Incidence of SARS-CoV-2 and all-cause mortality in persons with co-occurring substance use disorder and mental illness during the pandemic: a Norwegian cohort study.

Purpose: Most people were affected by the COVID-19 pandemic. Persons with co-occurring substance use disorder (SUD) and mental illness (MI) are already a marginalized group, with above average mortality. Thus, the study aim was to investigate SARS-CoV-2 incidence and mortality among persons with SUD/MI during the first two years of the pandemic.

Methods: This historical cohort study merged data from the Norwegian Patient Register, the Norwegian Surveillance System for Communicable Diseases and census data from Statistics Norway. We calculated crude mortality rates for persons with SUD and mild/moderate vs. severe MI and compared them to persons with physical illnesses or healthy controls. The incidence rate ratios for SARS-CoV-2 infection and mortality were estimated using Poisson regression models.

Results: Compared to healthy controls, the SARS-Cov-2-infection rate was marginally lower in persons with SUD and mild/moderate MI (IRR,1.19 [95%CI,1.09-1.30]) as in persons with physical illness (IRR,1.35 [95%CI, 1.23-1.47]), whereas persons with SUD and severe MI showed a lower rate compared to healthy controls. Crude mortality rates for persons with SUD/MI were substantially higher and increased much more during the pandemic than for persons with physical illnesses or healthy controls. The IRR for mortality in persons with SUD and mild/moderate MI was 10.61 (95%CI,7.19-15.67) and 11.44 (95%CI,7.50-17.45) for SUD and severe MI, compared to 5.03 (3.34-7.57]) for persons with physical illnesses only.

Conclusion: The analysis showed excess mortality during COVID-19-pandemic for SUD/MI, but without higher SARS-CoV-2 infection rates in this group. Consequently, excess mortality among persons with SUD/MI was not due to SARS-CoV-2 infection.