Three new observational studies from Scotland, Denmark, and the United States detail reduced hospitalizations and emergency department (ED) visits for Omicron COVID-19 infections relative to those caused by the Delta variant, as well as strong but waning third-dose vaccine effectiveness over time against Omicron.

Two-thirds reduction in hospitalizations

In Scotland, a team led by University of Edinburgh researchers conducted a test-negative case-control study of national COVID-19 infections among residents from Nov 1 to Dec 19, 2021, to estimate effectiveness of a third vaccine dose against symptomatic illness relative to that 25 weeks or more after receipt of the second dose. The study was published late last week in The Lancet Infectious Diseases.

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%).

Fifteen patients infected with Omicron were hospitalized, for an adjusted observed-to-expected admission ratio of 0.32. 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.

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