The COVID-19 vaccine and periods

Since we started receiving the Covid-19 vaccines last year, people who menstruate have been reporting changes to their periods, anecdotally, and officially: more than 30,000 reports of post vaccine menstrual irregularity were made to the UK medical regulator in 2021.

Changes reported were delayed periods, heavier periods, unexpected bleeding or a disrupted cycle.

In September 2021 the British Medical Journal (BMJ) published a report acknowledging a link between the vaccine and menstrual changes was ‘’plausible and should be investigated’’.

In the BMJ’s report, they that state one of the reasons why the vaccine may temporarily affect your cycle is not because of what is in the vaccine itself, but because of the body’s immune response to vaccination. This is reflected in similar studies that have found menstrual cycles can be affected by immune activation. Which all sounds perfectly plausible.

Factors including stress, lifestyle changes and other underlying conditions can also cause changes to people’s menstrual cycles, which also has to be taken into account. We were (and are) living through a pandemic, life was not (and still isn’t!) normal, and this could also have affected some people’s periods.

The research on this is limited and in its early stages, but recently a new study published in America in early 2022(in the journal Obstetrics and Gynaecology) found that the Covid-19 vaccine increases the menstrual cycle slightly. It showed there is indeed a slight change to a person’s cycle (a small increase of less than a day), and found effects of the vaccine were minimal and temporary - with menstrual cycles returning to normal shortly after.

The study looked at 3959 people with ‘normal cycle lengths’ who tracked their periods on the tracking app Natural Cycles. Out of the participants 2403 had received the Covid-19 vaccine (either Pfizer-BioNTech, Moderna, or Johnson and Johnson) and the remaining 1556 people had not received a vaccine. The researchers looked at three cycles before vaccination and three cycles after. They found a person’s cycle was .71 days longer after the first dose of Covid vaccine and 0.91 days longer after the second dose.

This evidence should reassure people. They found in the long run they are not seeing any long-term changes to menstrual cycles, and moving forward anyone who menstruates and is receiving the vaccine can be informed that they may see a slight short term change to their menstrual cycle length but do not have to worry about any long term effects.

Moving forward, a lot more research needs to be done into the effects of vaccines on menstrual cycles.

It is important to note that there is no evidence to suggest that the Covid vaccine affects fertility, and there is a huge amount of evidence proving Covid-19 vaccines are safe and effective.

If you do experience abnormal bleeding whether you have been vaccinated or not: speak to your GP or your local sexual health clinic.

What has your experience been? Did you notice any changes to your periods when getting the vaccine?

For more information, check out these articles:

Written by Helen Burkitt. Senior Sexual Health and Contraception Nurse
Last updated at: 05 December 2023