Editor’s Note: This month’s article was guest authored by Dr. Barbara Kuter, a visiting scientist at the Vaccine Education Center. We would like to thank her for addressing these timely and important issues related to COVID-19 vaccinations. We hope you find the article to be helpful as you address COVID-19-related concerns.

This article was updated on Dec. 15, 2022.

As COVID-19 vaccines are new, we continue to learn more about them, particularly in some subpopulations of individuals. Unfortunately, while studies are being completed, misinformation and unfounded theories abound. For example, in April 2021 a daycare in Miami refused to allow their employees to get vaccinated based on misperceptions that COVID-19-vaccinated adults could cause infertility in the children for whom they care. So, let’s take a look at what is known about COVID-19 vaccines and menstruation, fertility, pregnancy and breastfeeding.

COVID-19 vaccines and menstrual periods

  • Some women have taken to social media to share personal stories of changes in their menstrual cycles after receipt of the COVID-19 vaccine, reporting heavier bleeding or changes in the timing of onset of their periods. As summarized by Victoria Male (Science, Nov. 17, 2022), several studies evaluating whether menstrual cycles are affected by COVID-19 vaccination found that cycle length increased, but the average increase was less than one day following either the first or second dose. If the doses were given in the same cycle, the length was increased by about two days. In each case, the effects resolved within two cycles. The author pointed out that increases have been found after receipt of other vaccines as well, suggesting that the changes are not specific to the vaccine but caused by something else. Theories about the cause relate to immune responses triggering the changes, such as altering hormone levels or affecting specific immune cell types that also regulate the uterine lining.
  • In a small study of 177 unvaccinated women of childbearing age with clinically diagnosed COVID-19 (Li K., Chen G., Hou H., et al. Analysis of sex hormones and menstruation in COVID-19 women of child-bearing age. RBMO. 2021 Jan;42(1):260-267), 25% of the women reported changes in the volume of their periods, 20% reported lighter than usual periods, and 19% reported a longer than usual cycle during their illness. Those with multisystem involvement were more likely to experience these changes. The effects resolved within one to two months after recovery, and sex hormone levels during illness did not differ from those of healthy age-matched controls. Given that menstruation is controlled by the ovaries and is subject to a variety of external factors, the authors concluded that the transient effects of infection were the likely cause of these temporary changes.

The bottom line is that there is no reason for a woman to delay or schedule vaccination around her period.

COVID-19 vaccines and fertility

The ability of COVID-19 vaccines to affect the fertility of both women and men has also been questioned on social media, but these concerns are not substantiated by the available data. First, reproductive developmental studies conducted in animals for each of the currently authorized COVID-19 vaccines showed no impact on fertility. For example, preclinical animal studies found that animals that received the vaccine were as likely to become pregnant, deliver similarly sized litters, and have healthy pups.

Second, several lines of evidence in people also support the safety of COVID-19 vaccines when it comes to fertility. The arguments against COVID-19 vaccines affecting fertility were addressed by VEC Director, Dr. Paul Offit, in a May 10 op-ed published in The Hill:

  • The fear of fertility issues after receipt of a COVID-19 vaccine was born of the false notion that the spike protein on SARS-CoV-2, the virus that causes COVID-19, was similar to a protein on the surface of placental cells, called syncytin-1. It was suggested that an immune response against the spike protein might also cause an immune response against syncytin-1. But these two proteins aren't really that similar; they share only a small stretch of amino acids. As such, Dr. Offit wrote, it’s “the equivalent of saying that two people share the same social security number because both contain the number six.”
  • Natural infection with SARS-CoV-2 induces an immune response against the spike protein, so if the protein homology related to syncytin-1 was an issue, we would have been likely to see a decrease in birth rates given that about 100 million people in the U.S. have been naturally infected with the SARS-CoV-2 virus. If either female or male fertility were affected by COVID-19, fewer pregnancies would be expected. However, while the overall U.S. birth rate went down, rates in certain subgroups increased between 2019 and 2020. Overall decreases followed a several year trend that is complicated by economic and other factors.
  • During the phase 3 trials of the two FDA-authorized mRNA vaccines, about three dozen women became pregnant. If the vaccine affected fertility, it would be expected that all or most of those pregnancies would have occurred in the placebo group, but the pregnancies occurred equally among vaccinated and unvaccinated individuals.

Finally, these vaccines are processed near the injection site and do not cause hormonal or other biological changes that would be expected to affect either male or female fertility.

The bottom line is that no data indicate that male or female fertility is affected by COVID-19 disease or vaccination.

