Seroprevalence convergence masks persistent socioeconomic disparities in SARS-CoV-2 infection risk in Canada

Socioeconomic disparities in COVID-19 outcomes have been widely documented, but evidence regarding inequities in SARS-CoV-2 infection risk remains mixed. In Canada, infection-induced seroprevalence appeared to converge across socioeconomic strata by late 2022, raising questions about whether inequities in infection risk diminished during the Omicron period.

To assess whether apparent convergence in SARS-CoV-2 seroprevalence reflects true equity in infection risk or masks persistent socioeconomic disparities in force of infection.

We analysed serial cross-sectional SARS-CoV-2 seroprevalence data (anti-nucleocapsid antibodies) from Canadian Blood Services donors collected between April 2021 and April 2023 and stratified by area-level material deprivation quintile (Q1 = least deprived; Q5 = most deprived). We fitted a dynamic seronegative-seropositive model with sero-reversion to the full seroprevalence time series, estimating quintile-specific forces of infection before and after the emergence of the Omicron variant (January 2022). Models allowing differential Omicron-related amplification by material deprivation were compared using likelihood-based criteria.

During the pre-Omicron period, the most deprived quintile (Q5) experienced a 71% higher force of infection than the least deprived (Q1; incidence rate ratio (IRR): 1.71; 95% CI: 1.60-1.83). Following Omicron emergence, force of infection rose markedly in all quintiles. Because the pre-Omicron baseline was lowest in the least-deprived quintile, the relative increase during Omicron was largest there (48.5-fold) and smallest in the most-deprived quintile (31.8-fold), compressing the relative socioeconomic gradient (Q5 vs Q1 IRR: 1.12; 95% CI: 1.11-1.14). Despite this compression in relative terms, the most deprived populations continued to experience higher absolute force of infection throughout the Omicron period.

Convergence in SARS-CoV-2 seroprevalence across socioeconomic strata masked persistent inequalities in force of infection. Dynamic modelling demonstrates that apparent equity is attributable, in our modelling framework, to differential amplification of force of infection during the Omicron period rather than from elimination of underlying socioeconomic disparities.