A new study adds to the growing evidence that COVID-19 vaccines are safe for both pregnant people and people hoping to become pregnant.
The study, which looked at more than 2,000 couples in the United States and Canada, found "no adverse association" between getting vaccinated against COVID-19 and fertility, for both men and women.
On the other hand, men who contract COVID-19 may experience a temporary reduction in fertility. Couples who had a male partner test positive for COVID-19 within 60 days of their partner's menstrual cycle were 18% less likely to conceive in that cycle, according to the study, published on Jan. 20 in the American Journal of Epidemiology.
"The findings provide reassurance that vaccination for couples seeking pregnancy does not appear to impair fertility," Dr. Diana Bianchi, director of the National Institute of Health's Eunice Kennedy Shriver National Institute of Child Health and Human Development, which funded the study, said in a statement. "They also provide information for physicians who counsel patients hoping to conceive."
The myth that COVID-19 vaccines may negatively impact fertility was one that was spread largely on social media.
More and more research has now shown that not only do the vaccines not affect fertility, they also do not impact pregnancy.
A study released Jan. 4 by the Centers for Disease Control and Prevention (CDC) found no increased risk of preterm or low-weight birth among babies born to pregnant people who got a COVID-19 vaccine shot, compared to babies born to unvaccinated pregnant people.
The study's researchers at Yale University looked at the health data of more than 40,000 pregnant women and did not identify any safety issues with getting vaccinated while pregnant, no matter which trimester a woman was in when vaccinated, or how many vaccine doses she got during her pregnancy. Researchers noted most of the women included in the analysis were vaccinated in the second or third trimester, and the study didn't include booster doses.
In a health warning issued in September urging pregnant people to get vaccinated, the CDC said data shows there is also no increased risk for miscarriage linked to receiving a COVID-19 vaccine.
"Miscarriage rates after receiving a COVID-19 vaccine were similar to the expected rate of miscarriage," the CDC said at the time. "Additionally, previous findings from three safety monitoring systems did not find any safety concerns for pregnant people who were vaccinated late in pregnancy or for their babies."
In addition, two studies released last summer found Pfizer and Moderna's COVID-19 vaccines appear to be safe and effective for pregnant people, and were also found to likely offer protection to infants born to a vaccinated person.
In August, the CDC strengthened its recommendation for COVID-19 vaccination during pregnancy, citing new evidence of safety with the vaccines.
The nation's two leading health organizations focused on the care of pregnant people -- American College of Obstetricians and Gynecologists (ACOG) and the Society for Maternal-Fetal Medicine (SMFM) -- also issued new guidelines calling on all pregnant people to get vaccinated against COVID-19.
The World Health Organization (WHO) also says pregnant people can be vaccinated against COVID-19.
"Limited data are currently available to assess the safety of COVID-19 vaccines in pregnancy. However, based on what we know about the kinds of vaccines being used, there is no specific reason for concern," the WHO says on its website. "None of the COVID-19 vaccines authorized to date use live viruses, which are more likely to pose risks during pregnancy."
Both the Pfizer and Moderna vaccines use mRNA technology, which does not enter the nucleus of the cells and doesn't alter the human DNA; instead, it sends a genetic instruction manual that prompts cells to create proteins that look like part of the virus as a way for the body to learn and develop defenses against future infection.
They are the first mRNA vaccines, which are theoretically safe during pregnancy, because they do not contain a live virus.
The Johnson & Johnson vaccine uses an inactivated adenovirus vector, Ad26, that cannot replicate. The Ad26 vector carries a piece of DNA with instructions to make the SARS-CoV-2 spike protein that triggers an immune response.
This same type of vaccine has been authorized for Ebola, and has been studied extensively for other illnesses -- and for how it affects women who are pregnant or breastfeeding.
The CDC has concluded that pregnant people can receive the Johnson & Johnson one-shot vaccine after reviewing more than 200 pages of data provided by the company and the U.S. Food and Drug Administration (FDA).
Vaccine experts interviewed by ABC News said although pregnant women are advised against getting live-attenuated virus vaccines, such as the one for measles, mumps and rubella, because they can pose a theoretical risk of infection to the fetus, the Johnson & Johnson vaccine doesn't contain live virus and should be safe.
The COVID-19 virus has also proven to be more dangerous for pregnant people, especially if they are not vaccinated.
According to the CDC, COVID-19 causes a two-fold risk of admission into intensive care and a 70% increased risk of death for pregnant people.
A study led by researchers in Scotland, and published this month in Nature Medicine, found that unvaccinated pregnant people who contracted COVID-19 not only were at risk of more severe illness themselves, but also were more likely to experience pregnancy loss or preterm birth compared to other women.