COVID-19 vaccines and pregnancy 

Is it safe for a woman to get the COVID-19 vaccine when pregnant? While the data were originally limited, both the Centers for Disease Control and Prevention (CDC) and the American College of Obstetricians and Gynecologists (ACOG) recommended that pregnant women have access to the vaccine. Since the vaccines have been approved for use, more data have emerged that reinforce this recommendation. Here’s what we know:

  • COVID-19 infection during pregnancy can result in more severe disease compared with nonpregnant individuals of the same age. According to the CDC, pregnant women with COVID-19 are at increased risk of hospitalization, admission to the ICU, requirement for mechanical ventilation and death. Likewise, COVID-19-infected pregnant women may also be more likely to experience preterm birth or other adverse birth outcomes.
  • In the clinical trials for vaccines currently authorized for use in the U.S., women who became pregnant were no more likely to suffer adverse pregnancy outcomes than those in the unvaccinated group.
  • Once COVID-19 vaccines were authorized for use in the U.S., a safety monitoring system, called v-safe, was implemented to follow vaccinated individuals, including pregnant women. A v-safe pregnancy registry was established to specifically gather information on the health of pregnant women who were vaccinated. Early data were recently published by Shimabukuro and colleagues (Preliminary findings of mRNA Covid-19 vaccine safety in pregnant persons. CDC v-safe COVID-19 Pregnancy Registry Team. N Engl J Med. 2021 June 17). Among 3,958 participants, 827 had a completed pregnancy, with 115 (13.9%) resulting in a pregnancy loss and 712 (86.1%) resulting in a live birth; this rate of loss is what would be expected in an unvaccinated group. Likewise, neonatal outcomes, including preterm birth and small for gestational age, were also found at rates similar to background.

The bottom line is that data and experiences do not suggest reason for concern when receiving the COVID-19 vaccine during pregnancy.

COVID-19 vaccines and breastfeeding

Every mother’s motto is, “I only want what’s best for my child.” As such, the idea of getting any vaccine while breastfeeding may be concerning. Some moms may be concerned about the vaccine affecting their baby, themselves, their ability to produce breast milk, or all of these. While the clinical trials for currently approved COVID-19 vaccines did not include lactating women, the CDC, ACOG, and the Academy of Breastfeeding Medicine recommend that lactating women can receive the vaccine and that breastfeeding should not be stopped around the period of vaccination. Here’s why:

  • Neither mRNA nor the adenovirus-based vaccines contain live virus, so they do not replicate in the body. Based on how these vaccines are processed, there would be little reason to believe they would negatively affect lactating women or their breastfeeding babies.
  • A small study in Israel by Perl and colleagues (SARS-CoV-2-Specific Antibodies in Breast Milk After COVID-19 Vaccination of Breastfeeding Women. JAMA. 2021 May 18;325(19);2013-2014. DOI:10.1001/jama.2021.5782) evaluated breast milk antibodies and maternal and infant health during the study period.  The vaccine was well-tolerated and breast milk antibodies were detected. No serious adverse events were reported among mothers or their infants. Pain at the injection site was the most common complaint among the vaccinated moms.
  • A study by Pace and colleagues (Characterization of SARS-CoV-2 RNA, Antibodies, and Neutralizing Capacity in Milk Produced by Women with COVID-19. Clinical Science and Epidemiology. 2021 Feb 9;12(1):e03192-20) also showed that women naturally infected with COVID-19 did not transmit the virus via breast milk, but they did transmit neutralizing antibodies. Another study, by Shlomai and colleagues (Neonatal SARS-CoV-2 Infections in Breastfeeding Mothers. Pediatrics, 2021 May;147(5), e2020010918), showed that when newborns were separated from their SARS-CoV-2-infected mothers after birth but fed unpasteurized breast milk, they did not become infected, providing further evidence that the virus is not transmitted through breast milk. More information is needed to determine whether antibodies transmitted through breast milk from their mothers offer protection to breastfed infants.

The bottom line is that breastfeeding moms can safely be vaccinated against COVID-19. More information is needed to determine whether vaccine- or disease-induced antibodies transmitted in breast milk can protect breastfed infants.

Materials in this section are updated as new information and vaccines become available. The Vaccine Education Center staff regularly reviews materials for accuracy.

You should not consider the information in this site to be specific, professional medical advice for your personal health or for your family's personal health. You should not use it to replace any relationship with a physician or other qualified healthcare professional. For medical concerns, including decisions about vaccinations, medications and other treatments, you should always consult your physician or, in serious cases, seek immediate assistance from emergency personnel.