Longitudinal change in SARS-CoV-2 seroprevalence in 3-to 16-year-old children: The Augsburg Plus study

Abstract

Background: Currently, more than 30,200,000 COVID-19 cases have been diagnosed in Germany alone. However, data regarding prevalence of COVID-19 in children, both in Germany and internationally, are sparse. We sought to evaluate the number of infected children by measuring IgG antibodies.

Methods: Oropharyngeal swabs were collected between December 2020 and August 2021 to measure SARS-CoV-2, and capillary blood for the detection of SARS-CoV-2 antibodies (by rapid test NADAL® and filter paper test Euroimmun® ELISA); venous blood was taken for validation (Roche® ECLIA and recomLine Blot) in 365 German children aged 3-16 years from 30 schools and preschools. We used multiple serological tests because the filter paper test Euroimmun® ELISA performs better in terms of sensitivity and specificity than the rapid test NADAL®. The Roche® ECLIA test is used to detect SARS-CoV-2 spike protein, and the recomLine Blot test is used to rule out the possibility of infection by seasonal SARS-viruses and to test for specific SARS-CoV-2 proteins (NP, RBD and S1). In addition, one parent each (n = 336), and 4-5 teachers/caregivers (n = 90) per institution were tested for IgG antibodies from capillary blood samples. The total study duration was 4 months per child, including the first follow-up after 2 months and the second after 4 months.

Results: Of 364 children tested at baseline, 3.6% (n = 13) were positive for SARS-CoV-2 IgG antibodies using Euroimmun® ELISA. Seven children reported previously testing positive for SARS-CoV-2; each of these was confirmed by the Roche® Anti-SARS-CoV-2-ECLIA (antibody to spike protein 1) test. SARS-CoV-2 IgG antibodies persisted over a 4-month period, but levels decreased significantly (p = 0.004) within this timeframe. The median IgG values were 192.0 BAU/ml [127.2; 288.2], 123.6 BAU/ml [76.6; 187.7] and 89.9 BAU/ml [57.4; 144.2] at baseline, 2 months and 4 months after baseline, respectively. During the study period, no child tested positive for SARS-CoV-2 by oropharyngeal swab. A total of 4.3% of all parents and 3.7% of teachers/caregivers tested positive for IgG antibodies by Euroimmun® ELISA at baseline.

Conclusion: We noted a rather low seroprevalence in children despite an under-reporting of SARS-CoV-2 infections. Measurement of IgG antibodies derived from capillary blood appears to be a valid tool to detect asymptomatic infections in children. However, no asymptomatic active infection was detected during the study period of 4 months in the whole cohort. Further data on SARS-CoV-2 infections in children are needed, especially in the group of <5-year-olds, as there is currently no licensed vaccine for this age group in Germany. The Robert Koch Institute's Standing Commission on Vaccination (STIKO) recommended COVID-19 vaccination for 12-17 and 5-11 year olds in August 2021 and May 2022 respectively.

Fig 1. Flow diagram: Test-algorithm with number of tested children at baseline.

Fig 2. Before-and-after-plot: All children who tested positive for IgG-antibodies at least once during the study period.

Every child is represented as an ID-number in the plot (n = 17 at baseline, and n = 16 at 4 months after baseline). The child with ID 302 was only tested at baseline and two months after baseline, so there was no third IgG-antibody value available. The grey dashed line at y = 24.0 BAU/ml represents the threshold for categorization into positive and negative results (a test result is IgG-positive at a level of ≥24.0 BAU/ml). All seroconverted children are indicated by a red line.

Fig 3. Children who tested positive per school-type (absolute numbers).

Fig 4. Violinplot SARS-CoV-2-IgG antibodies in BAU/ml over 4 months (dried blood spot: Euroimmun® ELISA) (n = 332).

The Figure comes log10-scaled on the y-axis. Each violin presents one examination time point, stratified for positive and negative IgG antibody results. The grey dashed line at y = 24.0 BAU/ml indicates the threshold between IgG-positive and IgG-negative results (a test result is IgG-positive at a level of ≥24.0 BAU/ml, and negative at a level of <24.0 BAU/ml).

Fig 5. Violin plot of SARS-CoV-2-IgG antibodies in BAU/ml at baseline (Euroimmun® ELISA), stratified for symptomatic and asymptomatic children: Symptoms since the start of the pandemic.

Fig 6. Violin plot of SARS-CoV-2-IgG antibodies in BAU/ml at baseline (Euroimmun® ELISA), stratified for symptomatic and asymptomatic children: Symptoms four weeks before baseline examination.