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

Djaïleb A, Parker MF, Lavallée E, Stuible M, Durocher Y, Thériault M, Santerre K, Gilbert C, Boudreau D, Baz M, Masson JF, Langlois MA, Trottier S, Quaglia D, Pelletier JN. Longitudinal study on seroprevalence and immune response to SARS-CoV-2 in a population of food and retail workers through transformation of ELISA datasets. medRxiv. 2024 January 29. doi:

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

A CITF-funded study, published in preprint and not yet peer-reviewed, reported longitudinal data on humoral immunity collected over a year and a half from food and retail workers over key periods of the pandemic, including the initial vaccination campaigns and the emergence of the Omicron variant. The researchers found high rates of seroconversion (either due to infection or vaccination) in people from the four occupations studied and did not find that any one group had significantly higher levels of antibodies due to vaccination or infection overall. This study was led by Abdelhadi Djaïleb and Dr. Joelle N. Pelletier (both from the University of Montreal) in collaboration with Dr. Denis Boudreau (Université Laval) and Dr. Marc- André Langlois (University of Ottawa).

The researchers recruited 304 food and retail workers from Quebec (bars, restaurants, grocery, and hardware stores). Between April 2021 and October 2022, participants attended five evenly spaced visits wherein they provided blood samples; information on SARS-CoV-2 risk factors, symptoms, antigen or PCR test results, and vaccination status; and information on potential risk factors (demographic, socioeconomic, behavioral, clinical, occupational).

Samples were analyzed through a “centralised ELISA assay” performed at the University of Ottawa and an “in-house ELISA assay” performed at the University of Montreal. The National Research Council of Canada provided antigens against SARS-CoV-2 for all three strains (ancestral, Delta, and Omicron) needed for the ELISA assays. Both assays measured antibody levels against the ancestral strain of SARS-CoV-2 to determine vaccine-induced or infection-acquired seroconversion. The in-house assay also looked at cross-reactivity against Delta and Omicron variants.

Integration of a recent mathematical model allowed transformation of the ELISA dataset. This made it possible to normalize and statistically analyze the data using Welch’s ANOVA, which is only applicable to normally distributed data.

Key findings:

  • There was no overall statistical difference antibody levels/humoral response between the people in different occupations included in the study.
  • However, there were statistically higher levels of vaccine-induced antibodies for restaurant workers at visit 1 (mid-July 2021) and for both restaurant and hardware store workers at visit 2 (late October 2021), compared to workers in bars and grocery stores.
  • Anti-spike levels peaked in July/Aug 2021 and Feb-July 2022, which likely reflected high levels of vaccination coinciding with mass vaccination campaigns.
  • Before the appearance of Omicron, anti-spike antibody responses were the same against the Delta and ancestral strains, confirming previous studies showing effective cross-reactivity.
  • Omicron cross-reactivity for anti-spike IgG was weaker.
  • From the additional risk factors analysed, the only significant result was a higher ancestral anti-spike IgG response in non-smokers versus smokers.

This study is one of the few to include a cohort of highly vaccinated individuals without pre-existing severe health problems and who were not hospitalised with COVID-19, and to report longitudinal data from sampling the same individuals over a long period.