New Zealand modelling gives insight into Australian virus numbers

If extrapolated to Australia, the figures suggest between 65,000 and 174,000 deaths, and 1.74 million hospitalisations. That extrapolation assumes the numbers are directly comparable - an assumption that has not been confirmed by authorities on either side of the Tasman.

The modelling shows how quickly hospitals could be overrun, and explains the thinking behind New Zealand's lockdown. It also gives insight into why Australia has imposed increasingly severe measures to prevent the spread of the virus.

New Zealand health authorities based their "plan-for" scenario on Australian modelling, their report said.

Australian authorities have so far refused to release their data, but deputy chief medical officer Paul Kelly said the work would be "unlocked" later this week. He did not give futher details.

The New Zealand modelling considers what would happen with an infection rate (R0) of 1.5, when each infected person infects 1.5 others, and an R0 of 2.2.

It considers scenarios where the country is 25 per cent successful and 50 per cent successful in controlling the outbreak. But it points out that more intense control is plausible, including people keeping at home and closing off parts of the country. China's experiences suggested severe controls could result in less spread, with a higher than 50 per cent effectiveness, the report says.

New Zealand has since locked the country down and told everyone to stay home.

"If substantially uncontrolled spread of disease occurs in NZ during 2020, our 'plan-for' scenario based on the modelling work by our Australian colleagues sees 65 per cent of the population (3,230,000) contracting a COVID-19 infection, though only 34 per cent of the population (1,680,000) would probably be symptomatic (typically mild disease)," the report says, pointing out that the number could well be overestimated given it was not yet winter and control measures could be more effective than anticipated.

"Indeed, our 'plan-for' scenario is somewhat towards "worse case", and the most likely outcome for NZ is probably less severe."

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The report says New Zealand's "plan-for" scenario assumes the higher infection rate, R0 of 2.2, and 25 per cent success in halting the spread.

If that played out, the epidemic would peak in week 21. Sixty-five per cent of the country would be infected, with just over half that number showing symptoms. That would translate to 8.6 million Australians showing symptoms.

About 20 per cent of symptomatic cases would be severe and end up in hospital - 1.7 million in the Australian context.

Between 4 and 4.7 per cent of symptomatic cases would need intensive care - up to about 400,000 people if the figures are extrapolated to the Australian population.

Deaths would range from 0.75 per cent to 2 per cent of symptomatic cases under the "plan-for" scenario.

This scenario, assuming 2 per cent of people with symptoms die, anticipates deaths in every age group other than 0-9, although the numbers are comparatively low under the age of 50, with fewer than 1000 deaths in under-50s in New Zealand. The big impacts start from 50 and accelerate, with the modelling suggesting that as many as 9.3 per cent of over 80-year-olds in New Zealand could die. Overall, it anticipates 0.68 per cent of the country's population could die - which equates to 2 per cent of symptomatic people.

The New Zealand data also assumes people can transmit the virus for 48 hours before they show symptoms; Australia's advice so far is that people can transmit 24 hours before symptoms manifest.

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