The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) Omicron variant was first identified in South Africa and reported to the World Health Organization (WHO) on November 24, 2021. It was responsible for the sudden rise of cases in the USA in December 2021. The Omicron variant replaced its predecessor Delta variant, in a short span of only three weeks.
Study: A Quick Displacement of the SARS-CoV-2 variant Delta with Omicron: Unprecedented Spike in COVID-19 Cases Associated with Fewer Admissions and Comparable Upper Respiratory Viral Loads. Image Credit: G.Tbov/Shutterstock
A new study has been published on the medRxiv* preprint server, which studied whether clinical outcomes and viral loads differed between the Delta and Omicron variant infections during the period when both variants were co-circulating.
The unprecedented increase in coronavirus disease 2019 (COVID-19) cases caused by the Omicron variant triggered many concerns. It is important to understand the disease severity associated with Omicron infections and the virological determinants that contributed to its rapid and widespread predominance.
In a previous study in South Africa, Omicron infected patients showed a significant reduction in severe disease compared to Delta. In another study from Switzerland, researchers analyzed 18 Omicron samples from vaccinated individuals to show that Omicron had equivalent infectious viral titers compared to Delta. A third study showed that the peak viral RNA in Omicron infections was lower than the Delta variant.
However, no study assessed the clinical discrepancies of Omicron and Delta infected patients from the US. Prior studies also did not assess the viral loads and conduct cell culture studies to document the characteristics of both variants, stratified by vaccination status.
A new study
Researchers collected clinical specimens from patients who tested positive for SARS-CoV-2 between the last week of November and the end of December 2021. These samples were used for whole viral genome sequencing. Subsequently, the cycle threshold values (Ct) for viral RNA, the presence of infectious virus, and levels of respiratory IgG were measured, and clinical outcomes were obtained. Lastly, scientists compared the differences in each measure between variants stratified by vaccination status.
The Omicron variant displaced the Delta variant rapidly during the study period. More specifically, the transition happened in a short span of three weeks, by which time Omicron constituted 95% of the circulating lineages by the end of December 2021. One thousand one hundred twelve patients were infected with the Omicron variant and 910 by the Delta variant. It was observed that patients infected with Omicron were more likely to be vaccinated compared to patients with Delta but were less likely to be hospitalized, requiring ICU level care, or succumb to infection.
Scientists did not observe any significant difference in Ct values based on the lineage, irrespective of the vaccination status. In boosted patients, the recovery of infectious virus in cell culture was reduced, compared to fully vaccinated individuals without a booster dose of the vaccine and the unvaccinated, in the case of the Delta variant. However, no such difference was observed for the Omicron variant.
Overall, the Omicron variant has been associated with a significant increase in infections in fully and booster-vaccinated individuals but with fewer admissions and ICU-level care. Compared to the Delta variant, the admitted patients required similar amounts of oxygen and ICU level care. Viral loads were also similar in Omicron and Delta infected patients, regardless of the vaccination status.
One of the limitations of the current study is the retrospective nature of data collection which precluded the collection of baseline serum and respiratory IgG levels. Researchers also did not quantify viruses from clinical samples or perform antibody neutralization assays as part of this study. Further, the clinical data were compiled based on patients admitted to the Johns Hopkins Health System. It could well be the case that individuals tested positive in this system and sought healthcare elsewhere. Such individuals were not captured in this study, introducing a bias.
The current study showed a marked decrease in disease severity with Omicron compared to the Delta variant. However, an important finding was that Omicron led to significant increases in infections among fully vaccinated and individuals who had received the booster vaccination. There were no substantial differences in supplementary oxygen and ICU level care among the patients hospitalized post-infection with the Omicron and Delta variants. These results suggest that Omicron infections should not be underestimated and highlight the importance of taking infection control measures and raising public awareness.
medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.