Thousands of women spread across different countries reported temporary changes to their regular menstrual patterns after receiving the Covid-19 vaccine.

These changes include delays in menstrual cycle times, more menstrual blood counts than usual, to more significant pain in some parts of the body.

Dahye Yim, 30, was one of the women who experienced it. After getting her first Pfizer vaccine five months ago, she noticed something different with her menstrual cycle.

When getting menstruation, one of the PhD students from South Korea (South Korea) based in London claimed his body is easily tired, feels heavy, to feel like a migraine.

"After I received my second dose in September, I noticed a lump under the armpit and I was able to tell very easily that this was a side effect of the (vaccine) that was nothing to worry about. It helps me calm down," he told Al Jazeera, quoted Thursday (04/11/2021).

But Yim lamented the lack of scientifically supported information about the possible side effects of short-term vaccines to women.

"It would be helpful if this happened, including when I was looking for information about menstrual cycle irregularities as well. There's a lot of money out there for Covid research and I know there are priorities, but I think women's health is a big priority."

Initially, medical professionals did not make such claims. Most of them stated that the number of cases of vaccine effects on women's menstrual cycles was reported to be too low.

They even say the menstrual cycle changes could be caused by other factors such as pandemic-related stress. Of course this makes many women afraid to be vaccinated, especially plus the conspiracy theory that vaccines can cause infertility.

After much exclamation and insistence, now researchers are investigating the relationship between vaccines and women's menstrual cycles. The UK's Medicines and Healthcare Products Regulatory Agency (MHRA) is one of the bodies that will review this after receiving reports by more than 30,000 women and two U.S.-based medical researchers who collected more than 140,000 reports.

In fact, many participants in clinical trials of the Covid-19 vaccine claimed not to be asked about their menstrual cycles. Whereas in the past, irregularities in the menstrual cycle were noted as post-vaccination symptoms in other studies.

This also happened in human papillomavirus trials or HPV vaccines conducted in Japan in 2010 and 2013 that recorded "abnormal amounts of menstrual bleeding" in some participants. As well as in U.S.-based influenza vaccination trials that took place between 2013 and 2017.

But beyond a few records, there is little scientifically supported data or knowledge on how vaccines can affect a woman's menstrual cycle.

"There's a history of nausea around the period, and we absorb it from our environment," says Dr Victoria Male, lecturer in reproductive immunology at Imperial College London.

"There are a lot of people who don't call it periods and use terms like flow or time in a month. All of that goes into situations where we don't feel comfortable talking about menstruation."

She said that in the future, trial participants should be asked about their periods "as a standard question". This is a highlight of a problem that already existed before.

Analysts say structural gender imbalances around funding and a lack of representation of women, as researchers and participants, also play a role in this. Women were excluded from the study because this gender group was considered too complex to study because of their hormonal changes.

In the UK in recent years, less than 2.5% of publicly funded research has been used for projects around reproductive health. But figures show that one in three women suffer from reproductive health or gynaecological problems.

For women of color, the issue is life-threatening. Black women are four times more likely and women from Asian ethnic backgrounds are twice as likely to die in childbirth compared to white women.