Pregnant women are advised to get vaccinated against respiratory syncytial virus (RSV) to protect both themselves and their newborns, who are too young to be vaccinated. Current guidelines recommend getting the shot between 32 and 36 weeks of pregnancy, but is there an optimal timing within this slot that could further boost newborn protection?

RSV is a common respiratory virus that can cause serious infections in older adults and young children. The U.S Center for Disease Control and Prevention (CDC) recommends protecting infants from severe RSV with either a maternal vaccine during pregnancy or an antibody dose for the baby.

Researchers of a recent study found that getting vaccinated earlier in the recommended timeframe, closer to 32 weeks, may offer the best protection for newborns against RSV.

In an earlier research by the same team, the investigators found that the timing of COVID vaccination during pregnancy could affect the immune responses in the mother and transplacental antibody transfer to the fetus. The latest study tested how the timing of getting the RSV vaccine affected 124 pregnant women who received the shots between 32–36 weeks of pregnancy. To understand the level of protection in infants, the researchers also tested the antibodies in the umbilical codes of 29 infants born to these mothers.

"The investigators found that maternal RSV vaccination at least 5 weeks prior to delivery led to the most efficient transfer of maternal antibodies across the placenta to the newborn, compared with maternal vaccination at 2-3 or 3-4 weeks prior to delivery," the news release stated.

When comparing antibodies in maternal and cord blood after RSV vaccination to those of unvaccinated mothers, researchers found that vaccination led to significantly higher and longer-lasting RSV antibody levels in both mothers and their newborns.

"This work provides much-needed data to guide physicians in counseling patients about RSV vaccine timing during pregnancy. Our findings suggest that being vaccinated earlier within the approved timeframe allows for the most efficient placental transfer of antibodies to the newborn. They also may have implications for when the RSV monoclonal antibody, Nirsevimab, should be administered to newborns. Similar research should be conducted for other vaccines administered during pregnancy," said senior author Dr. Andrea Edlow in a news release.

Researchers say more studies are needed to find the minimum antibody levels needed to protect infants from RSV and to understand the added protection from breast milk of vaccinated mothers.

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