A single dose of maternal investigational Respiratory Syncytial Virus Vaccine (RSVPreF3) given during the late 2nd  or 3rd trimester of pregnancy was well-tolerated and exhibited an acceptable safety profile with respect to pregnancy-associated or neonatal adverse events or pregnancy outcomes.

This phase 2 randomized trial explored the safety and immunogenicity of RSVPreF3 in mothers and their infants. The RSV-particular antibody levels and neutralizing antibody (nAb) titers in babies born to vaccinated mothers were also assessed.

The safety of RSVPreF3 dosed 60 µg or RSVPreF3 dosed 120 µg was assessed during the late second or third trimester in a total of 213 healthy pregnant women aged 18 to 40 years. They were examined through 6 months’ post-partum and their offspring through initial months. The immunogenicity was assessed through day 43 post-partum and day 181 after birth.

RSVPreF3 was found to be acceptable as none of the severe adverse events for pregnant women or infants were associated with the use of vaccine under consideration. The main results were apparent in the 60 µg RSVPreF3 and 120 µg RSVPreF3 groups:

• There was an increase in nAb titers in mothers against RSV-A and RSV-B a month after vaccine administration. One month postvaccination, nAb titers in mothers raised 12.7- and 14.9-fold against RSV-A and 10.6- and 13.2-fold against RSV-B, respectively. At day 43, the nAb titers after the delivery continued to be 8.9 to 10.0 fold over the pre-vaccination levels (Figure 1):

• nAb titers were constantly higher than placebo
• In the 60 and 120 µg RSVPreF3 groups, the placental transfer ratios for anti-RSVPreF3 antibodies at birth were 1.62 and 1.90 respectively
• Babies had the maximum levels of nAb at birth and deteriorated through day 181 after the birth

RSVPreF3 maternal vaccination was well-tolerated and had an acceptable safety risk profile. It encouraged robust RSV-specific immune responses with positive antibody transfer to the infants from vaccinated mothers.