Maternal vaccination during pregnancy is likely the most effective way to protect infants.
Two large studies from Australia and the United States have added their weight to one of modern medicine's quieter but consequential questions: whether vaccinating a pregnant woman protects not only herself but the child she carries. The answer emerging from nearly 86,000 pregnancies is that it does — safely, and with measurable benefit extending into the earliest and most vulnerable months of an infant's life. In a world where newborns arrive immunologically unfinished, maternal vaccination may be among the most effective gifts a healthcare system can offer before a child draws its first breath.
- A lingering pandemic-era doubt — whether vaccines given during pregnancy could harm mother or baby — has now been tested against real-world data from tens of thousands of pregnancies, and the harm did not appear.
- The Australian finding is particularly pointed: giving flu and pertussis vaccines on the same day caused no increase in preterm birth, stillbirth, low birth weight, or any other adverse outcome compared to spacing them apart.
- The American data revealed a timing effect with clinical stakes — third-trimester COVID vaccination cut infant hospitalization risk by 65 percent, while vaccination before pregnancy offered almost no protection to the newborn at all.
- The mechanism is biological and time-sensitive: maternal antibodies cross the placenta and fade within six months, meaning the closer to birth a mother is vaccinated, the stronger the shield her infant inherits.
- Both research teams concluded their findings should give clinicians greater confidence to recommend pregnancy vaccination — a shift that could lift immunization rates among a group historically hesitant due to safety concerns.
Two studies published within weeks of each other have addressed a question that has shadowed maternal healthcare since the pandemic: is it safe to vaccinate pregnant women, and does it actually protect their babies? Drawing on data from Australia and the United States, both answer yes.
The Australian research focused on a practical clinical question — what happens when a pregnant woman receives flu and pertussis vaccines at the same appointment? Using data from nearly 7,000 women who received both shots simultaneously, researchers compared outcomes against women who received only one vaccine or spaced them across visits. The theoretical concern was that a combined immune response might trigger fever or other effects harmful to the pregnancy. It did not. Rates of preterm birth, low birth weight, stillbirth, and neonatal complications were statistically identical across groups — the first time such a direct comparison had been formally measured.
The American study tracked nearly 79,000 mothers and infants born between 2021 and 2023, asking not just whether COVID vaccination during pregnancy was protective, but when it mattered most. Of 3,648 infants who contracted COVID in their first six months, the data — adjusted for maternal age, weight, diabetes, breastfeeding, and other variables — showed that third-trimester vaccination reduced infant infection risk by around 19 percent compared to unvaccinated mothers, and by nearly 24 percent compared to mothers vaccinated before conception. For hospitalization, the effect was sharper still: third-trimester vaccination was associated with a 65 percent reduction in risk.
The reason timing matters is biological. Antibodies produced during pregnancy cross the placenta and circulate in the newborn's blood for the first months of life — a temporary but meaningful shield during a period when the infant's own immune system is still developing. Vaccination earlier in pregnancy, or before conception, means fewer fresh antibodies are available at birth. By six months, the maternal antibodies have largely faded regardless.
For clinicians, the studies offer something practical: a reduction in uncertainty. Both research teams concluded that maternal vaccination during pregnancy — not before, and not deferred — is likely the most effective strategy available for protecting infants who are too young to be vaccinated themselves.
Two new studies arriving within weeks of each other have settled a question that has lingered since the pandemic: whether vaccinating pregnant women against common infectious diseases is safe for both mother and baby. The answer, according to rigorous data from Australia and the United States, is yes—and the protection extends to newborns who cannot yet be vaccinated themselves.
The Australian research examined what happens when a pregnant woman receives two vaccines on the same day. Researchers pulled data on nearly 7,000 women who got flu and pertussis shots during pregnancy at the same visit, then compared them to an equal number of women who received only the pertussis vaccine or spread the two shots across different days. The concern, in theory, was straightforward: two vaccines at once might trigger a stronger immune response, including fever, that could harm the pregnancy or the developing baby. The data showed no such harm. Rates of preterm birth, small babies, and low birth weight were essentially identical between the two groups. The researchers noted this was the first time anyone had formally measured whether getting both shots together affected outcomes like premature rupture of membranes, preterm labor, stillbirth, bleeding after delivery, and low Apgar scores in newborns. None of these complications occurred at different rates.
