Laboratory procedures
Cord bloods were collected at birth in heparinized tubes, centrifuged and stored at -20°C until testing. IgG against RSV N protein were measured in 1:100 diluted plasma using an in-house enzyme-linked immunosorbent assay (ELISA) performed according to Roux et al.[27] and using recombinant N protein from RSV A long strain and anti-human polyvalent Ig G/A/M peroxidase antibody produced in goat (Sigma-Aldrich®, Saint-Louis, MA, US). In order to establish a standard curve, 8 two-fold dilutions (starting at 1:100) of a positive control (BEI Resources NR-4020, Manassas, VA, US) were used. Negative control was obtained from the IgM/IgG RSV ELISA kit (VIROTECH Diagnostics, Dietzenbach, HE, DE). Titers were calculated using a Michaelis-Menten equation applied to the values of the standard curve and expressed as relative unit (RU)/mL.