The Pfizer, Moderna and AstraZeneca Covid vaccines cause unexpected vaginal bleeding in older women and women on birth control, a study has confirmed.
While changes in bleeding and periods in menstruating women post-shot have been known for years, few studies have looked at the impact on women who don't normally menstruate, such as the elderly and those on birth control.
But a new study - that looked at data from more than 20,000 women in this category - found the risk of vaginal bleeding increased two to three times in the four weeks after Covid vaccination compared to before the shot.
In women entering menopause and premenopausal women, the risk was three to five times higher.
Researchers looked at data from August and September 2021.
Ninety-eight percent of the women included reported receiving their Covid vaccines in January 2021, meaning they had received the original Covid vaccine as opposed to any updated booster shots.
Additionally, in Norway, where the data was collected from, Covid vaccines used included those manufactured by Moderna, Pfizer-BioNTech and AstraZeneca.
When Covid vaccines were first rolled out, tens of thousands of women complained about late or unusually heavy periods.
Before formal research was conducted, anti-vaxxers latched onto the reports and used them to infuse fear in Americans that the vaccines caused infertility.
However, research later released showed while menstrual changes do occur following Covid vaccination, they are minor, temporary and do not impact fertility.
Experts are not entirely sure why changes in menstruation occur, but some believe the vaccine causes some of the body's tissue to become inflamed, causing changes to the lining of the uterus and hormone levels throughout the body.
While the recent study did not investigate why these women experienced unexplained vaginal bleeding, sometimes referred to as breakthrough bleeding, scientists did suggest it could be linked to the spike protein used in the shots.
Study author Kristine Blix from the Norwegian Institute of Public Health in Oslo, said: 'We had already, from the early pandemic, biweekly questionnaires going out to cohort participants to monitor effects of the pandemic.
'In the first questionnaire that covered COVID-19 vaccinations, sent in 2021, some women reported in free-text fields that they had experienced heavy menstrual bleeding.
'This urged us to ask for bleeding patterns in a structured manner.'
Researchers looked at data from nearly 22,000 women who had already experienced menopause, women in perimenopause, the time just before entering menopause, and non-menstruating premenopausal women, including some who were on long-term hormonal birth control.
They found 252 postmenopausal women (3.3 percent), 1,008 perimenopausal women (14.1 percent) and 924 premenopausal women (13.1 percent) reported unexplained or breakthrough vaginal bleeding throughout the entire year of 2021.
Of the women who reported this, about half in every group reported their bleeding occurred within the four weeks following their first and/or second vaccine dose.
The data showed: 45 percent of postmenopausal women, 51 percent of perimenopausal women and 55 percent of premenopausal women reported the unexplained bleeding.
Among these women, 28 percent of those in perimenopause characterized the bleeding as heavy, compared to 18 percent of both women who had finished menopause and who had not yet experienced menopause.
Women in the senior cohort of the study were aged 61 to 88 years old and were considered non menstruating. Other non menstruating females were aged 32 to 64 years. Respondents were then grouped into the three menopausal categories.
Vaginal bleeding in postmenopausal women can be a sign of a serious condition, such as cancer, and more women in this group than the others sought medical care after experiencing unexplained bleeding - 30.6 percent compared to 13.8 percent of perimenopausal women and 9.3 percent of premenopausal women.
The risk of breakthrough bleeding in the first four weeks after a dose of Moderna's vaccine was associated with a 32 percent increase compared to Pfizer-BioNTech's vaccine.
Heavy menstrual bleeding has since been added as a vaccine side effect.
The study, published this week in the journal Science Advances, analyzed data from an ongoing health survey called the Norwegian Mother, Father, and Child Cohort Study.
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