Severity and Symptom Characteristics between Omicron and Delta SARS-CoV-2 Variant Infections in the Australian Capital Territory: A Cross-Sectional Study

Abstract Background: Despite the widespread dominance of SARS-CoV-2 Delta (B.1.617.2 lineage) and Omicron (B.1.1.529 lineage) variants, there has been little information detailing differences in the presentation of symptoms and clinical characteristics. Characterising symptom and severity profile is important for understanding emerging SARS-CoV-2 variants. Methods: We conducted a retrospective cross-sectional study of all laboratory-confirmed SARS-CoV-2 infections notified in the Australian Capital Territory (ACT), Australia, between 12 August 2021 and 21 January 2022. Symptom data were ascertained through the initial case investigation interview, and by using either an automated survey or phone interview through routine public follow-up. The routine follow-up for the full isolation period collected individual symptom status and the specific symptoms experienced. We used validated vaccination status through the Australian Immunisation Register (AIR). We used a multivariable logistic regression model to estimate adjusted odds ratios (aOR) for hospitalisation with the Delta and Omicron SARS-CoV-2 variants. We compared the difference in proportions of individual symptoms experienced using Pearson’s χ2 tests, Wilcoxon rank sum test or Fisher’s exact tests. We considered results significantly different where p-value was less than 0.05. Results: We found that a higher proportion of individuals infected with Omicron (32% 144/452) were asymptomatic compared to those infected with Delta (12% 74/613). The most commonly reported symptoms for Delta infections are cough (62%, 381/613), headache (55% 338/613), fever (47% 290/613) and runny nose (47% 289/613). The most commonly reported symptoms for Omicron infections are runny nose (54% 242/452), cough (53% 240/452), sore throat (41% 181/452) and lethargy (39% 176/452). Cases infected with the Omicron variant had a lower odds of hospitalisation compared to cases infected with the Delta variant (OR 0.29, 95% CI 0.16-0.53). Conclusion: We found that symptom characteristics for Omicron and Delta SARS-CoV-2 infections were significantly different. Furthermore, we found that individuals infected with Omicron were more likely to report no symptoms at all, although vaccination status may have played a role in this cohort. Overall, infection with Delta was more likely to result in hospitalisation.