Non-menstruating women were more likely to experience unexpected vaginal bleeding after receiving COVID-19 vaccinations, according to a new study.

Researchers suggested it could have been connected to the SARS-CoV-2 spike protein in the vaccines. The study was published in Science Advances.

After vaccinations became widely available, many women reported heavier menstrual bleeding than normal. Researchers at the Norwegian Institute of Public Health in Oslo examined the data, particularly among women who do not have periods, such as those who have been through menopause or are taking contraceptives.

The researchers used an ongoing population health survey called the Norwegian Mother, Father, and Child Cohort Study, Nature reported. They examined more than 21,000 responses from postmenopausal, perimenopausal and non-menstruating premenopausal women. Some were on long-term hormonal contraceptives.