Two studies from Scotland and England analyzing rates of hospital treatment for those infected with Omicron suggest the coronavirus variant may cause less severe disease than previous variants, although scientists caution more research is needed.
Early data from Scotland indicates that there is a two-thirds reduction in hospitalization of double vaccinated young adults, compared with the Delta variant. In addition, data from England signals that Omicron infection is associated with a 15 to 20 percent reduced risk in requiring any hospital treatment, compared with Delta.
However, Neil Ferguson, director of the Centre for Global Infectious Disease Analysis at Imperial College London, cautioned that comparing the Scottish and English data would not be comparing apples with apples since they analyze different groups.
The Scottish data looks at people admitted to hospital, whereas the English data takes in more cases, including people who visit an accident and emergency department for just a few hours of monitoring before going home, he told journalists in a briefing Wednesday.
While limited, the data provide a first sign that Omicron may not be as virulent as the Delta strain. These datasets will be followed up over the coming weeks and will provide a clearer picture of the true risks of severe disease as time goes, the researchers said.
In the meantime, given how transmissible Omicron is, they urged that existing measures to curb the rates of infection are followed. Even if fewer people become seriously ill, with very high numbers of cases, those in need of hospital treatment could still overwhelm the health system.
These are the first studies to report on analysis of hospitalizations with Omicron; the U.K. Health Security Agency is also expected to release its data indicating similar results before Christmas.
British Prime Minister Boris Johnson on Tuesday ruled out additional restrictions before Christmas but said his government was keeping the data “under review.”
Preliminary results from Scotland, drawn from linked vaccination and hospitalization data for the entire Scottish population, have shown a 56 percent reduction in hospitalization in people aged 20 to 50 years with Omicron infection, compared with what would be expected with the Delta variant. Given the limitations of the data, the true value lies somewhere between 30 to 75 percent for this age group, the researchers pointed out.
Across all ages, the data suggests that Omicron is associated with a two-thirds reduction in the risk of COVID-19 hospitalization when compared to Delta. However, there have been very few cases of Omicron recorded in older populations, which skews the results.
With age being a risk factor for hospitalization for prior variants including Delta, it is not clear from this study if the lower hospitalization rates in older people are due to them being highly boosted with a third vaccine dose or if the variant causes milder disease.
The study also has some major statistical caveats. It assumes the time to hospital admission is the same for both variants — between five to 10 days after a positive test. If it turns out that Omicron takes more days before patients are hospitalized, then this study undercounts Omicron hospital cases. It also does not take into account if there is waning immunity from boosters.
Mark Woolhouse, professor of infectious disease epidemiology at the University of Edinburgh, told journalists in a briefing the data “only takes us so far, but it definitely takes us in the right direction.” He also cautioned that the size of the Omicron wave is “still the big unknown.”
In a second study, researchers at Imperial College London have analyzed data in England that included 56,000 cases of Omicron and 269,000 cases of Delta.
Their data suggests that individuals infected with Omicron are 15 to 20 percent less likely to attend hospital overall, and 40 to 45 percent less likely to be hospitalized for a night or more, compared with Delta cases.
The researchers also analyzed the impact of prior infection and found that those infected with Omicron after a previous infection were 50 to 60 percent less likely to be hospitalized, compared with people with no previous documented infection.
The researchers caution that insufficient time has passed since Omicron was first detected in England to analyze more severe outcomes such as intensive care unit admission or death.
“It is one step towards understanding the severity of Omicron, not the final analysis,” Ferguson said, adding: “It’s clearly good news to a degree.”
These datasets have been shared with the scientific modeling groups, which subsequently make recommendations to the government based on their projections.