Introduction: The COVID-19 pandemic continues to have a massive global impact. Understanding factors that predict severe disease and transmission is crucial for guiding public health interventions for COVID-19 and future pandemics. We aimed to determine the effects of index case viral load on household transmission, as well as determine risk factors for hospitalisation.

Methods: The analysis used data from participants with COVID-19 enrolled in the ConCorD-19 trial (NCT04552379) , a cluster randomised clinical trial of peginterferon beta-1a treatment. Logistic and linear regression analyses were used to assess associations between index case viral load and transmission to household contacts, after controlling for treatment allocation (n=539). Multivariate logistic regression (n=441) with univariate stepwise variable selection was used to identify risk factors for hospitalisation.

Results: High viral load (> log10 5.68 copies/mL) of index cases significantly increased odds of transmission to household contacts (OR=2.49, 95% CI=1.10-4.24), but not risk of hospitalisation (OR=1.16, 95% CI 0.82-1.65). Age (OR 1.05, 95% CI 1.02-1.09), BMI (OR=1.17, 95% CI=1.08-1.28), fever (OR=3.33, 95% CI=1.42-8.11) and being unvaccinated (OR=2.76, 95% CI=1.08-8.09) were associated with increased odds of hospitalisation. The AUC of the model was 0.82 (95% CI=0.74-0.89).

Conclusions: Index case viral load affected transmission but not disease severity in this cohort. The multivariate analysis identified BMI, age, fever, and vaccination status as independent risk factors for hospitalisation.