Introduction: Solid Organ Transplant (SOT) recipients are vulnerable to SARS-CoV-2 infection. Delta variant has been reported to have worse outcomes as compared to other variants to date. The Omicron variant has been reported more transmissible but less virulent in general population. However, this phenomenon has not been validated in SOT recipients.

Method: This is a retrospective cohort study, conducted in two phases, between July 1st and December 10th, 2021(Delta dominant phase), and January 1st and January 31st, 2022 (Omicron dominant phase) in one of the biggest SOT centers in the United States. Patient demographics including transplanted organ, immunosuppressive medication, vaccination status, and treatment modalities were obtained. We also compared clinical outcomes including hospitalization, ICU admission rate and mortality between two groups. Chi squared or fisher’s exact test were performed for categorical variables, whichever appropriate. For continuous variables, Mann Whitney U-test was performed. Multiple logistic regression model with stepwise backward elimination was developed to identify the risk factors for mortality. We put variables whose p value was less than 0.2. Of note, the institutional treatment protocol has not changed during the study period.

Result: We identified 64 and 95 SOT recipients while delta and omicron variants were dominant, respectively. The rate of kidney transplant, age, gender, African American race, and the rate of at least received one dose vaccination were not significantly different between two groups. Of note, the hospital admission rate (68.8% vs 36.2%, p<0.001), ICU admission rate (20.3% vs. 6.3%, p=0.012), and mortality (20.9% vs. 4.2%) were higher during the delta variant versus omicron variant phase. For the risk factor analysis for mortality, we put age, African American race, COVID-19 variant, kidney transplant and vaccination status into multiple logistic regression model. Finally, delta variant was the only statistical significant risk factor for mortality (p<0.001, Odds ratio 9.35, 95% confidence interval 2.52-34.5).

Conclusion: This is the first study comparing the outcomes between Delta variant dominant and Omicron variant dominant phase in SOT recipients. Even though there was no treatment protocol changed, the identified risk factor for mortality was delta variant dominant phase. However, still, the hospital admission rate (36.2%) should have been observed while Omicron variant phase and required significant aggressive treatment. Thus, we should prepare and may need to change the treatment protocol based on the variant in the future.