Table 1 Proposed mechanisms explaining why children have a generally milder course of SARS-CoV-2 disease than adults

• Different ACE-2 expression or function in children than in adults (ref. 16)

• Large thymic repertoire and sustained innate immunity (ref. 6, 21)

• Elevated B, T and NK lymphocytes (ref. 6, 19)

• Recurrent respiratory infections and vaccinations in the first years of life (trained immunity) (ref. 6, 19)

• Reduced exposure to tobacco smoke (ref. 17)

• Healthy respiratory system

• Less comorbidities than adults