Can COVID-19 vaccines impact menstrual cycles?

A web-based survey of more than 39,000 adults has found that 42% of respondents with regular menstrual cycles bled more heavily than usual after vaccination.

The study found that certain groups were more likely to experience heavy post-vaccination bleeding, including older premenopausal respondents, Hispanic or Latinx respondents, those who had previously been pregnant or had given birth, and those with conditions including endometriosis and polycystic ovary syndrome (PCOS).

In addition, some respondents who typically do not menstruate, including those on long-acting reversible contraceptives or gender-affirming hormones and those who have experienced menopause, reported ‘breakthrough bleeding’.

“Generally, changes to menstrual bleeding are not uncommon or dangerous, yet attention to these experiences is necessary to build trust in medicine,” Katharine Lee and colleagues wrote.

Medical experts’ dismissal of a possible connection between the vaccines and menstrual changes has fuelled greater concerns, leading vaccine-hesitant and anti-vaccine individuals and organisations to conflate these short-term changes with long-term fertility harm, according to the authors.

To address concerns about how SARS-CoV-2 vaccines could affect menstruation, Lee and team surveyed 39,129 fully vaccinated individuals — before boosters became available — who had not contracted COVID-19. Of the respondents, about 90% identified as women only, while 9.1% were categorised as gender diverse. After analysing their first three months of data, the researchers found that a significant proportion of respondents experienced some form of increased menstrual bleeding. They noted that many respondents observed these changes beginning more than a week after vaccination, which extends beyond the period when adverse symptom reporting is closely monitored during trials.

Non-causal associations

“While a very interesting paper because it is looking at something that, anecdotally, many women reported after receiving COVID-19 vaccines, the inherent weaknesses in the study design make it difficult to attribute too much weight to the findings,” said Gino Pecoraro, an Associate Professor of Obstetrics and Gynaecology at the University of Queensland and a Federal Councillor for the National Association of Specialist Obstetricians and Gynaecologists (NASOG). He is also a federal member of the Australian Medical Association (AMA) board and a practising obstetrician and gynaecologist in private practice in Brisbane.

“The nature of the design using women referred to the website questionnaire after receiving social media or traditional media links raises significant selection biases and makes interpretation difficult and needing caution,” Pecoraro said.

“As the authors themselves conclude ‘the associations described here are not causal but provide evidence to better study these trends further’. I believe it raises some important questions for future, better designed studies to look at. The article mentions there are very few studies looking at the effect of vaccination per se on menstrual cycles and this suggests it would be a useful effect to consider in future trials of vaccines.”

Fertility and pregnancy

Associate Professor Helen Petousis-Harris, who is a vaccinologist at the University of Auckland, said it is very common for women’s periods to vary, particularly during stressful times and changes to a period after receipt of a vaccine has been documented many times over more than 100 years — so the phenomenon is not new. “With respect to COVID-19 vaccines, there are now several studies, using a variety of approaches, that suggest that in the month following receipt of a dose of COVID-19 vaccine, some women have a small change to their period.

“For example, a US study found vaccinated women experienced, on average, several hours’ delay in getting their period after receiving a dose of COVID-19 vaccine. A Norwegian study also found women reported changes to their period after a dose of vaccine. The changes reported in all the studies were temporary. It would not be surprising if getting a vaccine throws a period out because it is well established that getting even a mild cold can delay a period. Interestingly, the authors of this more recent study noted that a delayed period was more likely to be associated with a vaccine reaction like fever.

“The important message is it is very clear that the vaccine has no adverse effects on fertility or pregnancy. There are many studies that have looked at the effect of the vaccine on both male and female fertility and pregnancy. The evidence shows that getting the vaccine has no negative impact on any of these things and it is important for protecting pregnant women and their infants. These new findings are of no surprise and certainly no reason to delay or avoid a COVID-19 vaccine.”

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