A 21-year-old girl living in remote California reported that two months after the first dose of the Covid-19 vaccine, which she received in March 2021, her mild period became extremely painful and doubled in duration, according to the Washington Post. After two menstrual cycles, the girl's period returned to normal, while the second dose of the vaccine worsened again.

The incident appeared not to be the only one. After the release of the Covid-19 vaccines, concern about menstrual changes was heightened. More and more women were experiencing menstrual changes or severe pain. Earlier research had shown that this was due to the stress and severe psychological ups and downs caused by the pandemic.

A new, very large international study published recently confirms the menstrual cycle changes in women due to vaccines. The study, which included almost 20.000 people around the world, showed that the Covid-19 vaccination can change the timing of the menstrual cycle. People who were vaccinated showed on average about one day delay in the occurrence of their period, compared to those who were not vaccinated.

Periodic changes in the period

The research was published on 27 September 2022 in the journal British Medical Journal and the data were drawn from a popular period tracking app. It's worth noting that the survey included women from all over the world, but most were from North America, Canada, Britain and Europe.

The researchers compared menstrual cycles between 14,936 participants who had been vaccinated with different vaccines and 4,686 who had not been vaccinated.

The data showed that women who had been vaccinated had a period with 0,71 day delay on average after the first dose of vaccine. However, people who received two doses of vaccine within a menstrual cycle showed greater disturbances, with the average increase in cycle duration being four days.

Alison Edelman, professor of obstetrics and gynecology at Oregon Health & Science University in Portland and lead author of the study, said for most women the effects were temporary and lasted for one cycle before returning to normal. He also reported that there was no evidence that the side effects of the period had impact on fertility.

Note that the study involved women who:

were not taking birth control,
had a regular cycle before they were vaccinated,
were aged between 18 and 45 years old.

In addition, unlike earlier studies, this one did not look at other side effects of the period, such as cramps.

The question that still puzzles us, however, is why vaccines affect a woman's menstrual cycle. To this question, the answer we get from researchers is not clear. As Edelman stated, the immune and reproductive systems are linked and inflammation or a strong immune response could cause period fluctuations.