Introduction
The global outbreak of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in December 2019 has been a worldwide concern. Taiwan, being an isolated island in East Asia, had a low incidence of SARS-CoV-2 infection until 2020.(1, 2) In Taiwan, the pandemic surged in April 2022 with the Omicron variant (BA.2.3.7) being identified as the major lineage.(3)
Pregnant individuals are encouraged to receive the SARS-CoV-2 vaccine because of the higher risk of adverse outcomes, such as death, intubation, admission to an intensive care unit, preterm birth, preeclampsia, venous thromboembolism, and other severe comorbidities.(4-6) The safety of SARS-CoV-2 vaccination during pregnancy has been supported by current evidence.(7, 8)
Newborns have immature immune systems and are vulnerable to infectious diseases. Neonatal passive immunity mainly depends on the maternal concentration of respective antibodies for most viral diseases.(9) Low or early disappearance of maternal-derived antibody concentrations can lead to insufficient protection for newborns. Maternal-derived, vertically transferred immunity provides protection for newborns against vaccine-preventable infectious diseases, such as measles, mumps, and rubella, which cannot be administered to infants before 12 months.(10) Similarly, no SARS-CoV-2 vaccine is currently available for newborns. For the window period before routine vaccination of the newborn, passive immunization for neonates from vertical transmission can be present for up to six months.(11)
Transplacental transfer of SARS-CoV-2 antibodies, specifically against the receptor binding domain (RBD) of the spike protein (anti-S) and nucleocapsid antigen (anti-N), has been observed in previous studies of both infected pregnant individuals and those vaccinated against COVID-19.(12-17) However, the relationship between the timing of vaccination or infection and transplacental antibody transfer is unclear.
Given that newborn protection from infectious diseases depends on antibodies acquired transplacentally,(9) we investigated the timing of transplacental passage of SARS-CoV-2 antibodies through vaccines and maternal infections. We focused on a specific, highly vaccinated pregnant population with or without SARS-CoV-2 infection, solely by the Omicron strain.