Anti-SARS-CoV-2 Antibody Levels in a Cohort of Health Care Workers Before and After the Omicron Wave in Canada.

INTRODUCTION

We measured anti-S immunoglobulin G (IgG) levels in a cohort of health care workers (HCWs) to explore factors affecting the levels of vaccine-induced IgG antibodies and their relationship with risk of incident SARS-CoV-2 infection throughout the first seven epidemic waves.

METHODS

A convenience sample of HCWs from one acute care hospital and four long-term care homes had anti-S SARS-CoV-2 IgG antibody levels at the beginning of the pandemic (T1) and during Omicron waves 5-7 (T2). Poisson analysis was conducted to assess predicted levels of antibodies by covariates (health and social conditions), number, timing and type of vaccines, as well as history of previous SARS-CoV-2 infection. Antibody levels assessed between October 2021 and August 2022 were also analyzed in relation to incident cases of Omicron infections.

RESULTS

Of the HCWs who provided one (n = 128) or two blood samples (n = 146), 53% were vaccine naïve at T1 and 1.4% were so at T2. The mean SARS-CoV-2 IgG concentration was 648 binding antibody units/mL at T1 and 1,913 binding antibody units/mL at T2. Income insufficiency and the presence of more than one chronic condition were associated with lower antibody levels at T2. Antibody levels were higher in HCWs with prior SARS-CoV-2 infection and increased with more vaccine doses received. Hybrid immunity elicited higher levels of antibodies in HCWs at T1 and T2. Waning of antibody levels over time was seen after vaccination with a third dose at T2. A correlation between antibody levels and subsequent risk of Omicron infection was not found.

CONCLUSIONS

Our results suggest that timing and prioritization of anti-SARS-CoV-2 vaccination needs to consider the health and socioeconomic factors of HCWs, and the waning effects of vaccines.