The study focuses on Estonian children under 18 who had COVID-19 from 27.02.2020 to 01.03.2023. The main aim is to find out the risk factors for severe COVID-19 in children. In order to do that the study includes their demographic data, vaccination status, and details about the severity of illness, such as hospitalization and long-term effects of COVID-19.

Data is collected for all children under 18 years old in Estonia who were diagnosed with COVID-19 between 27.02.2020 and 01.03.2024 (either with the ICD-10 codes U07.1/U07.2 and/or a positive SARS-CoV-2 test. The following data is collected for each patient: age at the time of COVID-19 diagnosis, gender, place of residence, and COVID-19 vaccination status.

In order to assess the severity of the disease, the following data is collected for each case: hospitalization due to COVID-19, length of hospitalization, need for intensive care, need for mechanical ventilation, fatal outcomes, and occurrence of long-term sequelae (post-COVID-19 condition and pediatric systemic inflammatory response syndrome). Additionally, data on any comorbid chronic and acute diseases of the children in the study group is collected. The outcome measures are compared across five different SARS-CoV-2 VOC periods: original, Alpha, Delta, Omicron BA.1/BA.2 and Omicron BA.4/BA.5.

Inclusion Criteria:

• SARS-CoV-2 positivity or COVID-19 diagnosis (ICD-10 codes U07.1, U07.2)
• COVID-19 diagnosis in the following time period: 27/Feb/2020 to 01/Mar/2023

Exclusion Criteria:

• age above 18 years

All the Estonian children and adolescents (under 18 yo) who received COVID-19 diagnosis or positive SARS-CoV-2 test in a 3 year period

The IPD collected throughout this trial will not be shared due to data protection laws.