The course of COVID-19 in school-aged children depends on the state of humoral immunity against diphtheria

Introduction: There is evidence that high titers of diphtheria antibodies are more often associated with an asymptomatic or less se- vere course of COVID-19. The aim of the study was to determine the features of the COVID-19 course in school-aged children depend- ing on the state of postvaccination immunity against diphtheria toxin. Methods. In total, 90 children aged 6 to 18 years were examined: 60 patients with confirmed SARS-CoV-2 infection and 30 children without signs of the disease (control group). There were 20 patients with a mild COVID-19 course, 31 patients with moderate disease severity and 9 patients with severe disease severity. All children were tested for the level of immunoglobulin G (IgG) against diphtheria toxin by immune-enzymatic analysis. Results. Total serum immunoglobulin G against diphtheria toxin was 0.431 (0.113; 0.828) IU/mL. Antitoxic immunoglobulin G was 2.5 times higher in children in the control group than in patients with SARS-CoV-2 infection. A decreased level of immunoglobulin G was associated with increased disease severity. In children with specific IgG 0.01-0.2 IU/mL revaccination is recommended; therefore, 4.2% of the children in the control group, 6.8% with a mild COVID-19 course, 50% with moderate disease severity and 100% of chil- dren with severe COVID-19 needed a booster dose of diphtheria toxoid. A moderate negative correlation was observed between the level of immunoglobulin G against diphtheria toxin and ESR (r=-0.38, p=0.004), CRP (r=-0.32, p=0.021), D-dimer (r =-0.35, p=0.009), duration of hyperthermia (r=-0.49, p=0.003), and duration of treatment (r=-0.43, p=0.012). Conclusions. Children with manifested SARS-CoV-2 infection had significantly lower levels of immunoglobulin G against diphtheria toxin compared to the control group, indicating a lower level of humoral immunity. Children with a mild course of COVID-19 had a significantly higher level of specific IgG against diphtheria toxin than those with a moderate and severe disease course.

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