The investigators will first examine the impact of the COVID-19 pandemic on childhood cancer patients in Sweden and Denmark, both regarding susceptibility to severe COVID-19 among long-term childhood cancer survivors, and, for those diagnosed with childhood cancer during the pandemic, whether the path through primary care to cancer diagnosis as well as short-term survival has changed. Second, the investigators will study childhood cancer incidence before, during and after the pandemic with the particular aim to test the hypothesis regarding an infectious disease etiology for acute lymphoblastic leukemia.

Inclusion Criteria for studying severe COVID-19 among childhood cancer survivors:

• Childhood cancer cases (age 0-19) diagnosed from 1971 in Sweden and Denmark who were alive five years after diagnosis
• Two different comparisons groups: five age- and sex-matched comparisons for each childhood cancer survivor as well as all siblings of the childhood cancer survivors.
• All childhood cancer survivors, matched comparisons, and siblings of the survivors should be 20+ at the start of 2020.

Inclusion Criteria for studying childhood cancer incidence and survival and number of contacts with healthcare before the childhood cancer diagnosis:

- All children (age 0-19) diagnosed with cancer from 2015 in Sweden and Denmark

Exclusion Criteria:

• No exclusion criteria

• Childhood cancer cases (age 0-19) diagnosed from 1971 in Sweden and Denmark who were alive five years after diagnosis + their matched comparisons AND all siblings of the childhood cancer survivors
• All children (age 0-19) diagnosed with cancer from 2015 in Sweden and Denmark