The seroprevalence of SARS-CoV-2-specific antibodies in Australian children: a cross sectional study

Background: Following reduction of public health and social measures concurrent with SARS-CoV-2 Omicron emergence in late 2021 in Australia, COVID-19 case notification rates rose rapidly. As rates of direct viral testing and reporting dropped, true infection rates were most likely to be underestimated. Objective: To better understand infection rates and immunity in this population, we aimed to estimate SARS-CoV-2 seroprevalence in Australians aged 0-19 years. Methods: We conducted a national cross sectional serosurvey from June 1, 2022, to August 31, 2022, in children aged 0-19 years undergoing an anesthetic procedure at eight tertiary pediatric hospitals. Participant questionnaires were administered, and blood samples tested using the Roche Elecsys Anti-SARS-CoV-2 total spike and nucleocapsid antibody assays. S and N seroprevalence adjusted for geographic and socioeconomic imbalances in the participant sample compared to the Australian population was estimated using multilevel regression and poststratification within a Bayesian framework. Results:  Blood was collected from 2,046 participants (median age: 6.6 years). Adjusted seroprevalence of spike-antibody was 92.1 % (95% credible interval (CrI) 91.0-93.3%) and nucleocapsid-antibody was 67.0% (95% CrI 64.6-69.3). In unvaccinated children spike and nucleocapsid antibody seroprevalences were 84.2% (95% CrI 81.9-86.5) and 67.1% (95%CrI 64.0-69.8), respectively. Seroprevalence increased with age but was similar across geographic distribution and socioeconomic quintiles. Conclusion:  Most Australian children and adolescents aged 0-19 years, across all jurisdictions were infected with SARS-CoV-2 by August 2022, suggesting rapid and uniform spread across the population in a very short time period. High seropositivity in unvaccinated children informed COVID-19 vaccine recommendations in Australia. Funding: Australian Government Department of Health and Aged Care. ### Competing Interest Statement All authors have no financial interests/personal relationships that which may be considered as potential competing interests with this study. Some authors have received institutional research grants and travel funding from the Australian Government, non-government or private organisations for other bodies of work. Dr Archana Koirala, Professor Kristine Macartney and Professor Nicholas Wood declares funding support from Australian Government Department of Health and Aged Care (DoHAC) to the institution National Centre for Immunisation Research and Surveillance (NCIRS). Dr Archana Koirala has received travel and accommodation grant to present and attend the World Society of Paediatric Infectious Diseases scientific meeting in 2023. Professor Kristine Macartney received payment as an expert witness for Australian Health Departments against proceedings against COVID-19 health regulations in 2021-2022. Professor Kristine Macartney declares institutinal directed funding support from the WHO and GAVI the Vaccine alliance, Welcome Trust and the Australian NHMR, to NCIRS and the University of Sydney payment as an expert witness for Australian health departments against proceedings against COVID-19 health regulations in 2021-2022, payment less than USD $5,000 for international travel for expert speaking engagements, non-pharmaceutical company sources. Professor Nicholas Wood, Professor Britta S von Ungern-Sternberg and Dr Brendan McMullan declare funding from the National Health and Medical Research Council (NHMRC) grants. Professor Britta S von Ungern-Sternberg declared funding support from Stan Perron Charitable Foundation. Professor Helen Marshall declares research grant from Pfizer to Women?s and Children?s Health Network (WCHN), no personal remuneration, for meningococcal research. Professor Peter Richmond declares research grants from Merck Sharpe & Dohme, GlaxoSmithKline (GSK) directed at the Telethon Kids Institute (TKI), no personal remuneration, on RSV, pneumococcal disease, varicella, meningococcal research and a COVID-19 vaccine study to assess immunogenicity and safety of homologous or heterologous vaccine schedules. Professor Peter Richmond, Professor Helen Marshall and Professor Nicholas Wood declare research grant from the Australian Government Department of Health and Aged Care to WCHN, TKI and University of Sydney for a COVID-19 DNA vaccine clinical trial Ms Alissa McMinn declares funding support from Pfizer provided support for flights/accommodation to attend the Public Health Association Australia Communicable Diseases & Immunisation Conference in 2023. Dr Ushma Wadia declares funding from Pfizer for flights and accommodation to attend Meningococcal Disease Vaccine Education in 2023. ### Funding Statement Yes ### Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes The details of the IRB/oversight body that provided approval or exemption for the research described are given below: Ethics approval for this national study was provided by the Sydney Children’s Hospital Network Human Research Ethics Committee (HREC 18/SCHN/72). I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable. Yes All relevant data are within the manuscript and its Supporting Information files. Line listed data cannot be shared publicly because of identification of Aboriginal and Torres Strait Islander and non-Indigenous children and adolescents in regional and remote Australian settings, in particular participants from remote communities.