In recent weeks, there have been a number of stories about the possibility of Covid-19 vaccines affecting periods.

Some 4,000 “period problems” were reported to the UK medicines regulator, the MHRA, up to 17 May, according to data obtained by the Sunday Times this weekend.

The MHRA says “a range of menstrual disorders have been reported after all three of the COVID-19 vaccines [AstraZeneca, Pfizer and Moderna] including heavier than usual periods, delayed periods and unexpected vaginal bleeding.”

And on 13 May, the BBC reported that several researchers had begun to collect their own evidence.

Here’s what we know so far.

‘Biologically plausible’

Dr Victoria Male, a reproductive immunologist at Imperial College London, told FactCheck: “It’s biologically plausible that COVID vaccines might have a short term effect on the menstrual cycle, particularly given that we know that some other vaccines do. But we don’t have good enough data to know for sure yet whether this is a real side-effect and, if so, how common it is.”

In a tweet on Sunday, Dr Male also points out that “almost all the reports are of short-term (one cycle, sometimes two) changes. So if you do get a funny period post-vaccine, it’s unlikely to bother you for more than a month.”

Cause or coincidence?

Scientists across all fields are keen to sort out correlation (where two events coincide) from causation (where one thing causes another). A number of experts say that it looks like reported changes to periods after vaccination are examples of the former.

Responding to the Sunday Times story, Dr Pat O’Brien, Vice President at the Royal College of Obstetricians and Gynaecologists, told the Science Media Centre: “many women will experience a temporary change in their periods from time to time during their lives.”

“And right now, many women in their 20s and 30s are having the COVID vaccine. So it seems inevitable that in some women these two events will coincide by chance,” he said.

In a series of tweets this weekend, Dr Male explained that this is one of the limitations of the MHRA’s Yellow Card scheme – which allows patients and doctors to record symptoms they’ve experienced after vaccination.

The system is good at “detecting serious side effects that don’t usually happen in the absence of vaccination” – for example, the extremely rare clotting incidents associated with the AstraZeneca jab, she wrote. But “what it is not so good at is detecting a change in the rate of non-serious events that sometimes happen anyway.” One example of this is “people experiencing a heavy period post-vaccine”, Dr Male says.

Dr Peter English, Retired Consultant in Communicable Disease Control, says: “Period problems are extremely common; so over any chosen time-frame you would expect many women to experience such issues. If the time frame includes the period following vaccination, the problems will be experienced after vaccination, just by chance.”

This point about chance is, Dr English says, “particularly important”. To work out whether vaccines cause menstrual changes, we have to compare the rate of period problems among vaccine recipients with the “background” rate in the population.

Here, the MHRA itself sheds some light: “The number of reports of menstrual disorders and vaginal bleeding is low in relation to both the number of females who have received COVID-19 vaccines to date and how common menstrual disorders are generally.”

In other words, from the evidence so far, the MHRA does not consider the number of reported “period problems” to be any higher than it would expect given the normal level of menstrual disorders we see in the wider population.

The regulator says it is “closely monitoring” the data and that “the current evidence does not suggest an increased risk of either menstrual disorders or unexpected vaginal bleeding following the vaccines.”

Other factors?

Dr Sue Ward, Vice President at the Royal College of Obstetricians and Gynaecologists, points out another factor that could be in play: “We’re aware some women have been reporting a change to their period cycle or symptoms during the pandemic. The degree to which changing hormone levels will affect someone is often informed by her psychological wellbeing at that time. We know that life events can make PMS symptoms feel worse and something as all-consuming and life-changing as a global pandemic could result in women experiencing their periods differently.”

She adds: “If you do notice any bleeding that is unusual for you, then we would recommend you contact your doctor.”

What about fertility?

Perhaps understandably, with discussion of the vaccine’s possible link to menstruation, some people have asked about the effect on fertility. On this point Dr O’Brien is clear: “We also want to stress that these perceived changes in menstrual cycle after having the COVID-19 vaccine should not be confused with an impact on fertility and the ability to have children.”

“There is no evidence to suggest that COVID-19 vaccines will affect fertility,” he says.

FactCheck verdict

There have been a number of reports of short-term “menstrual disorders”, including unusually heavy, delayed or unexpected periods, from recipients of all three Covid-19 vaccines in the UK.

However, the UK medicines regulator – as well as a number of other experts – say that there is not currently any definitive evidence that the jabs themselves have caused this. Changes to menstrual patterns are common, especially in women in their 20s and 30s, who have been most recently vaccinated.

Women who experience “any bleeding that is unusual for you” are advised to contact their doctor.

There is no evidence that Covid-19 vaccines affect fertility.