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.