Over 80% of kids and teenagers in Canada who tested positive for SARS-CoV-2 antibodies initially tested negative or were not tested at all, researchers found.
In a seroprevalence analysis involving 1,632 kids from daycare through secondary school, 82% of the children who tested positive for SARS-CoV-2 antibodies had previously either tested negative on RT-PCR or had never been tested, reported Kate Zinszer, PhD, of the University of Montreal School of Public Health, and colleagues.
Among these 95 children who were antibody-positive, 49 developed only mild symptoms and 46 were asymptomatic, the authors wrote in JAMA Network Open.
No differences were observed between age groups and seropositivity, though ethnic children had a higher chance of seropositivity versus white children (adjusted seroprevalence ratio 1.9, 95% CI 1.1-2.6), they added.
“The findings suggest that there was more transmission occurring in children compared with what was being detected by RT-PCR, although children experienced few or mild symptoms,” Zinszer and colleagues said.
The EnCORE (Enfants et COVID-19: Étude de séroprévalence) study examined 1,632 children ages 2 to 17 from four ethnically diverse neighborhoods in Montreal in October 2020 to March 2021. Participants came from 30 day cares (22% of kids), 22 primary schools (44%), and 11 secondary schools (34%).
Zinszer and colleagues analyzed finger-pricked dried blood spot (DBS) collections from participants using ELISA, while parents filled out an online questionnaire regarding “self-reported COVID-19 symptoms and tests.” Parents were instructed on how to collect and mail back DBS collections.
Primary outcome assessed the seroprevalence of SARS-CoV-2 antibodies.
Just over half of all participants were boys (51%), with a mean age of 9. The majority of children (79%) were normal weight or underweight and nearly all (94%) had no pre-existing medical conditions. Three-fourths of parents held a Bachelor’s degree or higher and 13% identified as ethnic minorities.
Seroprevalence was 5.8% (95% CI 4.6%-7.0%) overall, but increased over the course of the pandemic: 3.2% from October to November 2020 (95% CI 0.7%-5.8%) and 8.4% from March to April 2021 (95% CI 4.4%-12.4%).
Interestingly, 18% of seropositive participants had previously tested positive via RT-PCR, which the researchers described as further confirming “the value of serology as a tool for SARS-CoV-2 surveillance.”
Researchers reported that two children were hospitalized from a COVID-19 infection. There were 26 participants who tested positive via RT-PCR before DBS collection, and compared with the 881 participants who tested negative, “the only significantly different symptom […] was loss of smell,” they wrote.
The analysis had several limitations, the researchers acknowledged, including selection bias, as families who thought their children might have been exposed to COVID may have been more likely to enroll in the study, but requiring DBS collection might have made other families reluctant to enroll.
“It will be important to continue monitoring the serological status of children, particularly in the context of new COVID-19 variants of concern and in the absence of mass vaccination campaigns targeting young children,” Zinszer and colleagues concluded.
This study was supported by the Public Health Agency of Canada.
Zinszer did not report any conflicts of interest.
Coauthors reported industry ties to AbbVie, Seegene, MedImmune, Sanofi Pasteur, the Canadian Institutes of Health Research, and the Quebec Ministry of Health.