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18 August 2021

James Cook University scientists have developed a new modelling tool, which shows Australia needs to vaccinate at least 85 per cent of the population to achieve herd immunity.

As of July 1, about 6.2 million people have received at least one dose of vaccine, and just over 1.72 million, had received their second dose.

Australian Institute of Tropical Health and Medicine’s Professor of Infectious Diseases Epidemiology and Modelling Emma McBryde, said modelling had implications for the Federal Government’s national roll-out announced on July 3, 2021, to transition from suppression of COVID to a strategy of reopening and a return to normal life.

Professor McBryde’s group, which includes modellers from JCU, Monash University and the Australian National University, has developed a tool that would provide an optimal strategy to achieve herd immunity.

“Our modelling of the current plan with the delta virus suggests that we need around 85 per cent, or 21.5 million Australians, to be vaccinated to achieve herd immunity,” she said.     

Australian Prime Minister Scott Morrison has called for ‘good modelling’ which was needed in order to enter phase 2, of a “post vaccination phase" where Australia would need to reach a "threshold of vaccination" with a scientific, not political number.

“We have investigated herd immunity thresholds and estimated optimised vaccine distribution around the world for the original Wuhan strain (5), and have now repeated the exercise for the Delta variant for Australia.

“The first hurdle is to vaccinate to achieve herd immunity, however herd immunity has become more difficult to achieve with the Delta variant, as it is both more infectious and less amenable to vaccination,” said Professor McBryde. 

She said the Australian Institute of Tropical Health and Medicine’s modelling team was working with countries around our region to investigate herd immunity thresholds and optimise vaccine distribution around the world for the original Wuhan strain and have now repeated the exercise for the Delta variant.

“We show that assuming a reproduction number (the number of people infected by someone carrying the virus) for the Delta variant of 5, we would need to vaccinate 85% of the Australian population using the current strategy,” said Professor McBryde.

She said if vaccine coverage followed the current mixed strategy (AstraZeneca in the over 60s and Pfizer in under 60s) Australia could achieve herd immunity by vaccinating 85% of the Australian population, however this may not be achieved.

We have also shown that even without herd immunity, vaccinated people are protected against severe disease and much less likely to be hospitalised or die, said Professor McBryde.

Australia must begin to provide incentive following The US Centre for Disease Control and Prevention which now says if you are fully vaccinated, you can resume activities that you did prior to the pandemic, including travel.

She said the AITHM team concluded the Delta variant makes achieving herd immunity extremely challenging.

“But it’s important to remember that both vaccinated people and unvaccinated people will still have a reduced risk of infection and disease as Australia gets closer to herd immunity.”

A number of our assumptions about the emerging variants are uncertain, in particular just how infectious they are. Also we are not sure what the vaccine uptake will be or future age of vaccine eligibility. For this reason, we have developed an online tool which can be modified for different settings and updated as new data become available.

Check out the COVID-19 Vaccination modelling tool Professor McBryde's team has developed here:

Read more in Science Direct: Vaccines and variants: Modelling insights into emerging issues in COVID-19 epidemiology

and Vaccines and variants: Modelling insights into emerging issues in COVID-19 epidemiology. Paediatric Respiratory Reviews. doi:10.1016/j.prrv.2021.07.002

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