evidence: COVID-19 shows an increased number of cases and a greater risk of severe disease with increasing age5,6, a feature shared with the 2003 SARS epidemics7. This age gradient in reported cases, which has been observed from the earliest stages of the pandemic1, could result from children having decreased susceptibility to infection, a lower probability of showing disease on infection or a combination of both, compared with adults. Understanding the role of age in transmission and disease severity is critical for determining the likely impact of social-distancing interventions on SARS-CoV-2 transmission8, especially those aimed at schools, and for estimating the expected global disease burden. [...] First, age-varying susceptibility to infection by SARS-CoV-2, where children are less susceptible than adults to becoming infected on contact with an infectious person, would reduce cases among children. Decreased susceptibility could result from immune cross-protection from other coronaviruses9,10,11, or from non-specific protection resulting from recent infection by other respiratory viruses12, which children experience more frequently than adults13,14. Direct evidence for decreased susceptibility to SARS-CoV-2 in children has been mixed15,16, but if true could result in lower transmission in the population overall.