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Getting the Covid vaccine or booster shot in pregnancy is one of the most important steps you can take to protect your baby and yourself from serious illness. That’s for a couple reasons: Pregnancy is considered a high-risk state for Covid infection; and there is no plan to create a vaccine for infants under 6 months of age. (And, at the time of writing, the vaccine authorization for kids ages 6 months to 4 years has been delayed yet again.)
Vaccination in pregnancy can offer protection against Covid for both mother and baby—no matter when it’s received. Both the Centers for Disease Control and Prevention (CDC) and The American College of Obstetricians and Gynecologists (ACOG) stress that pregnant women get vaccinated as soon as possible, i.e., right now.
But new research may provide more information on when the placenta most efficiently transfers vaccine-provided antibody protection to the developing baby.
Research shows vaccination in pregnancy leads to strong antibody protection in babies
A recent study published in The Journal of the American Medical Association (JAMA) showed that vaccination with two doses of an mRNA vaccine during pregnancy resulted in antibody levels in infants that lasted until they were 6 months old at much higher rates than in babies born to unvaccinated, Covid-infected mothers.
The study, from Massachusetts General Hospital (MGH), looked at women who were either vaccinated with two doses of an mRNA vaccine or infected with the virus between week 20 to 32 of pregnancy.
They found that antibody levels (titers) were higher in the cord blood of vaccinated mothers at delivery. At 2 months of age, 98% of the infants born had detectable IgG antibody levels in their blood. At 6 months, 57% of the infants still had detectable levels of IgG, as compared with just 8% of infants born to unvaccinated, infected mothers.
A recent CDC study found that babies under 6 months old born to mothers who had received two doses of an mRNA vaccine during pregnancy were 61% less likely to be hospitalized due to Covid infection than those born to mothers who were not vaccinated.
"While it's still unclear just how high the titer needs to be to completely protect an infant from COVID, we know anti-spike IgG levels correlate with protection from serious illness," says Andrea Edlow, MD, MSc, a Maternal-Fetal Medicine specialist at MGH, director of the Edlow Lab in the Vincent Center for Reproductive Biology and co-senior author of the JAMA study, to ScienceDaily. "The durability of the antibody response here shows vaccination not only provides lasting protection for mothers but also antibodies that persist in a majority of infants to at least six months of age.”
But when those vaccinations were administered also matters.
When should you get the Covid vaccine or booster in pregnancy?
A 2021 study published in Obstetrics and Gynecology on 1,359 vaccinated pregnant women found that while antibody levels were detected at delivery regardless of vaccination timing, those vaccinated in the early third trimester had slightly higher antibody levels both in maternal blood and umbilical cord blood. Those who had only received one shot before delivery had the lowest levels of antibodies—though they did still have detectable levels.
Completing a full round of vaccination at any time during pregnancy before giving birth is the most important factor. But if you’re already fully vaccinated before becoming pregnant and looking to time your Covid booster shot in pregnancy to offer the most benefits to your infant once born, it seems that weeks 20 to 32 of pregnancy are when Shook, et al. (2022) pinpoint as an optimal time to receive the vaccine in pregnancy.
That equates to between the end of the second trimester and the beginning of the third trimester. In that window, antibody transfer via the placenta seems to reach peak levels.
“Individuals vaccinated or infected at 20 to 32 weeks’ gestation were enrolled because previous studies have demonstrated superior transplacental transfer of antibodies during this window compared with vaccination closer to delivery,” Shook, et al. (2022) write.
What if you’re pregnant and already had Covid?
Previous Covid infection conferred some benefit when combined with vaccination. Yang, et al. (2021) showed that antibody levels in women who had recovered from a previous Covid infection and had been vaccinated in early pregnancy were comparable to those who were uninfected but received the vaccine in the early third trimester.
Those who were eligible for a booster and received one in the third trimester had the highest antibody levels of all groups.
A note from Motherly
The bottom line? It’s important to receive both doses of the mRNA vaccine in pregnancy before giving birth in order to pass on antibody protection to your infant, and it’s best to do so sooner than later. If you have the option to time your booster dose, try to do so between weeks 20 to 32, when placental transfer is most effective.
"Pregnant women are at extremely high risk for serious complications from COVID," says Galit Alter, PhD, core member of the Ragon Institute of MGH, MIT and Harvard, and co-senior author of the JAMA study, to ScienceDaily. "And given the lag in development of COVID-19 vaccines for infants, these data should motivate mothers to get vaccinated and even boosted during pregnancy to empower their babies' defenses against COVID."
Shook LL, Atyeo CG, Yonker LM, et al. Durability of Anti-Spike Antibodies in Infants After Maternal COVID-19 Vaccination or Natural Infection. JAMA. Published online February 07, 2022. doi:10.1001/jama.2022.1206
Yang YJ, Murphy EA, Singh S, Sukhu AC, Wolfe I, Adurty S, Eng D, Yee J, Mohammed I, Zhao Z, Riley LE. Association of Gestational Age at Coronavirus Disease 2019 (COVID-19) Vaccination, History of Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) Infection, and a Vaccine Booster Dose With Maternal and Umbilical Cord Antibody Levels at Delivery. Obstetrics & Gynecology. 2021 Dec 28:10-97.
This post has been medically reviewed by Dr. Sarah Bjorkman, OB/GYN.