‘Inherently milder’: Australian state offers clues on Omicron

Hospital data from the Australian state of New South Wales (NSW) offers tentative evidence the Omicron variant is milder than previous coronavirus strains.

Early admission figures suggest Omicron could be half as likely to put people in hospital as the Delta variant, according to an analysis by Andrew Lilley, an economist at Diem Association.

Lilley, a PhD candidate at Harvard University, estimated the hospitalisation rate for COVID-19 had fallen from 6.9 percent to 3.6 percent in recent weeks as Omicron became the dominant strain in the state.

Lilley, who created a “hazard rate” for admissions using official statistics on hospital stays, told Al Jazeera he was “quite confident” the strain is inherently milder, although it would take more time to know the true hospitalisation rate for the mutation.

“The exact percentage is hard to estimate before we receive more data on the increase in cases that are breakthroughs,” said Lilley, who published his initial analysis on social media on Tuesday. “But it is extremely likely to be less virulent.”

Lilley, whose analysis has not been peer-reviewed, said the data suggested the less severe illness associated with Omicron is not only because it is more likely to reinfect people who have already had COVID-19.

Although a growing body of evidence from South Africa, Denmark and UK shows Omicron results in milder disease, scientists are divided on whether that is due to the variant being inherently milder or because it is more often associated with breakthrough infections.

‘Perfect laboratory’

New South Wales is considered a fruitful environment for observing the severity of the variant due to the low number of previously infected people, the steady number of Delta cases in recent weeks that makes for a stable baseline, and a low vaccine booster rate.

About 80 percent of cases in the state, where 94 percent of adults more than 16 years of age are double-vaccinated, are believed to be Omicron, according to health authorities.

“New South Wales is the perfect laboratory because we know that selectivity towards reinfections is not a factor in the lower hospitalisation rate for Omicron and because vaccination rates were so high that most Delta cases were already breakthroughs,” Lilley said.

The state reported 5,715 new COVID-19 cases on Thursday, the most of any Australian jurisdiction since the pandemic began, but only 347 patients in hospital and 45 in intensive care.

Catherine Bennett, a public health expert and epidemiologist at Deakin University in Melbourne, told Al Jazeera the analysis was “encouraging” although it was still “early days.”

“A lot of our early cases were linked to spreader events and so 20-30 year olds were over-represented; that will lead to lower hospitalisations rates anyway,” Bennett said.

“But arguably [it’s the] same for recent Delta cases too, and they will be spreading the virus to older contacts now as well without knowing. So the longer this trend continues, the more reassuring it is.”

Alexandra Martiniuk, a professor of epidemiology at the University of Sydney’s Faculty of Medicine and Health, told Al Jazeera that the many factors affecting hospitalisation rates made it difficult to draw conclusions about the severity of Omicron at this point.

“Until we have more data on fully vaccinated cases and their potential need for hospital, as well until we see more Omicron cases in the older age groups and in the unvaccinated, it will remain challenging to make any stable declarations about the case hospitalisation rate for Omicron in NSW,” Martiniuk said.