Early RSV vaccination in pregnancy boosts antibody transfer to newborns

Early RSV vaccination in pregnancy boosts antibody transfer to newborns | Image Credit: © kitsawet - © kitsawet - stock.adobe.com.

Neonates receive increased maternal antibodies through transplacental transfer when mothers receive respiratory syncytial virus (RSV) vaccination earlier in the approved period of 32- to 36-weeks' gestation, according to a recent study published in the American Journal of Obstetrics & Gynecology.¹

RSV burden in infants and preterm risks

Two percent to 3% of infants aged under 6 months are impacted by RSV, making it the most common source of disease and hospitalization in this population. Preterm infants, comprising 25% of pediatric RSV hospitalizations, face the highest risk of morbidity.² This has led organizations to recommend vaccination in pregnant women to reduce risks.¹

“As fewer than half the… trial participants received the… vaccine during the ultimately approved dosing window of 32 to 36 weeks (1628 of 3568 participants), data are lacking on the impact of timing of maternal vaccination within this window on transplacental transfer of maternal antibody, a correlate of infant protection,” wrote investigators.

Obtaining timing data

The prospective cohort study was conducted to determine the impact of gestational age at vaccination on transplacental transfer of maternal anti-RSV antibody in neonates. Participants included pregnant or 2-months postpartum adult women receiving the Abrysvo RSV vaccine while pregnant.

Vaccines were provided based on standardized clinical care and patient acceptance. Antipertussis antibody transfer from the Tdap vaccine received at under 30 weeks’ gestation was also assessed in vaccinated patients to provide a benchmark of placental transfer efficacy.

Additional measurements for antibody transfer

The cohort also contained 20 maternal-cord dyad samples obtained before the Abrysvo RSV vaccine was available to allow for comparison of RSV antibody responses from natural infection vs vaccination. Venipuncture was used to obtain maternal and umbilical vein blood samples, stored in EDTA tubes.

The former samples were obtained at admission, and the latter at birth. When evaluating antibodies, the cord plasma median fluorescence intensity (MFI) value was divided by the respective maternal plasma MFI value to measure transfer ratios.

Higher antibody levels with vaccination vs natural infection

Significantly increased median antibody levels against the RSV fusion protein (anti-RSV F) were reported in vaccinated patients vs the upper quartile of unvaccinated patients with prior RSV infection. In maternal blood, medians were 5.72 vs 4.82 log₁₀MFI, respectively, compared with 5.81 vs 5.03 log₁₀MFI, respectively, in cord blood.

The unvaccinated group presented with significantly increased anti-G antibody levels vs the vaccinated group, at 3.8 vs 4.2 log₁₀MFI, respectively, in maternal blood and 3.9 vs 4.3 log₁₀MFI, respectively, in cord blood. RSV B displayed similar trends.

Cord blood had significantly higher anti-F A₂ antibody levels vs maternal blood in efficient transplacental transfer, at 5.81 vs 5.72 log₁₀MFI, respectively. By 2 months of age, these levels significantly decreased to 5.35 log₁₀MFI.

Despite this decline, 2-month anti-F A₂ antibody levels were still expected to protect against medically attended lower respiratory tract illness with RSV at 90 days and potentially 180 days. Investigators also noted a similar trend for the RSV B strain.

Timing of vaccination influences transfer efficiency

A significant reduction in cord: maternal transfer ratios was noted in patients receiving vaccination 2 to 3 weeks before delivery vs 5 to 6 weeks, at 0.76 vs 1.35, respectively. This ratio further increased from vaccination more than 6 weeks before delivery, at 1.43.

Similar patterns were reported for vaccination 3 to 4 weeks before delivery vs over 6 weeks, at 0.92 vs 1.43, respectively. This highlighted significantly increased efficacy for antibody transfer to the fetus from vaccination earlier in the approved period of 32- to 36-weeks’ gestation.

“Our results suggest that maternal RSV vaccination at least 5 weeks prior to delivery is associated with optimal RSV-specific IgG transplacental transfer,” wrote investigators.

References:

1. Jasset OJ, Lopez Zapana PA, Bahadir Z, et al. Enhanced placental antibody transfer efficiency with longer interval between maternal respiratory syncytial virus vaccination and birth. Am J Obstet Gynecol. 2025;232:554.e1-15. doi:10.1016/j.ajog.2024.10.053
2. Wang X, Li Y, Shi T, Bont LJ, et al. Global disease burden of and risk factors for acute lower respiratory infections caused by respiratory syncytial virus in preterm infants and young children in 2019: a systematic review and meta-analysis of aggregated and individual participant data. Lancet. 2024;403(10433):1241-1253. doi:10.1016/S0140-6736(24)00138-7