SARS-CoV-2 specific T cell responses are lower in children and increase with age and time after infection

Cohen, Carolyn A.; Li, Athena P. Y.; Hachim, Asmaa; Hui, David S. C.; Kwan, Mike Y. W.; Tsang, Owen T. Y.; Chiu, Susan S.; Chan, Wai Hung; Yau, Yat Sun; Kavian, Niloufar; Ma, Fionn N. L.; Lau, Eric H. Y.; Cheng, Samuel M. S.; Poon, Leo L. M.; Peiris, Malik; Valkenburg, Sophie A.

SARS-CoV-2 infection of children leads to a mild illness and the immunological differences with adults are unclear. Here, we report SARS-CoV-2 specific T cell responses in infected adults and children and find that the acute and memory CD4(+) T cell responses to structural SARS-CoV-2 proteins increase with age, whereas CD8(+) T cell responses increase with time post-infection. Infected children have lower CD4(+) and CD8(+) T cell responses to SARS-CoV-2 structural and ORF1ab proteins when compared with infected adults, comparable T cell polyfunctionality and reduced CD4(+) T cell effector memory. Compared with adults, children have lower levels of antibodies to β-coronaviruses, indicating differing baseline immunity. Total T follicular helper responses are increased, whilst monocyte numbers are reduced, indicating rapid adaptive co-ordination of the T and B cell responses and differing levels of inflammation. Therefore, reduced prior β-coronavirus immunity and reduced T cell activation in children might drive milder COVID-19 pathogenesis.