EXPERT REACTION: Can COVID-19 vaccines impact your menstrual cycle? 

I agree it is important to gather data around this topic because effects on menstruation of any drug or intervention is hardly ever researched. For example, the original trials of the vaccine asked about fever and arm soreness and many other potential side effects, but not a single question on menses.

Unfortunately, this is not high quality research and I would not put a lot of credence on its findings. In the introduction, they call it an exploratory study, and that is all it is, they are exploring the effect of the vaccine on menses. It is a descriptive study, where they describe a large case series of almost 40,000 people who got the vaccine, and the key finding was that you were equally likely to have no change in your next period as you were to have a heavier next period.

However, this is not true research per se, in the sense that there is no comparison group. Ideally, you ask people to track their bleeding for six months, for example, and compare those who had the vaccine to those who did not. But here, they only track people who got the vaccine. One cannot conclude that the increase or decrease in the bleeding or the irregular pattern of bleeding the person experienced is related to the vaccine or to something else. There are so many factors that affect menses. I have written more on this topic here.

The limitations of this paper include self-reported symptoms and diagnosed women’s health conditions (not confirmed by any medical notes), and recall bias (the survey is done in retrospect). If someone thinks the vaccine is ‘bad’ then they may be more likely to report their next period was heavier.

As a practicing gynaecologist reading any research paper on menses as it relates to the Covid-19 vaccine, my important takeaway message is that if one menstrual cycle is a bit heavier or lighter, or comes early or late, usually this is no cause for concern; but if this becomes a pattern, then do seek medical help.

Menstrual disturbance is not commonly assessed in vaccination safety studies. This paper reports an internet survey on self-reported menstrual disturbance following COVID-19 vaccination in almost 40000 people. Within this group, menstrual disturbance was common following COVID vaccination. This is plausible as we know menses are affected by illness and stress.

However, caution is needed to what extent this data reflects the overall population. As study participants have responded to social media advertisements, there is likely selection bias with people who have noticed menstrual changes more likely to respond.

84% of participants were white ethnicity and 10% were gender diverse, which is not reflective of the overall population. Participants with underlying gynaecological conditions were more likely to report menstrual disturbance following vaccination - these conditions themselves are associated with menstrual disturbance, so it would be important to clarify whether the rate of menstrual disturbance was changed by COVID vaccination.

This study contrasts with a prospective study of people tracking their menstrual cycles pre- and post-COVID vaccine in both vaccinated and unvaccinated cohorts. This allows direct comparison pre- and post-COVID vaccine and between the two different groups, i.e., vaccinated and unvaccinated. Reassuringly, this study found no change in length of menses and a clinically insignificant change (less than one day) in the length of the menstrual cycles."

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.

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.

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.

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 phenomena 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 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.