Observational studies from the United States, Scotland and Denmark have revealed reduced hospitalizations due to the Omicron COVID-19 variant as compared to Delta variant. It also revealed reduced emergency department (ED) visits due to Omicron. However, studies also observed that Omicron evaded third-dose vaccine effectiveness over a period of time. Two-thirds reduction in hospitalizations was observed in Scotland due to Omicron, according to a study led by University of Edinburgh researchers based on a test-negative case-control study of national COVID-19 infections among residents from November 1 to December 19, 2021.
The study which was published recently in The Lancet Infectious Diseases was aimed at assessing effectiveness of a third vaccine dose against symptomatic illness relative to 25 weeks or more after receipt of the second dose. By study end, 23,840 Omicron infections were reported, primarily among those 20 to 39 years old (49.2%). The proportion of possible Omicron reinfections was more than 10 times that of Delta cases (7.6% vs 0.7%).
For an adjusted observed-to-expected admission ratio of 0.32, fifteen patients infected with Omicron were hospitalized. A third COVID-19 vaccine dose was tied to a 57% reduction in the risk of symptomatic Omicron infection, compared with patients at least 25 weeks after the second dose. “These early national data suggest that omicron is associated with a two-thirds reduction in the risk of COVID-19 hospitalisation compared with delta,” the researchers wrote.”
Although offering the greatest protection against delta, the booster dose of vaccination offers substantial additional protection against the risk of symptomatic COVID-19 for omicron compared with 25 weeks or more after the second vaccine dose.” The authors noted that a combination of an increased risk of viral spread and Omicron immune evasion could mean that any benefit of reduced hospital admission rates could be exceeded by higher rates of infection in the community.
“Incorporation of our data on the risk for hospitalisation within modelling output could inform decisions by policy makers regarding the speed, range, nature, and duration of societal measures that otherwise would be needed to control the risk of spread of infection and minimise the risk of overwhelming health system capacity,” they wrote.
In the study in Denmark, also published in The Lancet Infectious Diseases, Statens Serum Institute researchers reviewed the 188,980 COVID-19 infections reported in the national COVID-19 surveillance system from November 21 to December 19, 2021. A total of 38,669 cases (20.5%) were caused by Omicron.
Relative to Delta, Omicron was linked to an adjusted relative risk (aRR) of hospitalization of 0.64, with 0.6% of Omicron patients admitted versus 1.5% of 150,311 Delta patients, for a 36% lower likelihood of hospitalization for Omicron relative to Delta. “We found a significantly lower risk of hospitalisation with omicron infection compared with delta infection among both vaccinated and unvaccinated individuals, suggesting an inherent reduced severity of omicron,” the researchers wrote.
