Clinical outcomes among SARS-CoV-2 omicron-infected adults according to prior infection and vaccination history in Iran: A retrospective registry-based study.

Publication date: Jun 20, 2026

Omicron subvariants of SARS-CoV-2 have challenged immunity derived from prior infection and vaccination. Evidence from Middle Eastern populations using heterogeneous vaccine platforms remains limited. We evaluated associations between pre-existing immunity profiles and clinical outcomes among laboratory-confirmed Omicron-infected adults in Iran. We conducted a retrospective registry-based study in Hamedan Province, Iran, including 4674 adults with laboratory-confirmed COVID-19 between March 2022 and March 2023. Participants were classified into four pre-existing immunity groups: no documented immunity (V – Inf-), vaccination only (V + Inf-), prior infection only (V – Inf+), and hybrid immunity (V + Inf+). The primary outcomes were hospitalization and mortality among infected individuals. Multivariable logistic regression models adjusted for age, sex, comorbidities, and calendar period corresponding to predominant Omicron subvariant circulation. Among infected individuals, hybrid immunity (V + Inf+) was associated with lower observed hospitalization and mortality than vaccination only (V + Inf-). Vaccination only was associated with the highest observed hospitalization and mortality proportions. Prior infection only (V – Inf+) was also associated with more favorable outcomes than vaccination only. Associations were attenuated after adjustment for age and clinical covariates but remained directionally consistent. Among adults with laboratory-confirmed Omicron infection, pre-existing hybrid immunity was associated with more favorable clinical outcomes than vaccination only. Because analyses were conditioned on infection, the findings should be interpreted as differences in outcomes among cases rather than protection against acquiring infection.

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