Introduction: There is evidence that high titers of diphtheria antibodies are more often associated with an asymptomatic or less severe course of COVID-19. The aim of the study was to determine the features of the COVID-19 course in school-aged children depending 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 children 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.

Abstract
Lyme disease is an infectious disease caused by bacteria of the Borrelia burgdorferi sensu lato (Bbsl) complex and is characterized by predominant lesions of the skin, cardiovascular system, nervous system and musculoskeletal system. We have described a clinical case of first- degree atrioventricular block in a 14-year-old boy caused by Lyme borreliosis. The disease started with the manifestations of cardiovascular system involvement. The patient and his parents did not recall observing a tick bite or manifestation of erythema migrans (EM). The boy was prescribed doxycycline. Three weeks after antibiotic therapy a second ECG examination was performed and showed no abnormalities.