A Comparison Between Omicron and Earlier COVID-19 Variants' Disease Severity in the Milan Area, Italy

Abstract

Background: In the context of the fourth wave of the COVID-19 pandemic in Italy, which occurred in correspondence with the outbreak of the Omicron variant, it became fundamental to assess differences in the risk of severe disease between the Omicron variant and the earlier SARS-CoV-2 variants that were still in circulation despite Omicron becoming prevalent.

Methods: We collected data on 2,267 genotyped PCR-positive swab tests and assessed whether the presence of symptoms, risk of hospitalization, and recovery times were significantly different between Omicron and the earlier variants. Multivariable models adjusted for sex, age class, citizenship, comorbidities, and symptomatology allowed assessing the difference in outcomes between Omicron and the earlier variants according to vaccination status and timing of administration.

Results: Compared to the earlier variants in the same period, Omicron was less symptomatic, resulted in fewer hospital admissions for those who were unvaccinated and for those who were already immunized after the booster dose, and was associated with quicker recovery, yet not in subjects with three vaccination doses.

Conclusion: Despite being milder, Omicron's higher transmissibility and vaccine resistance should not lead to underrating its damage potential, especially with regard to hospital and health service saturation.

Keywords: COVID-19; Omicron; SARS-CoV-2; hospitalization; negativization; symptoms; vaccination; variants.

Figure 1

Smoothed weekly trend of share of SARS-CoV-2 sequences that are Omicron variant in Italy and worldwide (Italy excluded). Source: our elaboration of GISAID data (https://www.gisaid.org/).

Figure 2

Forest plot of COVID-19-related symptoms in Omicron vs. other variants (ref.). Logistic regression models adjusted for sex, age class (18–39, 40–49, 50–59, 60–69, and 70+), citizenship (Italian, foreigner), number of comorbidities (0, 1+), and vaccination status (unvaccinated; one dose; two doses 7–179 dbpt*; 2 doses ≥ 180 dbpt*; 3 doses <7 dbpt*; 3 doses ≥ 7 dbpt*). ^*dbpt, days before positive test.

Figure 3

Average adjusted predictions (AAPs) of (A) hospitalization and (B) recovery time in Omicron vs. other variants by vaccination status (unvaccinated; one dose; two doses 7–179 dbpt*; 2 doses ≥ 180 dbpt*; 3 doses <7 dbpt*; 3 doses ≥ 7 dbpt*). Models were adjusted for sex, age class (18–39, 40–49, 50–59, 60–69, and 70+), citizenship (Italian, foreigner), number of comorbidities (0, 1+), and number of symptoms (0–9). *dbpt, days before positive test.