How a COVID-19 vaccine can (and cannot) affect menstruation, mammograms and more

As Canada ramps up its rollout of the COVID-19 vaccines, questions continue to arise about the potential side effects reported around the world.

Following reports of blood clots from the AstraZeneca-Oxford and Johnson & Johnson vaccines, many women have wondered whether they are at higher risk — especially if they take hormonal birth control, which is also associated with clotting.

Experts say more evidence is needed and that it's unclear whether women are at higher risk of developing blood clots than men. But four health experts told CBC News there are key findings women can keep in mind when getting the vaccine — particularly if they are pregnant, booking a mammogram or a fertility treatment, or wondering whether it can disrupt their menstrual cycle.

Here is what the experts said about how the COVID-19 vaccines can — and cannot — impact women's health.

Can the vaccine affect my period?

The question of whether a COVID-19 vaccine could impact the menstrual cycle was prompted after an American professor of anthropology tweeted that she'd experienced an unusually heavy period after being immunized.

Her tweet prompted hundreds of other women to report that they too had experienced disruptions to their cycles — from missed periods to spotting to lighter or heavier bleeding than usual.

Neuroscientist Liisa Galea, a professor of psychology at the University of British Columbia and a scientific adviser at the Women's Health Research Institute, said that while it's possible a vaccine could impact menstruation, it simply hasn't been studied enough to know for sure.

"One of the receptors through which SARS-CoV-2 enters our bodies is in the uterine lining. I'm wondering if maybe if you get the vaccine at the certain time in your cycle, you might notice some differences," she said.

Galea has also heard anecdotal reports about women noticing changes in their period, but said it's much more likely that the general stress of living through a global pandemic might be responsible for variations in menstruation that women are reporting.

"The important thing is it's not going to be long lasting. You might see a disruption or some spotting, a little hiccup, but as far as I can tell, there's no evidence that your cycle is going to change past that."

Jerilynn Prior, a professor of endocrinology at the University of British Columbia, agreed that while the research is scarce, it's unlikely that the vaccine would have such an immediate and tangible effect on the menstrual cycle.

"The variability of the menstrual cycle, even within one woman, is quite large in general," she said. Prior added that people who menstruate tend to be "hypervigilant" about their physiology after undergoing any medical procedure — including receiving a vaccine.

Prior recommended that women track their periods and ovulation for a month before and after receiving the vaccine if they are interested in noting any changes.

Can the vaccine affect my mammogram results?

Experts say it's possible, but not a cause for concern.

Monika Naus, a medical director of the Communicable Diseases and Immunization Service at the BC Centre for Disease Control, said vaccines of all kinds can cause swelling in the lymph nodes located in the armpit on the side of the body where the injection takes place.

Because of their proximity to breast tissue, Naus said swollen lymph nodes can sometimes appear as white blobs on a mammogram.

"There can be a false positive, and some concern that there's an abnormality there," she said.

Naus recommended that women wait four to six weeks after they've been vaccinated to go in for a routine mammogram — though if they detect a breast lump or have concerns about their breast health, they should get screened immediately.

Prior said an experienced mammographer would likely be able to distinguish between a swollen lymph node and an abnormality in breast tissue, especially if they are warned ahead of time that a patient was recently vaccinated against COVID-19.

The Canadian Association of Radiologists and Canadian Society of Breast Imaging said a radiologist would also book a follow up to ensure any abnormalities can be attributed to the vaccine.

Can I get vaccinated if I'm pregnant or undergoing fertility treatments?

Although pregnant women were initially excluded from clinical trials, the Canadian Society of Obstetrics and Gynecology and the National Advisory Committee on Immunization have said the vaccine is considered safe for them.

Dr. Kathleen Ross, a family physician practising obstetrics at Royal Columbian Hospital in New Westminster, B.C., said it's especially important that pregnant women get the vaccine given that they are at higher risk of developing serious illness from COVID-19 should they fall ill.

"The vaccines are completely safe, they're not going to change your DNA, they're not going to cause any changes to the fetus. When a vaccine is offered to you, please, step up and get vaccinated," she said.

The COVID-19 vaccines have also been deemed safe for women undergoing fertility treatments. But the American Society for Reproductive Medicine has recommended that patients scheduled for procedures like egg retrieval or intrauterine insemination avoid getting a COVID-19 vaccine within three days of a procedure.

The society says the recommendation is not because the vaccine is unsafe, but because the vaccine could induce side effects like fever, chills, and fatigue, which may be difficult to distinguish from a post-surgery infection.

The Canadian Fertility and Andrology Society recommends the same, writing the benefits of the vaccine far outweigh any known risks.

It suggests women discuss a treatment timeline with their doctors, given that Canadian provinces have extended the time frame between the first and second doses of the vaccine.