Methods
A prospective cohort study was conducted at the National Cheng Kung University Hospital (NCKUH), a medical center in Southern Taiwan, between June 6, 2022 and September 20, 2022. Eligible participants included the following: pregnant women who delivered at the NCKUH; women with or without a primary SARS-CoV-2 infection officially reported to the Central Epidemic Command Center (CECC) of Taiwan; women with cord blood sample available upon delivery and maternal blood sample available within three days from delivery; and participants who provided informed consent. Our study population did not have a second SARS-CoV-2 infection.
The participants were categorized based on the timing of their last vaccination, including during the third trimester (T3), second trimester (T2), and before and during the first trimester (T1). The diagnosis of SARS-CoV-2 was confirmed by real-time polymerase chain reaction (RT-PCR) or antigen testing of respiratory tract specimens, according to the guidelines of the CECC of Taiwan. The study was approved by the Institutional Review Board of NCKUH (IRB: B-ER-111-164), and all the participants provided informed consent.
Clinical and laboratory data were collected using questionnaires and electronic medical records. Cord blood was obtained from the umbilical cord immediately after delivery. Blood samples were centrifuged at 1000 g for 10 min at room temperature, and plasma samples were aliquoted into dedicated pre-coded tubes and stored at –80°C until analysis.
Serological Testing
Severe acute respiratory syndrome coronavirus 2 antibody levels were measured in the maternal and fetal cord plasma using the Elecsys Anti-SARS-CoV-2 and Anti-SARS-CoV-2 S kits (Roche Diagnostics, Switzerland). Immunoglobulin M and G antibodies against the nucleocapsid (anti-N) and receptor-binding domain of the spike (anti-S) of SARS-CoV-2 were detected. The cut-off for positivity was ≥1.0 or the anti-N assay and ≥0.8 U/mL for the anti-S_RBD assay. The value of the anti-S_RBD assay between 0.4–250 U/mL represents the linear range, and samples >250 U/mL were diluted with a Diluent Universal provided by the kit. The cPassTM surrogate virus neutralization test (GenScript Biotech Corp., USA) was conducted to measure neutralizing antibody (nAb) titers against different SARS-CoV-2 variants. Ninety-six-well plates coated with recombinant hACE2 and recombinant S-RBD from Wuhan and Omicron BA.2 variants were used in the experiments, according to the manufacturer’s instructions. An inhibition percentage ≥30% was considered seropositive for SARS-CoV-2 nAbs. Additionally, the percent inhibition of the Wuhan strain was converted to IU/mL using a previously proposed formula.(18)