By Professor George Milne

Early Delta variant modelling results by researchers at The University of Western Australia indicate that Australia will need to continue to rely on hard lockdowns to control COVID-19 outbreaks until vaccination coverage is greater than 90 per cent.

Professor George Milne from the Department of Computer Science and Software Engineering and his team conducted computer modelling experiments to analyse a range of vaccination strategies against the COVID-19 Delta variant.

The study found that to arrest a Delta variant outbreak, as is occurring in Sydney, it was necessary to vaccinate 12-to-17 year olds in addition to adults, to vaccinate at least 90 per cent of these age groups, and to give a third Pfizer dose to boost immunity in those previously vaccinated with the AstraZeneca vaccine.

Such high vaccination rates would prevent a Delta variant epidemic occurring without the need for strict lockdown measures. With lower vaccination rates, and without the Pfizer boost, lockdown measures would be required.

The modelling is based on Australia’s current vaccination policy, Pfizer for those under 59 years, and AstraZeneca for 60 and above, with the Pfizer vaccine having assumed 88 per cent effectiveness against Delta transmission, and AstraZeneca 60 per cent.

Professor Milne said the higher transmissibility of the Delta variant, and the fact that the AstraZeneca was significantly less effective against Delta when compared to earlier SARS-CoV-2 strains, meant  vaccination coverage needed to be far higher than their original COVID-19 vaccination modelling showed.

“Our prior vaccination modelling, based on the Wuhan and Alpha strains, showed that lockdown could be avoided at an achievable 70 per cent coverage,” Professor Milne said.

”But with a Delta variant outbreak and no rapid activation of lockdown measures, even at 90 per cent vaccine coverage in those aged 12 and older, we estimate Australia’s current vaccination approach may result in over 100,000 COVID-19 cases, if an outbreak similar to that which occurred in greater Melbourne last year occurred in its population of 5.1 million.”

The modelling also indicated that giving a third ”booster” dose of the Pfizer vaccine to those previously vaccinated with the AstraZeneca vaccine, would significantly reduce the scale of such a Delta variant outbreak.

This vaccination strategy was shown to reduce case numbers during future COVID-19 Delta outbreaks to a “manageable” level at 90 per cent vaccination coverage, without the need for lockdown. Importantly, application of this immunity-boosting strategy was also shown to reduce infections in the elderly by more than 87 per cent, when compared with Australia’s current, age-specific, vaccine allocation program.

Professor Milne said the results suggest that Australia would need to adopt such a third dose Pfizer vaccination approach in those previously vaccinated with AstraZeneca.

“Recent trial data coming from Europe suggests that using the Pfizer vaccine as a second dose to those who have had a first dose of AstraZeneca, appears highly effective in creating immunity in those vaccinated,” he said.

”It is realistic to assume that this increased immunity effect will also be achieved by a three dose boosting approach, two AstraZenca doses followed by a Pfizer booster.

”This is a strategy that Australia should strongly consider adopting, to more rapidly increase immunity levels within our population.

”As Australia will not satisfy the above conditions until mid-2022, if at all, strict Stage 4 lockdown measures are required, and these measures need to be activated as soon as community transmission of the Delta variant is detected. This has not occurred in Sydney.”

The UWA COVID-19 modelling research, carried out by Professor Milne’s team, is funded by the Western Australia Government’s Future Health Research and Innovation (FHRI) Fund Focus Grant program.

The aim of the UWA research is to conduct modelling analyses to determine effective and cost-effective responses to infectious disease epidemics and pandemics, specifically with application to the COVID-19 pandemic.

(Professor George Milne is Professor at University of Western Australia)


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