Omicron less severe than Delta but more easily evades boosters

COVID vaccinator with syringes

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.

36% lower odds of a hospital stay

In the study in Denmark, also published in The Lancet Infectious Diseases, Statens Serum Institut researchers reviewed the 188,980 COVID-19 infections reported in the national COVID-19 surveillance system from Nov 21 to Dec 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.

Among the 124,313 Danes (65.8%) who received two doses of COVID-19 vaccine, the aRR of hospitalization for Omicron infection was 0.24, much lower than that among those who were unvaccinated or had received only one dose.

In a similar comparison by vaccination status, the RR of hospitalization was 0.57 among patients who received no or only one dose of vaccine, while it was 0.71 among those who received two doses, and 0.50 among recipients of three doses.

"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. "Our results could guide modelling of the effect of the ongoing global omicron wave and thus health-care system preparedness."

The study was also presented at the European Congress of Clinical Microbiology and Infectious Diseases, which ends tomorrow in Portugal.

Importance of ongoing COVID surveillance

In a commentary on both studies, Cornelia Adlhoch, DVM, PhD, and Helena de Carvalho Gomes, MD, MPH, both of the European Centre for Disease Prevention and Control, underscored the critical nature of ongoing COVID-19 surveillance systems that enable studies such as the ones from Scotland and Denmark.

"How long will countries be able to keep such comprehensive systems active for SARS-CoV-2 surveillance?" they asked. "Many countries in Europe are already reducing testing availability and limiting sequencing for various reasons, including availability of resources."

They called for definition of the type and level of detail needed for data on a local, national, regional, and global level to enable preparation for the next SARS-CoV-2 variant and future pandemic threats.

Waning efficacy of 3 vaccine doses against severe Omicron

The US study, conducted by Kaiser Permanente Southern California and Pfizer researchers, estimated the effectiveness and duration of two and three doses of Pfizer's COVID-19 vaccine against Delta and Omicron infections among adult Californians from Dec 1, 2021, to Feb 6, 2022. The study was published in The Lancet Respiratory Medicine.

The analysis included 11,123 hospital admissions and ED visits. Effectiveness of two doses of the Pfizer vaccine against Omicron was 41% against hospitalization and 31% against an ED visit at least 9 months after the second dose.

After three doses, effectiveness against Omicron hospitalization rose to 85% within 3 months but declined to 55% afterward. Similarly, effectiveness against Omicron ED admission was 77% within 3 months but dropped to 53% thereafter.

"Trends in waning against SARS-CoV-2 outcomes due to the delta variant were generally similar, but with higher effectiveness estimates at each timepoint than those seen for the omicron variant," the authors wrote.

In a Kaiser Permanente news release, lead author Sara Tartof, PhD, said that while three doses of vaccine were substantially more effective than two doses against Omicron, they were less effective against Omicron than against Delta or other SARS-CoV-2 strains.

"Additional doses of current, adapted, or novel COVID-19 vaccines may be needed to maintain high levels of protection against subsequent waves of COVID-19 caused by omicron or future variants with similar potential to escape protection," she said.

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