How does the booster jab affect your menstrual cycle? Study links longer periods to vaccine

As the booster jab rollout continues in the UK, it has become clear that everyone has slightly different reactions to the Covid vaccine. A study has now revealed the effects it can have on your menstrual cycle, so here’s everything you need to know

Find out how the Covid vaccine may affect menstural cycles
Find out how the Covid vaccine may affect menstural cycles

As Covid cases rise, with case numbers soaring to 142,224 cases on Monday, January 10, 2022, the UK’s booster jab rollout continues in a bid to protect people from severe symptoms of the virus.

And, while these range from unpleasant to bizarre, with symptoms of Omicron including rashes, night sweats and sleep paralysis, the side effects of the vaccine remain minimal to most.

However, you may have seen posts on social media with users discussing how their menstrual cycles appear to have been affected by the Covid booster jab, with claims of post-vaccine periods lasting for longer or being severely delayed.

People have claimed that the Covid vaccine has affected their menstrual cycles on social media

While more studies are being funded to look into the effects of the Covid vaccine on women and menstrual cycles, several studies have now been released.

So, here’s everything you need to know about the correlation between the Covid jab and periods.

How does the Covid booster jab affect your menstrual cycle?

While it has been widely debated, a study has shown that the Covid vaccine does affect your menstrual cycle. However, the changes are believed to be minimal and should not be cause for concern.

An initial investigation, by the UK’s Medicines and Healthcare Products Regulatory Agency (MHRA), looked into over 30,000 reports to its ‘MHRA’s yellow card surveillance scheme for adverse drug reactions’ scheme.

From the reports, the side effects people listed included heavier and later periods, and unexpected bleeding after three doses.

Studies have now found a correlation between the Covid vaccine and slightly longer menstrual cycles



Lucy Lettice, Chief Operating Officer and Co-founder of eco-friendly period care brand &SISTERS echoed the prevalence of this concern.

She told The Mirror: “Almost 50% of the customers I speak to regularly are sharing concerns about regular and more irregular periods since having the vaccine, which has been reflected in sales of our heavy absorbency pads and tampons over the last year.

“We’re glad we can help manage the bleed but ultimately, we recommend that our sisters always contact their GPs to understand the true root cause further.”

After the analysis of the reports, the MHRA stated: “That evaluation of yellow card reports does not support a link between changes to menstrual periods and covid-19 vaccines since the number of reports is low relative to both the number of people vaccinated and the prevalence of menstrual disorders generally.”

While the MHRA found that some women experienced heavier periods, it did not support a ‘link’ between changes to menstrual cycles and the Covid-19 vaccine


Getty Images/Science Photo Library RF)

Following this study, fertility tracking app Natural Cycles delved deeper into the issue and found that people can experience changes in their menstrual cycles following the vaccine. The study analysed the app data 3,959 individuals, of which 2,403 were vaccinated and 1,556 were not.

Fortunately, the research found that the changes are minimal and there was an average change of less than one day in cycle duration following the vaccine (Pfizer, Moderna and Johnson & Johnson).

However, there was no change to the period length and, in cases where users of the app had two vaccine doses in the same menstrual cycle, the average change in cycle length was an increase of two days at most.

These changes also tend to stabilise in the next cycle and are temporary. So, though there is potential for some change after getting a vaccine dose, experts believe there is nothing to worry about.

Additionally, roughly 5% of vaccinated women analysed had a clinically significant change in their cycle of more than eight days, but this was similar to that of unvaccinated women analysed.

Do I need to worry about getting the Covid booster jab as a woman?

As the Natural Cycles app study showed, following research into the menstrual cycle changes of almost 4,000 women, there is little to worry about when getting the Covid booster jab as a woman.

The research looked into the cycle changes of almost 4,000 women and found that the average change in the duration was ‘less than one day’


Getty Images)

While there has been worry about the correlations between the vaccine and fertility, pregnancy and menstrual cycles, scientists and government officials have confirmed that it is safe for females.

Dr Victoria Male, from Imperial College London, wrote in the British Medical Journal: “Most people who report a change to their period after vaccination find that it returns to normal the following cycle and, importantly, there is no evidence that covid-19 vaccination adversely affects fertility.

“In clinical trials, unintended pregnancies occurred at similar rates in vaccinated and unvaccinated groups. In assisted reproduction clinics, fertility measures and pregnancy rates are similar in vaccinated and unvaccinated patients.”

And, as &SISTER’s Lucy Lettice said, if you do notice any significant abnormalities to your menstrual cycle or periods, it’s important to speak to your GP as soon as possible to get to the bottom of the reasons for the changes.

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