One of the most persistent concerns about potential adverse effects from the Covid-19 vaccines has been whether it will affect the reproductive system, particularly fertility. Two new studies directly address concerns about the Covid-19 vaccines and possible effects related to menstruation, fertility, or other reproductive health. One study, published on January 5th in Obstetrics and Gynecology, found that the vaccines did not have an appreciable impact on menstrual cycles. The other, an NIH-funded study appearing on January 20th in the American Journal of Epidemiology, found no difference in the likelihood of conception among couples based on their vaccination status.

While worries about vaccines and fertility are as old as vaccines themselves, the specific concern linked to Covid vaccines dates back to early claims by several fringe scientists who suggested theoretical reasons that the spike protein might interfere with reproduction. Since then, many scientists have pointed out why these concerns were unfounded, and the clinical trials did not suggest any impact on fertility.  

In addition, several organizations have already issued statements that Covid vaccines are not connected to fertility health, including the American College of Obstetricians and Gynecologists, the American Society for Reproductive Medicine, and the Society of Maternal-Fetal Medicine. The two new studies provide more weight to those statements.

The first study addresses the issue tangentially by reporting on findings about Covid-19 vaccines’ impact on menstrual cycles. Early in the vaccine rollout, anecdotal evidence suggested an uptick in breakthrough bleeding even among people taking hormone therapy that should have suppressed periods, such as people taking hormonal contraception or transgender individuals taking hormones that should have prevented periods.

Other people had entered perimenopause and not had a cycle in a long time until one suddenly appeared in the weeks after getting vaccinated. Examples of other anecdotes included people with otherwise normal cycles who experienced early or particularly heavy periods after getting vaccinated, prompting an investigation into whether there was a link between vaccination and heavier, earlier, or longer periods.

For the new study, researchers analyzed data from six menstrual cycles—three before and three post-vaccination—in 3,959 people, about two-thirds vaccinated. Over half the vaccinated people got the Pfizer vaccine, a third got Moderna, and 7% got Johnson & Johnson. The researchers did detect a slight difference in cycle length, but the variation was less than one day, suggesting no significant impact. While the study doesn’t directly discuss fertility effects, menstruation is one indicator of potential fertility health, so the findings should offer reassurance to those worried about it.

The second NIH-funded study compared more than 2,000 couples with varying infection and vaccination (one dose, two doses, no doses) status and taking into account a wide range of characteristics that might also affect fertility, including the following:

• Age.

• Educational attainment.

• Household income.

• Current smoking status.

• Private health insurance.

• Rotating shift work.

• Night shift work.

• Body mass index (BMI).

• Intercourse frequency.

• Intentional activities to improve chances of conception.

• Sleep duration.

• 10-item Perceived Stress Scale score.

• Major Depression Inventory score.

• Pap smear in the past three years.

• History of infertility.

• Parity (number of previous children).

• Irregular menstrual cycles.

• Menstrual cycle length.

• Geographic region of residence.

• Last method of contraception.

• Occupation in the healthcare industry.

• Race/ethnicity.

• Time since December 14, 2020.

• Tested positive for SARS-CoV-2.

Nearly three-quarters of the women had received one vaccine dose, and 58% were fully vaccinated with two doses. Among fully vaccinated women, 78% of their partners had received at least one vaccine dose. Among the 42% of unvaccinated women, only 8% of their partners had received at least one dose of the vaccine. Regarding Covid infections, 9% of vaccinated women’s partners and 8% of unvaccinated women’s partners had a previous infection with SARS-CoV-2. Of the 8% of women who had a Covid infection, 62% of their partners also had Covid.

In calculating all the variations of these partnerings (the female partner was vaccinated while the male was not, and vice versa and both partners were vaccinated or unvaccinated), the researchers found no difference in the likelihood of conceiving across couples. Instead of finding a link to vaccination, the researchers found that conception was slightly less likely if the male partner had a Covid infection two months before his partner’s menstrual cycle, adding evidence to previous research suggesting that Covid infection could affect male fertility.  

These studies can help healthcare providers respond directly to patients’ concerns about Covid vaccines and fertility. For those who want a more in-depth discussion of the fertility study, epidemiologist Katelyn Jetelina gives a thorough overview in her newsletter.