There is an urgent need to understand why the course of the coronavirus that started in late 2019 (COVID-19) is affecting different groups of individuals with varying severity during the ongoing global pandemic. Greater knowledge of the disease, which is caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV2), will help us to prioritise our limited health resources. Because the virus is new, and no vaccine is yet available, everyone is naïve and susceptible to being infected with SARS-CoV2. The virus will continue to spread until an effective vaccine exists or sufficient members of our global population have been infected to establish herd immunity. At the moment, the best way to minimise loss of life and severe cases requiring intensive care is to try and shelter vulnerable groups of individuals and slow down the spread of the virus. The Korean Center for Disease Control and Prevention has probably tested the most extensive and therefore the most representative population. It reported that, up to 20 March, 6.3% of all cases that tested positive for COVID-19 were children under 19 years of age. The data from Korea are frequently being updated. 1 It is still unclear how many of the population who have been tested belonged to the same age group, so enrichment of COVID-19 within this young adult age group is uncertain. However, a general pattern has been reported from multiple countries and that is that children who test positive for COVID-19 experience a mild form of the disease. This means that children and younger adults who do not have underlying conditions, such as impaired lung function or immunosuppression, have a much lower risk of severe forms of COVID-19 than other age groups. The reasons for this mild COVID-19 disease in children remain elusive, and multiple hypotheses exist. This editorial discusses some of those theories.

On a general note, the immune systems of children and adults are different, both with respect to their composition and functional responsiveness. 2 In addition, there are differences in the immune systems of very young children, preschool children and teenagers. During the first weeks of life, the human newborn infant is exposed to a range of novel environmental exposures and undergoes dramatic changes. 3 Another difference between newborn infants and older children is the presence of some maternal antibodies during the first months of life. These antibodies do not include novel viruses such as SARS-CoV2. 4 One possible explanation for the milder COVID-19 disease presentation in children is that children have a qualitatively different response to the SARS-CoV2 virus to adults. Another possibility is that the presence of other simultaneous