Cord blood IgG for respiratory syncytial virus and subsequent infection during the COVID-19 pandemic.

The resurgence of respiratory syncytial virus (RSV) infection among Japanese infants during the coronavirus disease 2019 pandemic might be due to a decrease in cord blood anti-RSV immunoglobulin G (IgG) positivity resulting from reduced maternal RSV exposure. This study examined changes in the positivity before and during the pandemic to clarify the relationship between this positivity and infantile/severe RSV infections. Data from a prospective cohort study conducted in Tokyo, Japan, involving mother-child pairs of infants born between February 2019 and August 2022 were reanalyzed. Cord blood anti-RSV IgG levels measured by enzyme-linked immunosorbent assay were classified as positive, gray zone, and negative. We examined the relationship between antibody positivity and infantile (≤12 months old) and/or severe RSV infections as diagnosed by pediatricians. A total of 319 families participated. There was a significant decrease in cord blood anti-RSV IgG positivity from 65.1% in 2019 to 50.9% in 2022 (P = .02). Among infants followed up until their 2nd birthday, 43 (33.1%) were diagnosed with RSV infections. Of these cases, 5 were infantile/severe and occurred among the 50 infants classified as negative or gray zone, while none were observed among the 80 infants classified as positive (P = .004). Infantile/severe RSV infection was identified in 4 (16.7%) born during the pandemic, representing a significantly higher rate than 1 (0.9%) born before the pandemic (P < .001). This study revealed an association between lower cord blood anti-RSV IgG positivity during the coronavirus disease 2019 pandemic compared to before the pandemic, and an increased risk of subsequent infant/severe RSV infections.