The dynamics of humoral and cellular immunity in recovered children and adolescents
SARS-CoV-2 specific humoral immunity was found to decay over time, but not T cell immunity (Fig. 3). Linear regression analysis showed that the level of SARS-CoV-2 RBD IgG was significantly associated with recovery time (p=6.31e-07, R2=0.5808) (Fig. 3A), but not with the specific CD4+ (p=0.783) or CD8+ (p=0.915) T cell responses (Fig. 3B). SARS-CoV-2 RBD IgG had a fast decay rate (-0.0377 anti-RBD IgG ratio/day) while CD4+ (-0.0022%/day) and CD8+ (-0.0001%/day) T cell responses persist over time, including the patient with the longest follow-up time at 219 days who had undetectable anti-RBD IgG but persistent SARS-CoV-2 specific CD4+ and CD8+ T-cell response. The average SARS-CoV-2 RBD IgG half-life (t1/2) decay was 121.6 days, and the presence of antibodies was estimated to last for 237.7 days or 7.9 months. The same estimation was not applicable to CD4+ and CD8+ T cell responses because of the lack of association with time.
Age is a factor associated with the measured RBD IgG level and T cell activation magnitudes in recoveredchildren and adolescents
Fifteen patients were younger than 12 years and 16 patients were 12 years or older. The results demonstrated differences in the immune responses to SARS-CoV-2 between older and younger children. In comparison to children older than 12 years, the younger patients had a significantly higher level of SARS-CoV-2 RBD IgG ratio (p=0.041) (Fig 4A). While the frequency of CD4+ T cells reactive to mixed M, N and S peptide pool was similar between the age groups (Cohen’s d=0.071) (Fig 4B(i)), the frequency of S-peptide specific CD4+ T cells was higher in younger children (Cohen’s d=0.3058) (Fig 4B(ii)). Correlative analysis showed that the four patients with highest level of anti-RBD IgG and S-peptide specific CD4+ T cells were all from the younger age group (Fig 4D(i)). In contrast, no difference was observed between the two age groups in SARS-CoV-2 S-reactive CD8+ T cells (Cohen’s d=0.03164) (Fig 4C and 4D(ii)).