Evaluation of Clinical and Immune Responses in Recovered Children with Mild COVID-19

Abstract

The coronavirus disease 2019 (COVID-19) has spread globally and variants continue to emerge, with children are accounting for a growing share of COVID-19 cases. However, the establishment of immune memory and the long-term health consequences in asymptomatic or mildly symptomatic children after severe acute respiratory syndrome coronavirus 2 infection are not fully understood. We collected clinical data and whole blood samples from discharged children for 6-8 months after symptom onset among 0-to-14-year-old children. Representative inflammation signs returned to normal in all age ranges. The infants and young children (0-4 years old) had lung lesions that persisted for 6-8 months and were less responsive for antigen-specific IgG secretion. In the 5-to-14-year-old group, lung imaging abnormalities gradually recovered, and the IgG-specific antibody response was strongest. In addition, we found a robust IgM⁺ memory B cell response in all age. Memory T cells specific for the spike or nucleocapsid protein were generated, with no significant difference in IFN-γ response among all ages. Our study highlights that although lung lesions caused by COVID-19 can last for at least 6-8 months in infants and young children, most children have detectable residual neutralizing antibodies and specific cellular immune responses at this stage.

Keywords: SARS-CoV-2; clinical; immune responses; mild COVID-19; recovered children.

Figure 1

SARS-CoV-2-specific antibody response in children recovered for 6–8 months: (A) The absolute positive numbers of individuals with anti-S IgG and anti-N IgG antibody titers of un-detected (N), 1:200 (low), 1:400 (moderate), and 1:800 (high). (B) Neutralizing antibody titers were determined according to different dilutions. (C,D) Differences in antibody levels between different age groups. (E) Correlations between different antibodies and the correlation between antibodies over time. Red indicates a positive correlation, blue indicates a negative correlation, and transparency represents a change in the p value; the greater the p value is, the greater the line transparency. Each dot represents one donor. Data were analyzed using unpaired, two-tailed t-tests (ns, p > 0.05). Error bars represent the mean and SD. RC, recovered children; HC, healthy control; WAC, weeks after confirmation; S, spike; N, nucleocapsid.

Figure 2

The immune response returns to homeostasis in children with mild COVID-19 at 8 months: (A) Changes in white blood cells (WBC), neutrophils and lymphocytes percentages after acute infection in children. (B) The tSNE plots shows the clustering of PBMCs. (C–E) The absolute counts of CD3⁺ (C), CD4⁺ (D), and CD8⁺ T cells (E) in recovered and healthy children. (F) The ratio of the CD4⁺/CD8⁺ cell population among different age groups. (G) The absolute counts of CD19⁺ B cells in recovered and healthy children. Each dot represents one donor. Data were analyzed using unpaired, two-tailed t-tests, *p < 0.05. Error bars represent the mean and SD. RC, recovered children; HC, healthy control; S, spike; N, nucleocapsid.

Figure 3

SARS-CoV-2-specific memory T cells were detected in children with mild COVID-19: (A) Comparisons of the frequency of helper T cell immune memory subsets in different age groups. (B) Comparisons of the frequencies of cytotoxic T cell immune memory subsets in different age groups. (C) Spike- and nucleocapsid-specific IFN-γ memory T cell counts in recovered and healthy children. (D,E) Comparisons of spike- and nucleocapsid-specific IFN-γ memory T cell counts by age category in recovered and healthy children. (F) Correlations among the frequencies of different memory T cell subsets, WAC and IFN-γ responses. Red indicates a positive correlation, blue indicates a negative correlation, and transparency represents a change in the p value; the greater the p-value is, the greater the line transparency. Each dot represents one donor. Data were analyzed using unpaired, two-tailed t-tests (ns, p > 0.05, * p < 0.05, ** p < 0.01, and *** p < 0.001, and **** p < 0.0001). Error bars represent the mean and SD. RC, recovered children; HC, healthy control; WAC, weeks after confirmation; S, spike; N, nucleocapsid.

Figure 4

SARS-CoV-2-specific memory B cells show a weak response in children with mild COVID-19: (A) Comparisons of the memory B cell frequency of recovered and healthy children in different age groups. (B,C) Spike- and nucleocapsid-specific IgG⁺, IgM⁺ and memory B cell counts in recovered and healthy children. (D–G) Comparisons of spike- and nucleocapsid-specific IgG⁺, IgM⁺ memory B cell counts by age category in recovered and healthy children. Each dot represents one donor. (H) Comparisons of spike- and nucleocapsid-specific antibody memory B cell numbers by age category in recovered and healthy children. Red indicates a positive correlation, blue indicates a negative correlation, and transparency represents a change in the p value; the greater the p value is, the greater the line transparency. Data were analyzed using unpaired, two-tailed t-tests (ns, p > 0.05, * p < 0.05, ** p < 0.01, and *** p < 0.001, and **** p < 0.0001). Error bars represent the mean and SD. RC, recovered children; HC, healthy control; WAC, weeks after confirmation; S, spike; N, nucleocapsid.