At the start of the COVID-19 pandemic, there was uncertainty surrounding the diagnosis, prevalence, phenotype, duration, and treatment of Long COVID. This study aimed to (A) describe the clinical phenotype of post-COVID symptomatology in children and young people (CYP) with laboratory-confirmed SARS-CoV-2 infection compared with test-negative controls, (B) produce an operational research definition of Long COVID in CYP, and (C) establish its prevalence in CYP.

In total 219,175 CYP aged 11-17 years who had a positive (n=91,014) or negative (n=128,161) PCR test for SARS-CoV-2 between September 2020 and March 2021 in England were invited to participate. Test-positive and test-negative CYP were matched, at study invitation, on month of test, age, sex, and geographical region. 31,012 consenting CYP enrolled into the study at 3-, 6- or 12-months after their index-PCR test and, depending on when they enrolled, they were also invited to fill in follow-up questionnaires at 6-, 12-, and 24-months post index-test. The overall response rate was 14.1%, with retention across sweeps varying from 36.6% to 54.1%.

A sub-study was set up in January 2022 when the Omicron variant was dominant. In the sub-study an additional 5,135 CYP who were PCR positive for the first time in January 2021 were invited, along with 4,507 who were reinfected during this period, and 5,157 who remained PCR-negative. 3,046 consenting CYP enrolled into the sub-study and filled in questionnaires at 0-, 3-, and 6- and 12-months after testing.

The datasets include repeat self-reported information on CYP's physical and mental health over time, using validated scales. For the main sample, flexible survey weights have been developed to re-weight analyses to be nationally representative of CYP in England.

Further information is available on the UCL Long COVID in Children and Young People (The CLoCk Study) webpages.

Suitable data analysis software
The depositor provides these data in R format (.rda). Users are strongly advised to analyse them in R, as transfer to other formats may result in unforeseen issues.

Supplementary information. Text S1, Bayesian age, period and cohort model. Detailed description of the BAPC models. Text S1A, Random walk of first and second order. Description of the different type of parameters assumptions for the age, period and cohort effects. Text S1B, Estimation, prediction and comparison. Description of the methods to estimate BAPC models, how to compare between different models and how to compute projections into the future. Text S1C, BAPC model for CHD mortality in England and Wales. Specific methods and assumptions used for the CHD mortality BAPC model in England and Wales. Text S2, Mean absolute percent error. Description of the type of error measurement used to compare scenarios and models. Text S3, References. References used in the Supplementary information section. (DOCX)

The statistic shows the total population in Canada from 2020 to 2024, with projections up until 2030. In 2024, the total population in Canada amounted to about 41.14 million inhabitants. Population of Canada Canada ranks second among the largest countries in the world in terms of area size, right behind Russia, despite having a relatively low total population. The reason for this is that most of Canada remains uninhabited due to inhospitable conditions. Approximately 90 percent of all Canadians live within about 160 km of the U.S. border because of better living conditions and larger cities. On a year to year basis, Canada’s total population has continued to increase, although not dramatically. Population growth as of 2012 has amounted to its highest values in the past decade, reaching a peak in 2009, but was unstable and constantly fluctuating. Simultaneously, Canada’s fertility rate dropped slightly between 2009 and 2011, after experiencing a decade high birth rate in 2008. Standard of living in Canada has remained stable and has kept the country as one of the top 20 countries with the highest Human Development Index rating. The Human Development Index (HDI) measures quality of life based on several indicators, such as life expectancy at birth, literacy rate, education levels and gross national income per capita. Canada has a relatively high life expectancy compared to many other international countries, earning a spot in the top 20 countries and beating out countries such as the United States and the UK. From an economic standpoint, Canada has been slowly recovering from the 2008 financial crisis. Unemployment has gradually decreased, after reaching a decade high in 2009. Additionally, GDP has dramatically increased since 2009 and is expected to continue to increase for the next several years.