Do COVID-19 vaccines affect menstruation and fertility?

Ever since the pandemic started there have been news headlines speculating about the effect of the vaccines on our reproductive system, but what does the science tell us?

  • 11 June 2021
  • 4 min read
Photo by Marcin Jozwiak on Unsplash
Photo by Marcin Jozwiak on Unsplash


If you’ve been lucky enough to have had a COVID-19 vaccine, you may well have had some of the common side effects including a headache, sore arm, fever or other flu-like symptoms. Now, anecdotal evidence is starting to emerge that it could affect your periods too, but this is currently being researched to see whether there is a conclusive link. The good news is that even if there were a connection, it seems to be temporary. Scientists say there is no evidence that the vaccines affect fertility, i.e. the ability to get pregnant and to deliver a child.

“There is absolutely no scientific evidence or truth behind this concern that vaccines somehow interfere with fertility, either in men or in women”

Surveying changes to periods

Anthropologist Dr Kathryn Clancy at the University of Illinois spoke on Twitter about how her period arrived early and was heavier than usual one week after her first dose of the Moderna vaccine. She received so many responses from others reporting changes to their menstrual cycle that she launched a survey asking for people who have been vaccinated to share their menstrual experiences.

The HPV (human papillomavirus) vaccine and the flu vaccine have been reported to affect menstrual cycles temporarily so it wouldn’t be surprising if COVID-19 vaccines do so briefly either. Immune cells are at work in the creating and then breaking down the lining of the uterus that happens during menstruation; vaccines produce inflammatory molecules called cytokines and interferons that stimulate immune cells, including potentially in the uterus. This might cause the lining to shed sooner or more intensively than usual, causing changes to the menstrual cycle. Some trans men and post-menopausal women who don't normally have periods also contacted Dr Clancy to say they had experienced post-vaccine menstrual bleeding.

It’s important to note that many other factors including stress, anxiety and nutrition can affect menstruation and it can be hard to draw a through-line of cause and effect between vaccination and periods. What is also crucial is that despite half the global population having periods at some point in their life, there is still a lot we don’t know about menstruation as it is not studied in enough detail.

The vaccine doesn’t affect fertility 

So far the changes in menstrual cycles have mainly been reported through social media. There hasn't been a controlled scientific study to understand the origin of any potential effect of vaccination on periods. The UK’s Yellow Card reporting system is trying to capture all possible reactions and side effects from various drugs or vaccines (including COVID-19 vaccines) and people are encouraged to report any changes in menstruation there to aid future research studies on a possible causal relationship. 

Even if a link between COVID-19 vaccines and a temporary blip in menstrual cycles is eventually shown, this does not have any implications for getting pregnant or having a healthy pregnancy. 

A study of more than 35,000 people who were pregnant when they received their vaccine showed that there are no increased risks of severe side effects or adverse pregnancy outcomes from either the Moderna or Pfizer/BioNTech vaccines. The study of participants aged 16 to 54 years was undertaken by scientists at the US Centers for Disease Control and Prevention and published in the New England Journal of Medicine in April.

That vaccines could affect fertility is a “common myth” said Dr Soumya Swaminathan, Chief Scientist at the World Health Organization (WHO). She adds “There is absolutely no scientific evidence or truth behind this concern that vaccines somehow interfere with fertility, either in men or in women, because what vaccines do is they stimulate an immune response against that particular protein or antigen of that virus or bacteria. … there is no way in which they could interfere with the functioning of the reproductive organs in either men or women.”

Research also shows that other vaccines that may temporarily affect menstruation such as the HPV vaccine, does not affect fertility.

Not only that but the vaccine is especially important for pregnant women. The American College of Obstetricians and Gynecologists advises that giving the vaccine to pregnant women is vital given that COVID-19 is more likely to be severe in pregnancy, affecting not just the mother but the baby, as pregnancies are more likely to be premature. When pregnant women get vaccinated, they can pass on antibodies to their fetuses, which would give them extra protection too. The UK’s Joint Committee on Vaccination and Immunisation (JCVI) updated its guidance a month ago to say that pregnant and breastfeeding women should be offered the vaccine.