Times, 4/16; Holohan, The Atlantic, 4/12).