CD40LG and susceptibility to pneumonia measurement: COVID-19-specific immunoglobulin M (IgM) levels were considerably higher in children with pneumonia (147.6 ± 75.8 IU/mL) compared to those without (10.9 ± 52.1 IU/mL, p < 0.001), indicating a more recent infection; the levels of COVID-19-specific immunoglobulin G (IgG) did not differ significantly (p = 0.255).