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CHU Sainte-Justine
Wednesday, February 23 2022

How many children in the Greater Montreal Area are protected against COVID-19?

A surveillance exercise by a CHU Sainte-Justine team

The Omicron variant is considered predominant in Quebec since December 20, 2021. Given its high transmissibility, the number of children who have been infected with COVID-19 has increased significantly.

To estimate the proportion of children in the Greater Montreal area who have already contracted SARS-CoV-2 (COVID-19 virus), the National Director of Public Health (Chief medical officer of health), Dr. Luc Boileau, has mandated a team from the CHU Sainte-Justine to conduct surveillance of the proportion of children under 18 years of age with an antibody response to COVID-19 (seroprevalence).

In summary, we are trying to determine how many children are protected against COVID-19 either following a natural infection or through vaccination, in the Greater Montreal Area.

Our methods

Starting on January 26, 2021, we have been collecting residual blood samples from children who were seen at the CHU Sainte-Justine emergency room or came to the test center who had a blood sample taken. These residual samples, which would normally be destroyed, are anonymized and sent to Héma-Québec's laboratory for serological research of antibodies against SARS-CoV-2.

A person infected with COVID-19 develops two types of antibodies (IgG): antibodies to the nucleocapsid protein (N) and antibodies to the spike protein (S). However, a vaccinated person only develops antibodies to the S protein (Fig. 1).

SARS-CoV-2 virus

IgG antibodies appear, on average, 2 weeks after infection or vaccination.

Our results

Between January 26 and February 10, 2022, 641 samples were collected and sent to Héma-Québec for testing of antibodies: 285 samples from children aged 0 to 4 years, 188 samples from children aged 5 to 12 years and 168 samples from adolescents aged 12 to 17 years. Because the samples are anonymized, we do not know the reason for consultation of these children.

Children aged 0-4 years are currently not vaccinated. Excluding children younger than 6 months of age who may still have maternal antibodies (transferred to the baby by the mother late in pregnancy to protect him or her), nearly one-third (32.5%) have developed antibodies to SARS-CoV-2 N protein – meaning that nearly one-third (32.5%) of children 6 months to 4 years of age have had COVID-19 in the months prior to our analysis.

Among infants under 6 months of age, nearly 30% had COVID-19 or their mothers had COVID-19, which is reflected by the presence of IgG against protein N. The higher proportion of infants less than 6 months of age with antibodies to protein S (67%) indicates that some proportion of mothers had been vaccinated.

The 5–11-year-old group showed the highest rate of antibodies to protein N (43% had COVID-19) while this proportion was relatively lower in the 12–17-year-old group (about 34%). When vaccinated and naturally infected adolescents are combined, nearly 97% had antibodies to protein S, therefore having at least partial protection against COVID-19.

The difference between the proportion of children aged 6 months to 4 years with antibodies to protein N (32.5%) and S (40.1%) is explained by the fact that the human body tends to have a better response to protein S. It also tends to lose its antibodies to protein N over time, which does not happen as much or as quickly with protein S.

Proportion of children with antibodies COVID-19
in the Greater Montreal Area

Although we suspect that a large proportion of children acquired their COVID-19 infection recently, antibodies to the N protein are not specific to the Omicron variant and can sometimes persist for several months or even more than a year.

Therefore, we do not know with certainty what proportion of children acquired their infection during the Omicron wave. Furthermore, in vaccinated children, it is possible to have had COVID-19 without detectable increase in antibodies to the N protein. The levels of antibodies to the N protein in the 5-11 and 12-17 age groups therefore represent minimum levels that are probably underestimates.

Our conclusions and next steps

A sizable proportion of children under 18 years of age have had COVID-19. As children have been more affected since September 2021, and even more so since the Omicron wave, it is likely that most infections have occurred with the Delta and, even more so, with the Omicron variants.

It will be interesting to know the proportion of children with positive N protein serology who have had COVID-19 documented by PCR test. It will also be interesting, once this exercise is completed, to repeat a seroprevalence in a few months to follow the evolution of the disease in the population.


  • Caroline Quach, MD , MSc, Pediatric Infectious Diseases Specialist and Medical Microbiologist
  • Christian Renaud, MD, Pediatric Infectious Diseases Specialist and Medical Microbiologist (Héma-Québec)
  • Émilie Vallières, MD, PhD, Pediatric Infectious Diseases Specialist and Medical Microbiologist
  • Marc Desforges, PhD
    Clinical Department of Laboratory Medicine, Division of Microbiology, CHU Sainte-Justine and Université de Montréal


  • Zineb Laghdir, Research Coordinator
  • Research Team : Louise Wang, Verinsa Mouajou, Marie-Ève Benoît, Research Assistants
    CHU Sainte-Justine Research Centre, C. Quach’s team


  • Ms. Leila Rabaamad, Core Laboratory, CHU Sainte-Justine
  • Ms. Renée Bazin (Héma-Québec)
  • Ms. Josée Perreault (Héma-Québec)
  • Ms. Evi-Jane Kay Molloy, Laurie-Jane Cloutier-Gagnon, Graphic Designers
  • Ms. Lucie Dufresne, Assistant to the Director
  • Ms. Anne-Julie Ouellet, Director
    CHU Sainte-Justine Communications and Public Relations Department

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Updated on 9/28/2022
Created on 2/23/2022
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