Rate of Vertical Transmission of Coronavirus Disease 2019 and Effect of Maternal Vaccination: Evidence from a Multicentre Prospective Cohort Study in Japan

Background: Coronavirus disease 2019 (COVID-19) has been a serious pandemic. Vertical transmission of COVID-19 has been reported, but the transmission rate is unclear, and the clinical features of congenital and neonatal COVID-19 are poorly understood. Maternal vaccination against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has been recommended, but the efficacy of maternally transferred antibodies is unknown.Methods: This multicentre, prospective observational study included a comprehensive testing protocol with appropriate precautions. Mothers and their infants were enrolled and classified into an active infection group (mothers with COVID-19 at delivery), a post-infection group (mothers recovered from COVID-19 during pregnancy), and a vaccination group (mothers vaccinated during pregnancy). Specimens were collected and clinical examinations were performed for 6 months starting at delivery. Polymerase chain reaction assays, anti-SARS-CoV-2 antibody measurements, and clinical examinations were conducted.Findings: Sixty-nine mothers and 70 infants were enrolled. SARS-CoV-2 was detected in the blood, amniotic fluid, placenta, and breast milk of mothers in the active infection group and in the blood, placenta, and vagina in mothers in the post-infection group. Neither SARS-CoV-2 nor IgM was detected in any specimen from infants. No anomalies, functional disorders, or developmental problems were observed in the first 6 months in infants. Compared with the post-infection group, there was a significant increase in maternally transferred antibody titres in the vaccination group according to number of vaccinations, which was maintained for longer than the first month of life.Interpretation: Vertical transmission of COVID-19 was not observed despite a comprehensive testing protocol. SARS-CoV-2 was detected more often in the placenta, which appeared to prevent vertical transmission. Maternal vaccination increased maternally transferred antibody titres in infants and was considered immunologically efficient. Vaccination during pregnancy is very efficient in preventing congenital, neonatal, and infantile COVID-19.Funding: Kawano Masanori Memorial Public Interest Incorporated Foundation for Promotion of Pediatrics.Declaration of Interest: The authors have no conflicts of interest relevant to this article to disclose.Ethical Approval: This study had a multicentre, prospective observational design and was approved by the ethical committees of eight participating institutions and hospitals in metropolitan areas in Japan (National Center of Global Health and Medicine, Tokyo Metropolitan Children's Medical Center, Tokyo Metropolitan Tama Medical Center, Toho University Omori Medical Center, Koshigaya Municipal Hospital, The University of Tokyo Hospital, Nihon University School of Medicine, and Tokyo Metropolitan Toshima Hospital). Mothers and their babies were enrolled between April 2020 and March 2023 after maternal informed consent was obtained.