Delta more deadly than national plan assumes

The research team whose work underpins Australia’s pandemic exit strategy believes the Delta strain of the virus is significantly more deadly than they assumed when the national plan was adopted.

While the science has been settled for a while about Delta being more infectious than previous variants of concern, at the time a Doherty Institute-led team was commissioned to provide COVID modelling for national cabinet it was less clear whether it was also likely to cause more severe illness.

Scott Morrison releases the Doherty Institute modelling underpinning the national plan.Credit: Alex Ellinghausen

That debate appears at an end, with both the Doherty and the Burnet institutes adopting the findings of a Canadian study showing unvaccinated people infected with Delta are nearly twice as likely to end up in intensive care and one-and-a-half times more likely to die as people who caught the Alpha variant.

The Doherty modelling, prepared in July and updated last month, assumed Delta was no more virulent than Alpha.

University of Melbourne Professor James McCaw, a member of the Doherty team, confirmed that assumption would be ditched.

“At the time the Doherty work was done it was very clear that Delta spread more quickly and that is incorporated in the work, but the evidence that Delta was more severe in terms of clinical outcomes was either scant or certainly, unsettled in the international community,” he said on Tuesday.

“As scientists we update our understanding based on available evidence. It is now far more clear that Delta is more severe and so work and forecasts of potential hospitalisations, when we get to that stage, would account for the increased severity of Delta.”

The implications of this for the national plan, which advocates the staged lifting of lockdown restrictions once 70 per cent and 80 per cent of the population are fully vaccinated, are yet to be worked through.

The changed assumption about Delta, the COVID-19 strain driving the current outbreaks in NSW and Victoria, is already reflected in Burnet Institute modelling released on Sunday by the Andrews government showing that, even with a cautious easing of lockdown, demand on hospital and intensive care beds could outstrip supply and up to 3000 people may die from the virus.

Burnet Institute deputy director Margaret Hellard said she felt “comfortable” referencing the Canadian research, which was consistent with evidence emerging on the ground in Sydney and Melbourne hospitals.

“I am not saying we are absolutely right or absolutely wrong,” Professor Hellard said. “We have used the most reasonable information available to us.”

Although the overall death rate from COVID in Australia is lower this year than last year, that is due to the high levels of vaccination within vulnerable communities.

The Canadian study, by University of Toronto epidemiologists David Fisman and Ashleigh Tuite, looked at the outcomes of more than 200,000 people in Ontario infected with COVID in the first half of this year. At the time, Delta was an emerging variant but not yet the dominant strain of the North American epidemic.

The study found that people infected with Delta were twice as likely to end up in hospital, three times as likely to require intensive care and 2.32 times more likely to die from COVID than people infected with the original strain of the virus.

The study also compared the health outcomes of people with Delta to those infected with other variants of concern, such as the Alpha strain. The results showed people with Delta were 1.49 times more likely to be admitted to hospital, 1.86 times more likely to end up in ICU and 1.51 times more likely to die.

The Doherty team considered the newly published Canadian research when it was preparing its modelling for national cabinet but at the time, chose not to adopt its assumptions.

“Given recent emergence of Delta variants, there is presently very limited evidence of their severity relevant to antecedent strains,” the Doherty report noted.

“While early reports from Scotland and Canada suggest clinical outcomes might be worse than for Alpha variants, it is important to note that infections in these settings are skewed towards unvaccinated population groups in whom other risk determinants may also differ, potentially confounding and inflating early estimates of severity.”

Dr Freya Shearer, a member of the Doherty research team, confirmed the Canadian assumptions about the virulence of the Delta strain would be built into future modelling.

“That specific paper is part of an evidence review we have done in the last month that we would be incorporating into our analyses going forward,” she said.

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