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. The new findings in this research update are of no surprise and certainly no reason to delay or avoid a Covid-19 vaccine.

This topic highlights the importance of collecting data about menstruation during clinical trials - so that it can be ascertained and communicated  early. Despite these potential minor disruptions to periods, there is substantial data on the vaccine's effects on fertility and pregnancy, and it strongly supports vaccination safety.

The author of this commentary summarises research that links COVID-19 vaccination with a short and temporary increase in menstrual cycle length. Possible mechanisms as to how the immune response from the COVID vaccine could temporarily change menstrual cycles are postulated.

Menstrual disturbance is common. It is normal for menstrual cycles to vary by up to 8 days per cycle and menstrual cycles are also affected by physical and psychological stressors. Routinely including questions about menstrual cycles in future vaccine trials means that any impact on periods could be assessed prospectively. Having good quality data allows us to reassure people. The author makes an interesting point that this information could also assist in understanding  whether the immune response against disease is affected by menstrual cycles.

Traditionally vaccine trials (and many trials of other treatments and interventions) have not addressed impacts on reproductive health and have excluded pregnant people.  This was also evident when, despite being at higher risk of complications of the disease, pregnant people were excluded from early trials of the COVID-19 vaccine.  This needs to change and trials of future vaccines should specifically address reproductive issues, so as not to exclude a proportion of the population and to let people make informed decisions about their healthcare.