Do COVID vaccines affect menstruation?

July 2, 2021

As more and more people are offered the vaccine, there are increasing numbers of reports in the media about whether menstrual cycles might be affected by vaccines offered to combat COVID-19.

We spoke to Professor Emma Duncan from King’s College London, who specialises in genetics and endocrinology, to find out what might be going on here, and her thoughts on whether vaccines are having any effect on periods.

If you’d like to watch her discussing vaccines and menstruation, please check out COVID Question Time #6 on our YouTube channel.

How many people have changes to their periods after COVID vaccination?

We’ve received app reports from around 3,000 ZOE COVID Study users who described changes to their periods after vaccination. These included early or unexpected bleeding, missed or late periods, and particularly heavy or uncomfortable periods, for example. 

By the end of April when women aged 40 and over – and younger women who were health care workers or in other care roles - had been offered the vaccine, over 657,000 app users who menstruate had been vaccinated. However, fewer than 0.1% had logged an irregularity in menstruation within 3 months of receiving a vaccination. 

Regarding this, Emma explains that “the average age of menopause in the UK is 51, but anywhere between 45-55 is considered usual. Leading up to the menopause, periods become increasingly irregular. So in these early days, with the vaccine rollout including many perimenopausal women who may be experiencing irregular periods anyway, it’s difficult to be sure that these changes were due to the vaccine itself.”  

As of the end of June, now that all adult people who menstruate have been offered the vaccine, reports of an irregularity to the menstrual cycle have increased slightly but were still less than 0.5% of all vaccinated women using the app. 

Emma reassures us that ‘’these rates are still very low” and adds “there are many reasons why women experience period irregularities.”


Why is my period irregular after receiving the COVID vaccine?

Emma tells us that sex hormones are very responsive to what’s going on in life generally. “Nobody who is busy running away from a lion stops to reproduce. Your entire physiology is focused on dealing with the stress in hand.”

At times of stress and anxiety you often see effects on sex hormones - which control menstruation. The pandemic has been a time of unprecedented stress and anxiety for all of us and for many women a time of increased domestic violence, so it’s natural that many people who ovulate may be seeing some disruption to our regular menstrual cycles.

Can COVID vaccines change when my period comes?

Some women are reporting slightly earlier and some women are reporting slightly later periods. However the timing of menstruation tends to be influenced by events that happen a couple of weeks earlier. 

Emma explains that “in a typical 28 day cycle, ovulation will usually occur on day 14. The egg is released from the follicle and travels down the fallopian tubes towards the uterus. The empty follicle then becomes a ‘corpus luteum’ and secretes various hormones, particularly progesterone, which promotes gestation by preparing the uterus for pregnancy. If the egg isn’t fertilised, the corpus luteum degenerates, the hormone levels fall, and the uterus lining breaks down and is shed, which is what we experience as a period. All of these events take place in the fortnight before actual menstruation.”

Why am I experiencing PMS symptoms after COVID vaccination?

At different points in the menstrual cycle, women experience different levels of chemical messengers called cytokines. These are signalling molecules made by cells of the immune system, that help mediate and regulate immunity and inflammation.  

These messenger molecules can contribute towards PMS symptoms, along with other hormones that vary across the cycle. They also play a role in the cramping and discomfort experienced by many women at the time of their periods also.

Emma explains that “when you have a vaccination, cytokines can also be increased, which could lead to similar symptoms that you might normally experience at different times across your menstrual cycle. This too wouldn’t be surprising given that the same messengers might be involved, so it can be difficult to distinguish between the two.”

Why has my menstrual flow changed after COVID vaccination?

Some women have reported having a particularly heavy period and others an unusually light one. Emma explains that “one possible reason might be that the uterus lining itself also contains the same chemical messengers, cytokines, that regulate inflammation; and if these are affected by immunisation some women might notice some changes in bleeding.”  

Emma also mentions that “some women may be taking NSAIDs (non-steroidal anti-inflammatories) like ibuprofen or aspirin to help deal with vaccine after-effects like fever or aches and pains, so these too may make a small difference in menstrual flow.”

Those taking oral contraceptives, a typically good indicator and regulator of the menstrual cycle, have also reported irregular bleeding. Emma explains “that the most common reason for women on oral contraceptives to have breakthrough bleeding is unreliable absorption of the pill (from forgetting to take it to diarrhoea to antibiotic use).  Given that about 1% of women fall pregnant on the pill every year, this is not unusual.”

Are any other vaccines known to affect periods?

There have been reports of period irregularity with other vaccines, such as that for the human papillomavirus (HPV). Emma explains that any irregularity usually settles down quickly, in a month or two and doesn’t have any long term effects on fertility, menstrual cycles or subsequent pregnancies.

Emma also says that “the big studies of the vaccines for COVID-19 did look at these issues in women taking part. Fewer than 0.1% of these study participants, who were a majority female, reported changes in their periods with the vaccine – and some of the women who received the placebo also had similar symptoms.” 

So it does occur, occasionally, but not in very many women.

What can I do if I’m worried about COVID vaccines?

Firstly, Emma says that “if you’re experiencing any unexpected bleeding, missed periods and heavy and uncomfortable periods, they’re real and they’re distressing events. However based on your reporting and other reporting from women around the world, trial data from vaccines and previous data from other types of vaccination, these issues appear to be uncommon. If they do occur they appear to be very unlikely to persist for a long time, at most one or two cycles.”

“The most important thing is to accept vaccination when offered the opportunity because this is the main way we’re going to end the pandemic together”. However, Emma advises that if you’re experiencing irregular bleeding or disruption to your periods for more than one or two cycles, you should talk about this with your GP. She also adds that “anyone who has had a period (or bleed, of any duration) after 12 months without menstruation should see their GP, irrespective of whether this is a ‘one-off’ or a continuing problem.”

Periods are a much more complex picture than we are able to explore with the ZOE COVID Study app currently, but we want to thank you for continuing to contribute daily, as it’s incredibly important to build up a nationwide picture of trending symptoms and hotspots. Your contributions are invaluable and crucial in helping us to continue our research.

Stay safe, and keep logging.

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