High Seroprevalence of Anti-SARS-CoV-2 Antibodies in Children in Vietnam: An Observational, Hospital-Based Study

Background: The robustness of sero-surveillance has delineated the high burden of SARS-CoV-2 infection in children; however, these existing data showed wide variation. This study aimed to identify the serostatus of antibodies against SARS-CoV-2 and associated factors among children following the fourth pandemic wave in Vietnam. Methods: A cross-sectional study was conducted at Vietnam National Children’s Hospital (VNCH) between March 13 and April 3, 2022. Thus, 4032 eligible children seeking medical care for any medical condition not related to acute COVID-19 infection were tested for IgG SARS-CoV-2 antibodies by ADVIA Centaur(®) SARS-CoV-2 IgG (sCOVG) assay using the residuals of routine blood samples. Results: The median age of enrolled children was 39 (IQR = 14–82) months. The overall seropositive prevalence was 59.2% (95%CI = 57.6–60.7) and the median antibody titer was 4.78 (IQR 2.38–9.57) UI/mL. The risk of seropositivity and the median antibody titer were not related to gender (58.6% versus 60.1%, 4.9 versus 4.6 UI/mL, all p > 0.05). Children aged ≤12 months were likely to be seropositive compared to children aged 36 to <60 months (59.2% versus 57.5%, p = 0.49) and those aged ≥144 months (59.2% versus 65.5%, p = 0.16). Children aged ≥144 months exhibited a significantly higher titer of protective COVID-19 antibodies than other age groups (p < 0.001). In multivariate logistic regression, we observed independent factors associated with SARS-CoV-2 seropositivity, including the age 13 to <36 months (OR = 1.29, 95%CI = 1.06–1.56, p = 0.01), 60 to <144 months (OR = 0.79, 95%CI = 0.67–0.95, p = 0.01), ≥144 months (OR = 1.84, 95%CI = 1.21–2.8, p = 0.005), the presence of infected household members (OR = 2.36, 95%CI = 2.06–2.70, p < 0.001), participants from Hanoi (OR = 1.54, 95%CI = 1.34–1.77, p < 0.001), underlying conditions (OR = 0.71, 95%CI = 0.60–0.85, p ≤ 0.001), and using corticosteroids or immunosuppressants (OR = 0.64, 95%CI = 0.48–0.86, p = 0.003). Conclusions: This study highlights a high seroprevalence of antibodies against SARS-CoV-2 among children seeking medical care for non-acute COVID-19-related conditions in a tertiary children’s hospital in Hanoi, Vietnam. In the context of reopening in-person schools and future emerging COVID-19 variants, this point will also be a key message about the necessity of “rush-out” immunization coverage for children, especially those under the age of five years.