A group from Stanford University School of Medicine, etc. has reported about passive immunity of infants born to mothers infected with SARS-CoV-2.
https://pubmed.ncbi.nlm.nih.gov/33972953/
An important aspect of immunity against infectious pathogens in young infants relies on effective maternal antibody production, transfer of maternal antibodies across the placenta to the fetus, and persistence of passive immunity in the infant. Authors have investigated SARS-CoV-2 antibody transplacental transfer ratios with respect to the timing of maternal infection during gestation, antibody response to SARS-CoV-2 infection in the newborns, and persistence of passively- and actively-acquired SARS-CoV-2 antibodies in infants.
The study enrolled 145 mothers with SARS-CoV-2 infection and 147 of their infants. There was a significant positive correlation between IgG levels in the 125 paired maternal and cord blood samples (Rs=0.93, p<0·0001). Transplacental IgG transfer ratios were calculated in 77 IgG positive mother-infant dyads, and the median transfer ratio was 1.0 (95% CI 0.86-1.09). The transfer ratio was significantly higher in the mothers who were severe-critically symptomatic (n=4) compared to mothers who were asymptomatic (n=23) (1.6 vs. 1.0, p=0.003) or mild-moderately symptomatic (n=50) (1.6 vs. 0.9, p=0.002). The transfer ratios based on time elapsed from the first maternal positive PCR to delivery were 0.6 (<60 days, n=22), 1.2 (60-180 days, n=27), and 0.9 (>180 days, n=5). These studies demonstrate that cross-placental SARS-CoV-2 IgG transfer occurs throughout gestation, and a higher transfer efficiency is achieved when infection onset is more than two months prior to delivery. These findings have important implications in determining optimal timing of vaccination in pregnant mothers and infants.