Data reassuring on menstrual changes after Covid-19 vaccine, researchers find

Evidence suggests any alterations to women’s cycles ‘short-lived and small’

In the study of women, the most commonly reported post-vaccination change was a heavier than normal period. Photograph: Carlos Junior

Any menstrual changes after Covid-19 vaccination are short-lived and small compared with natural variation in normal cycles, according to an assessment of the latest evidence published by the British Medical Journal (BMJ).

Many women have reported changes to their periods after vaccination. Two new observational studies on the issue provide reassuring data, said Dr Victoria Male, a reproductive specialist at Imperial College London.

The first study drew on data from 3,959 Americans who logged at least six consecutive cycles on a tracking app. Of these, 2,403 were vaccinated while the remainder acted as a control group.

After accounting for other factors, the first dose of vaccine had no effect on timing of the subsequent period, while the second dose was associated with a delay of 0.45 days.


Most affected were the 358 individuals who received both doses of the vaccine in the same cycle, experiencing a 2.32-day delay to their next period – a scenario unlikely to have occurred in Ireland because of vaccine scheduling practices.

Clinically significant

Among this US group, 11 per cent experienced a change in cycle length of more than eight days, which is considered clinically significant, compared with 4 per cent in the unvaccinated group. In all groups, cycle lengths returned to normal by two cycles after vaccination.

A second study asked a group of 5,688 Norwegians whether they had experienced specific menstrual changes such as unexpected breakthrough bleeding or worse than normal period pain in the cycles before and after each vaccine dose.

The high level of variation in normal cycles is underlined by the initial finding that 38 per cent of participants reported at least one change from normal in pre-vaccination cycles, which increased to 39 per cent after the first vaccine dose and 41 per cent after the second. The most commonly reported post-vaccination change was a heavier than normal period.

‘Natural variation’

“The findings from both these studies are reassuring: changes to the menstrual cycle do occur following vaccination, but they are small compared with natural variation and quickly reverse,” Dr Male said, adding that much of the public concern around this issue arises from misinformation about Covid-19 vaccines causing female infertility.

“Although we already have evidence that this is not the case – and evidence also suggests that infection with Covid-19 may reduce sperm count and quality – a deeper understanding of the effects of both infection and vaccination on fertility will enable better counselling of patients for whom this is of particular concern.”

The two studies assessed are a step in the right direction but there is still much to learn, Dr Male said. For example, understanding the way in which post-vaccination menstrual changes occur, determining whether any group is particularly vulnerable so they can be counselled appropriately, and better defining the extent and persistence of these changes, she added.

“The widespread public interest in this topic highlights how pressing a concern this is for the public. It’s time we started listening to them.”

Kevin O'Sullivan

Kevin O'Sullivan

Kevin O'Sullivan is Environment and Science Editor and former editor of The Irish Times