This is a summary, written by members of the CITF Secretariat, of:

Azeez R, Lotoski L, Winsor GL, Arnold CR, Galipeau Y, Pelchat M, Goguen S, Simons E, Moraes TJ, Mandhane PJ, Turvey SE, Bolotin S, Patrick DM, Bullard J, Lix LM, Doucas N, Rodriguez N, Brinkman FSL, Subbarao P, M-A, Azad MB. Predictors of SARS-CoV-2 anti-Spike IgG antibody levels following two COVID-19 vaccine doses among children and adults in the Canadian CHILD Cohort. medRxiv 2023.09.06.23294696; doi:

The results and/or conclusions contained in the research do not necessarily reflect the views of all CITF members.

A CITF-supported study, published in preprint and not yet peer-reviewed, found that two doses of mRNA vaccine (Pfizer or Moderna) produce robust antibody responses three months post-vaccination in both children and adults. Additionally, hybrid immunity (immunity derived from the combination of SARS-CoV-2 infection and vaccination) conferred stronger immune responses in both age groups when compared to vaccination alone. The study was led by Dr. Meghan Azad (University of Manitoba) and her MSc student Dr. Rilwan Azeez, on behalf of the Canadian CHILD cohort. Serological analyses were led by Dr. Marc-Andre Langlois (University of Ottawa).

A subset including 51 double-vaccinated children (mean age 12 years, 46% female) and 995 adults (mean age 44 years, 60% female) from the Canadian CHILD cohort were studied. Dried blood spots were collected over two time points (March 2021 to September 2021; October 2021 to January 2022). Among adults, 66% received an mRNA vaccine for both doses (47% Pfizer, 11% Moderna, and 8% a combination of the two), while 30% received a combination of mRNA and viral vector vaccines, and 4% received viral vector vaccines only. Virtually all children (99%) received the Pfizer mRNA vaccine for both doses.

Key findings:

  • Among those who received two vaccine doses, children produced significantly higher median antibody levels than adults. For both age groups, the highest antibody levels were observed approximately three months after the second dose. Antibody levels did not differ by sex.
  • Although more adults (14%) had had COVID-19 than children (6%), the percentages did not differ much by biological sex in either age group.
  • Hybrid immunity was associated with a higher and more rapid production of antibodies than vaccination alone. After receiving two vaccine doses, antibody levels peaked within the first month among previously infected participants, compared to four months post-vaccination among infection-na├»ve individuals (participants who did not have a prior SARS-CoV-2 infection).
  • Overall, higher post-vaccination SARS-CoV-2 IgG antibody levels were associated with prior infection to SARS-CoV-2, recent vaccination, age under 18 years, and receiving mRNA vaccines, while no associations were observed for differences in biological sex or time interval between doses.