The American study took a different angle, tracking nearly 79,000 mothers and their infants born between mid-2021 and mid-2023. The researchers wanted to know when maternal COVID vaccination offered the most protection. They sorted mothers into groups based on timing: vaccinated during pregnancy, vaccinated in the three months before pregnancy, vaccinated six to twelve months before, and so on. Among the infants in the study, 3,648 tested positive for COVID in their first six months of life. The pattern was striking. Babies born to unvaccinated mothers accounted for 943 of those cases. Babies born to mothers vaccinated during pregnancy accounted for 1,861 cases—but this larger absolute number reflected the much larger group of vaccinated mothers in the study. When the researchers adjusted for all the variables that might affect infection risk—maternal age, race, weight, diabetes, socioeconomic status, and whether the baby was breastfed—vaccination during pregnancy, particularly in the third trimester, showed real protective power. Third-trimester vaccination reduced the risk of infant COVID infection by roughly 19 percent compared to unvaccinated mothers, and by nearly 24 percent compared to mothers vaccinated before becoming pregnant.
The protection against severe disease was even more pronounced. Only 76 infants required hospitalization for COVID. Of those, 33 were born to unvaccinated mothers. When researchers calculated the effectiveness of maternal vaccination at preventing hospitalization, third-trimester vaccination showed a 65 percent reduction in risk compared to no vaccination. Vaccination at any point during pregnancy reduced hospitalization risk by 53 percent. Vaccination before pregnancy, by contrast, offered almost no protection against infant infection or hospitalization.
The timing matters because of how maternal antibodies work. When a woman is vaccinated during pregnancy, her immune system produces antibodies that cross the placenta and are present in high concentrations at birth. These antibodies circulate in the newborn's bloodstream for the first months of life, offering a temporary shield against infection. By the time a baby is six months old, these maternal antibodies have largely faded. Vaccination earlier in pregnancy or before conception means fewer fresh antibodies are available at the moment of birth.
For clinicians, these findings remove a layer of uncertainty. The Australian researchers wrote that their evidence "may help clinicians more confidently recommend pregnancy vaccination concomitantly and potentially provide more opportunities to optimise uptake." The American team concluded that because infants under six months cannot be vaccinated and pre-pregnancy vaccination does not appear protective, "maternal vaccination during pregnancy is likely the most effective way to protect infants against covid." The studies suggest that the theoretical risks of maternal vaccination—risks that have circulated in conversations since the pandemic began—do not materialize in practice. What does materialize is protection: for the pregnant woman herself, and for the infant in those vulnerable first months of life when the immune system is still learning to recognize threats.
Citas Notables
The evidence presented may help clinicians more confidently recommend pregnancy vaccination concomitantly and potentially provide more opportunities to optimise uptake.— Australian researchers, JAMA Network Open
Because infants aged less than six months are too young to be vaccinated themselves and pre-pregnancy maternal vaccination does not appear to be protective, maternal vaccination during pregnancy is likely the most effective way to protect infants against covid.— American researchers, Pediatrics
La Conversación del Hearth Otra perspectiva de la historia
Why did researchers need to study what happens when two vaccines are given on the same day? Isn't that already standard practice?
It should be, but the pandemic created doubt. People started asking whether combining vaccines might overwhelm a pregnant woman's immune system and cause harm. No one had formally measured it before. The Australian team wanted to close that gap with real data.
And they found nothing to worry about?
Correct. Preterm birth, small babies, low birth weight—all the outcomes they tracked—occurred at the same rates whether women got one vaccine or two on the same day. The researchers were even able to measure things no one had looked at before, like whether the vaccines affected the placental membranes or caused bleeding after delivery.
The American study seems to say timing matters enormously. Why is third trimester so much better than vaccination before pregnancy?
Because of antibodies. When you vaccinate during pregnancy, the mother's immune system produces fresh antibodies that cross the placenta and are present in high concentration when the baby is born. Those antibodies protect the newborn for the first months of life. If you vaccinate before pregnancy, those antibodies have already started to fade by the time the baby arrives.
So a woman vaccinated a year before getting pregnant offers her newborn almost no protection?
That's what the data showed. The protection was negligible. But vaccinate her in the third trimester, and her newborn has meaningful defense against infection and especially against hospitalization.
What changes now that these studies exist?
Clinicians can recommend pregnancy vaccination with more confidence. They have evidence, not theory. And that confidence matters—it translates into more women actually getting vaccinated, which means more newborns protected.