Governance Organization

CHA – Children’s Hospital Academic Medical Organization

Project Title

CHA-21-001 – Natural Evolution of Serum Antibodies in Children and Adults with Confirmed SARS-CoV-2 and Household Contacts

Project Highlights

SUMMARY: Using combined funding from IFPOC, Ontario COVID-19 RRF and PSI, we conducted a case-ascertained cohort study of 175 households in the Ottawa region to assess household transmission of SARS-CoV-2 using antibody surveillance. Households were eligible for enrollment if an adult or child member tested positive for SARS-CoV-2 by reverse transcriptase polymerase chain reaction (RT-PCR) (index participant). In addition to the index participant, enrollment of a minimum of one additional household member was required. At 3-time points (enrollment, 2- and 6-months), questionnaires were administered and SARS-CoV-2 testing was completed. ELISA assays evaluated SARS-CoV-2-specific IgA, IgM and IgG against the spike-trimer and nucleocapsid protein.

UPDATE: Enrollment of the cohort is complete and data from the first time point has been analyzed. We have submitted 2 manuscripts from this project to date (under consideration at CMAJ and JAMA Pediatrics; submitted Oct 2021, Nov 2021 respectively).

RESULTS: We enrolled 175 index participants (72 children, 103 adults) and 487 household contacts (695 total study participants) from Sept 2020-Apr 2021. 76% of households enrolled all members. Antibody testing was completed on 97% index participants and 94% household contacts. 239 household contacts were SARS-CoV-2 antibody-positive, indicating a secondary attack rate of 49% (95%CI: 43,55). 17% of infected household contacts were asymptomatic. Asymptomatic infection among child and household contacts was similar (OR=1.3; 95% CI:0.8,2.1).

Transmission by age: Adult index participants were more likely than child index participants to transmit (OR=2.2; 95%CI: 1.4,3.9). In households where the index participant was a child, 37% (95%CI: 28,48) of members had antibodies. The youngest children (0-3 years) transmitted to a lower proportion of household contacts (19%; 95%CI: 7,41) than did 4-11-year-olds (42%) and 12-17-year-olds (39%). In households with adult index participants, 57% (95%CI: 49,64) of members had antibodies. For index participants of all ages, the secondary risk to pediatric versus adult household contacts was similar (OR=0.8; 95%Cl: 0.6,1.1).

Risk factors for transmission: Odds of transmission increased with every additional member with known infection (OR, 3.0; 95% CI 2.1 to 4.2). Lower-density households were associated with less transmission (OR=0.8; 95%CI: 0.7,1.0). The odds of an adult transmitting to a child were higher, compared with a child transmitting to another child (OR=2.0; 95%CI: 1.0,4.2). Isolation of the index participant and indoor masking were not associated with decreased transmission. The odds of transmission from symptomatic and from asymptomatic index participants were similar (OR=1.4; 95%CI: 0.6,3.1).

Non-seroconverters: 332 index participants and household contacts (161 children, 171 adults) tested positive for SARS-CoV-2 by RT-PCR prior to study enrollment. 43 (13%, 95%CI:9.7,17.0) did not seroconvert, 63% of whom were children. Individuals who were asymptomatic at testing were no more or less likely to seroconvert (OR=0.4; 95%CI:0.1,1.2). Seroconversion was not associated with time since infection (≤30d vs >30d) (OR=0.9; 95%CI:0.4,1.8). Children aged 0-3 years had lower odds of seroconversion.

IMPACT: We found high transmission by both children and adults, with a secondary attack rate of ~50%. We believe our findings have important implications for public health policy. While household transmission has been identified as a major driver of the pandemic, second only to superspreader events, there have been only limited data on the role children play in transmission. We found children spread infection to approximately one-third of household members; child and adult household contacts were equally likely to become infected from the infected child. Many Canadian provinces still do not require school-aged children to wear masks in indoor settings or while playing sports. Further, in many jurisdictions, unvaccinated children are permitted to circulate in the community even if they are a close contact of a COVID-positive case. This policy may reflect a lack of understanding of how children may transmit the virus, even when asymptomatic. Although vaccines for 5-11 year-olds have recently received approval, a survey published by the Kaiser Family Foundation found only 27% of parents plan to vaccinate their child. If substantial proportions of the pediatric population remain unvaccinated, the pandemic has the potential to continue through many subsequent waves of infection. Providing evidence on how children transmit SARS-CoV-2 has the potential to significantly impact public health policies. Our findings related to individuals testing positive by RT-PCR but who did not develop an antibody response support emerging evidence of uneven antibody responses to SARS-CoV-2 infection, especially in children, and highlight the importance of vaccination even among those previously infected.


Risks and Complications

Primary Project Lead for Contact

Maala Bhatt


Secondary Project Lead for Contact

Roger Zemek


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