Can the COVID-19 vaccine affect your menstrual cycle?

The race to get vaccinated is well and truly underway. We all know what side effects to expect after COVID-19 vaccine: a sore arm, a fever and a day off work to sleep away the fatigue. However, there are anecdotal reports coming out of the US and UK where people have experienced unexpected changes to their menstrual cycle after their jab.

Photo by Tonik on Unsplash

People have commonly been reporting changes such as heavier, earlier, and more painful periods. There have also been reports of experiencing delayed or absent periods after a COVID-19 vaccination. These anecdotal reports have mostly come out of the UK where nearly 45 million of them are fully vaccinated. But more recently, the ABC has written an article stating that they have had several women have contacted them to say they have experienced changes in their menstrual cycle in Australia too.

Dr Kate Clancy, an associate professor at University of Illinois is a human reproductive ecologist and decided to raise the question on twitter after experiencing a disruption to her own menstrual cycle:

“A colleague told me she has heard from others that their periods were heavy post-vax. I’m curious whether other menstruators have noticed changes too? I’m a week and a half out from dose 1 of Moderna, got my period maybe a day or so early, and am gushing like I’m in my 20s again.”
Dr. Kate Clancy 🏳️‍🌈 (@KateClancy)

After she received an overwhelming response, she collaborated with post-doctoral researcher, Dr Katharine Lee. Together, they have developed an online survey to understand the menstrual experiences of people after they have had a COVID-19 vaccine. So, are these changes in bleeding patterns a cause for concern?

Plausible Link

Dr Victoria Male is a lecturer in reproductive immunology and recently published an article in the British Medical Journal on menstrual changes after the COVID-19 vaccine and states “a link is plausible and should be investigated”. Dr Male has found that menstrual cycle changes are not limited to the type of vaccine and have been reported after both the mRNA vaccines (Pfizer and Moderna) and the viral vector vaccines (AstraZeneca). This information suggests that perhaps it is the immune response after the vaccine that elicits these unexpected changes.

The uterine lining is made up of many immune cells which take part in shedding the wall during a period. The large immune response after a vaccination could influence the numerous hormones that control the menstrual cycle, therefore leading to short term changes in bleeding. Unfortunately, this mechanism isn’t fully understood yet and more research into immune system mediation of hormones may help us understand it.

Why Didn’t the Clinical Trials Notice Anything?

Unfortunately, the early COVID-19 vaccine trials did not ask many questions about menstrual cyles. Questions are often directed towards arm soreness, headache, fever, or chills. This raises some concerns as many people feel like there was missed information and the trials were lacking. However, clinical trials were effective in picking up severe issues that cause major health concerns. But because there is no hospitalisation or illness after menstrual changes and no specific questions about their periods, then perhaps it was missed.

In addition, a US study of over 35,000 pregnant women has shown that there are no changes in miscarriage or birth abnormality rates in vaccinated and unvaccinated women. There is also nothing to suggest the vaccines affect fertility. However, if a woman becomes infected with COVID-19 during her pregnancy then she is putting herself and her newborn at a higher risk of being admitted into the ICU.

The Takeaway

It is important to not confuse short term disruptions in your regular cycle to disruptions in fertility. Menstrual cycles can vary month to month and be influenced by a range of factors, especially stress. The ongoing stress of lockdowns, restrictions and the uncertain may also be contributing to the short-term changes seen in menstrual cycles after a vaccination. Therefore, these menstrual changes we may experience are often short-lived and most likely not a cause for concern.

If you would like further resources, please read this article by the Australian Government Department of Health written by health experts:




2 Responses to “Can the COVID-19 vaccine affect your menstrual cycle?”

  1. Xinran Huang says:

    Hi Motanna,
    Thanks for the insightful post. I got my period a few days earlier after getting my jab, but didn’t think much into it since the menstrual cycle can be affected by so many factors.
    Do you think there will be formal investigations into the effect of COVID vaccines on menstrual cycles? (Is it worth it? Is it possible?) I do hope that this doesn’t cause further vaccine hesitancy!

  2. Inez Beadell says:

    Hi Montanna, really interesting article and not something I was previously aware of. I’ve heard of some people refraining from getting vaccinated due to fertility concerns – do you think this is related to the mechanism affecting periods?