Norwegian scientists have discovered an unusual side effect in COVID-19-vaccinated women who don’t menstruate: atypical vaginal bleeding after receiving the Moderna and Pfizer vaccines.
A study conducted over nine months and involving almost 22,000 women revealed that 3.3 percent of postmenopausal, 14.1 percent of perimenopausal, and 13.1 percent of premenopausal women experienced unexpected bleeding post-vaccination.
The majority of these bleeds occurred within the first four weeks after receiving the vaccine. Postmenopausal women had a two to three times higher risk of bleeding during the 28 days following vaccination, while perimenopausal and premenopausal women showed a three- to fivefold elevated risk.
Interestingly, the study also noted differences in susceptibility between the Moderna and Pfizer vaccines. Premenopausal women were at a 32 percent higher risk of vaginal bleeding after a dose of the Moderna vaccine compared to the Pfizer vaccine.
However, it’s important to note that the study did not investigate the specific reasons for this unexplained bleeding, and there is no conclusive evidence to suggest that the vaccines directly caused it.
One theory posited by the researchers is that the spike protein of the SARS-CoV-2 virus, used in both the Moderna and Pfizer vaccines, could be a potential contributor to the bleeding. This hypothesis is supported by the higher risk observed after the Moderna vaccine in premenopausal women.
Possible mechanisms for inducing the bleeding may involve a spike protein-related immune response or endometrial expression of angiotensin-converting enzyme 2 (ACE2) receptors, which serve as the entry point for the virus.
While previous studies have also observed vaccine-induced vaginal bleeding, there have been conflicting findings. A 2023 study published in Vaccine found that premenopausal women vaccinated for COVID-19 were no more likely to report irregular menstrual cycles or heavier bleeds after the shot compared to unvaccinated women. However, the authors did acknowledge a slight difference in cycle length.
Irregular vaginal bleeding can have various causes, ranging from stress to underlying medical conditions like endometriosis, pelvic inflammatory disease, and polycystic ovary syndrome. Additionally, certain medications and pregnancy complications can lead to irregular bleeding.
Despite the observed side effect, the U.S. Food and Drug Administration (FDA) continues to recommend COVID-19 vaccination, emphasizing its critical role in public health and protecting against severe consequences of the virus.