SARS-CoV-2 induced immune perturbations in infants vary with disease severity and differ from adults' responses.

Differences in immune profiles of children and adults with COVID-19 have been previously described. However, no systematic studies have been reported from infants hospitalized with severe disease. We applied a multidimensional approach to decipher the immune responses of SARS-CoV-2 infected infants (n = 26; 10 subacute, 11 moderate and 5 severe disease; median age = 1.6 months) and matched controls (n = 14; median age = 2 months). Single cell (scRNA-seq) profiling of PBMCs revealed substantial alterations in cell composition in SARS-CoV-2 infected infants; with most cell-types switching to an interferon-stimulated gene (ISG) state including: (i) CD14 monocytes co-expressing ISGs and inflammasome-related molecules, (ii) ISG naive CD4 T cells, (iii) ISG proliferating cytotoxic CD8 T cells, and (iv) ISG naive and transitional B cells. We observe increased serum concentrations of both interferons and inflammatory cytokines in infected infants. Antibody responses to SARS-CoV-2 are also consistently detected in the absence of anti-IFN autoantibodies. Compared with infected adults, infants display a similar ISG signature in monocytes but a markedly enhanced ISG signature in T and B cells. These findings provide insights into the distinct immune responses to SARS-CoV-2 in the first year of life and underscore the importance of further defining the unique features of early life immunity.