Many women have reported changes in their periods after vaccination against Covid-19 and the US National Institutes of Health has allocated $1.67 million to investigate a possible relationship.
Two new observational studies provide reassuring data suggesting that any changes are short-lived and small compared to the natural variation in normal cycles.
Dr Victoria Male, a reproductive specialist at Imperial College London, takes a closer look at these new findings in The BMJ .
The first study was based on data from 3,959 Americans who recorded at least six consecutive cycles on a menstrual cycle tracking app. Of them, 2,403 were vaccinated and the rest acted as a control group.
After taking other factors into account, the first dose of the vaccine had no effect on the timing of the next menstruation, while the second dose was associated with a delay of 0.45 days.
The most affected were the 358 people who received both doses of the vaccine in the same cycle, experiencing a delay of 2.32 days until their next period. In this group, 11% experienced a change in cycle length of more than 8 days, which is considered clinically significant, compared to 4% in the unvaccinated group. In all groups, cycle length returned to normal two cycles after vaccination.
A second study asked a group of 5,688 Norwegian women if they had experienced specific menstrual changes (such as unexpected intermittent bleeding or worse-than-normal menstrual pain) in the cycles before and after each dose of the vaccine.
The high level of variation in normal cycles is underlined by the initial finding that 38% of participants reported at least one change from normal in pre-vaccination cycles, which increased to 39% after vaccination. first dose of the vaccine and 41% after the second.
The most commonly reported change after vaccination was a heavier than normal period.
The results of these two studies are reassuring: changes in the menstrual cycle occur after vaccination, but they are small compared to the natural variation and are quickly reversed, says Male.
However, he notes that under the UK’s vaccination schedule, it is impossible to receive both doses of the vaccine in the same cycle, so the changes seen in the US and Norway do not necessarily occur here.
A study using data from British users of the same menstrual cycle tracking app as the US study is expected to clarify this point soon, he notes.
Meanwhile, the UK’s medicines regulator (MHRA) says current evidence does not support a link between changes in menstrual periods and Covid vaccination in the UK, and continues to advise that anyone If you notice a change in your periods that persists for several cycles, or if you have new vaginal bleeding after menopause, be treated according to standard clinical pathways.
Much of the public concern around this issue arises from misinformation that Covid-19 vaccines cause female infertility, Male adds.
While we already have evidence that this is not the case - and evidence also suggests that Covid-19 infection can reduce sperm count and quality - a deeper understanding of the effects of both infection and vaccination on fertility will allow better counseling for patients who are particularly concerned about this, he writes.
These studies represent a step in the right direction, but there is still much to learn, he says. For example, understand how menstrual changes occur after vaccination, determine if any group is especially vulnerable so that they can be appropriately counseled, and better define the extent and persistence of these changes.
"The broad public interest in this issue highlights how pressing this concern is for the population. It is time for us to start listening to them," he concludes.
