Seroprevalence Convergence Does Not Reflect Transmission Equity: Persistent Socioeconomic Disparities in COVID-19 Force of Infection in Canada

BackgroundSocioeconomic disparities in COVID-19 outcomes have been widely documented, but evidence regarding inequities in SARS-CoV-2 transmission remains mixed. In Canada, infection-induced seroprevalence appeared to converge across socioeconomic strata by late 2022, raising questions about whether transmission inequities diminished during the Omicron period. AimTo assess whether apparent convergence in SARS-CoV-2 seroprevalence reflects true equity in transmission or masks persistent socioeconomic disparities in force of infection. MethodsWe 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. We fitted a dynamic susceptible-infected 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 socioeconomic status were compared using likelihood-based criteria. ResultsDuring the pre-Omicron period, force of infection increased monotonically with material deprivation; the most deprived quintile experienced a 71% higher force of infection than the least deprived (incidence rate ratio (IRR): 1.71; 95% CI: 1.60-1.83). Following Omicron emergence, force of infection increased markedly in all quintiles but by differing magnitudes. Relative increases were largest in the least deprived quintile (48.5-fold) and smallest in the most deprived quintile (31.8-fold), resulting in compression of the socioeconomic gradient (Q5 vs Q1 IRR: 1.12; 95% CI: 1.11-1.14). Despite this compression, materially deprived populations continued to experience elevated transmission risk. ConclusionConvergence in SARS-CoV-2 seroprevalence across socioeconomic strata masked persistent inequalities in force of infection. Dynamic modelling demonstrates that apparent equity arose from differential amplification of transmission during the Omicron period rather than from elimination of underlying socioeconomic disparities.