Abstract: Background: The surge of COVID-19 hospitalizations in January 2022, at the onset of the Omicron period and before widespread population immunity, was a window in which to study the impact of vaccination status and other factors on disease severity in Omicron infections. Methods: This was a case-control study of adults hospitalized with COVID-19 during January 2022 that evaluated risk factors for severe disease. Cases were patients with severe COVID-19, defined in three ways: (1) requiring supplemental oxygen > 24 hours if not on home oxygen, (2) requiring high-level oxygen: either high flow of > 15 L/min, noninvasive ventilation, or mechanical ventilation (MV), and (3) the composite of MV or hospital death. Controls were patients hospitalized with COVID-19 who did not meet the respective case definition. Results: Among the 379 patients not on home oxygen before admission, 133 required supplemental oxygen. Among all 393 patients meeting the study criteria, 58 required high-level oxygen, 22 required MV, and 28 died. Factors associated with all case definitions were (1) absence of full vaccination compared to booster vaccination: requiring oxygen, odds ratio [OR], 7.86 [95% CI, 3.78-16.4]; high-level oxygen, OR, 5.17 [95% CI, 2.01-13.3]; MV or hospital death, OR, 5.21 [ 95% CI, 1.63-16.6]; (2) immunocompromised status: requiring oxygen, OR, 2.79 [95% CI, 1.38-5.63]; high-level oxygen, OR, 3.79 [95% CI 1.66-8.67]; MV or hospital death, OR 4.63 [95% CI 1.72-12.5], and (3) age > 75 years compared to 50 to 64 years: requiring oxygen, OR 2.85, [95% CI 1.43-6.07]; high-level oxygen, OR 4.02 [95% CI 1.47-11.0]; MV or hospital death, OR 5.68 [95% CI 1.49-21.7]. Other factors associated with requiring oxygen were obesity: body mass index (BMI) 35.0-39.9 kg/m2, OR, 2.4 [95% CI 1.09-5.26]; BMI > 40.0 kg/m2, OR, 3.29 [95% CI 1.34-8.10] compared to BMI 65 years, and body mass index > 35 kg/m2, but not the absence of a booster if fully vaccinated. This is further evidence that immune memory mitigates disease severity in Omicron infections.
