Experts consider limitations of Doherty Institute modelling as COVID case numbers climb

By Melissa Coade

Thursday August 5, 2021

(Image: Adobe/James Thew)

Experts have underscored the limitations of modelling underpinning Australia’s COVID response plan, noting there are risks that demand for the vaccine could outstrip supply, and the difficulty of measuring vaccine effects on transmission.

With people in South East Queensland days into what authorities hope will be another short lockdown, and Sydney gunning for 6 million people to be vaccinated by the end of August as three more local government areas are subject to hard lockdown, the pressure is on to get everybody through one of Australia’s most trying times in the pandemic so far.

Responding to government decision-making about what action it will take to combat outbreaks of the Delta variant across the country, infectious diseases and vaccine expert Professor Robert Booy said some ‘outside the box’ thinking was needed to improve how accessible the jab was to people who needed it most.

Booy, who is an honorary professor at the University of Sydney and has consulted for vaccine manufacturers CSL, GlaxoSmithKline, Merck Sharp & Dohme and Sanofi Pasteur, said that as circumstances changed in Australia, the threat the virus posed to people in their 30s would increase.

“The risk of dying from COVID-19 in your 30s is four times higher than for a teenager, the risk of dying from COVID-19 in your 50s is almost 40 times higher than a teenager,” Booy said.

“The priority remains to get at-risk people vaccinated rapidly.”

The government call for more young people in NSW to step forward and get a shot was met with a barrage of messages from frustrated citizens who claim they either cannot get the online booking system to work, or feel unfairly directed to receive a vaccine when other age groups are eligible for the mRNA Pfizer shot. 

“The COVID-19 vaccine eligibility checker [is] still unable to provide for ALL. Few [people] around got confused and just wait with no hope,” Twitter user @thamtana1 wrote.

Another person, @jack_csmith, responded to the NSW Health tweet by asking the government not to ‘peer pressure young people’ into getting the AstraZeneca shot when the state was not locked down properly.

On Wednesday NSW Health reported that two people had died with COVID-19 overnight, including a 27-year-old man from Liverpool. The other death was a woman in her 80s. 

NSW premier Gladys Berejiklian said that the death of the 27-year-old showed how deadly the virus could be for younger groups in the community. She appealed to more people to get vaccinated and confirmed that while the state had achieved four million shots so far, she hoped to see that number reach another two million by the end of the month. 

“Sadly, a male in his 20s passed away at home with COVID from southwestern Sydney, overnight. Obviously, our deepest condolences to his family and loved ones,” the premier said. 

“It demonstrates again, how this disease is lethal, how it affects people of all ages. Unfortunately, the gentleman wasn’t vaccinated and, as we understand it, the death happened quite suddenly.”

“It’s so important for the community to come forward and get vaccinated,” Berejiklian added.

To date, a total of 925 people have died in Australia with COVID-19 since the start of the global pandemic.

Time to consider a change to vaccination approach?

Professor Booy offered a scenario where on the last day of August, two patients aged 30 and 50 were booked in for the vaccine.

“If there was only one COVID-19 vaccine left, who should get the shot?” he asked.

Do you vaccinate the 30-year-old to prevent transmission better or do you vaccinate the 50-year-old who is at greater risk of disease?

“The person who is 30 may be a schoolteacher or a delivery person for essential food. Such professions are important to be vaccinated. But the decision to make by professionals is clearly nuanced.”

Booy noted that teenagers were ‘at very low risk of complications’. For healthy children aged 12 to 15-years, there was presently no recommendation for routine vaccination, he said. 

“We need to think more outside the box. Efforts to dramatically improve supply need to continue so that current demand and need can be met.” 

At Tuesday’ press conference with the prime minister on Tuesday, the Doherty Institute’s director of epidemiology Professor Jodie McVernon also acknowledged the role people played in transmitting the virus. She said modelling recommended a strategy that brought forward the immunisation of people aged 30-39 to the beginning of September, and those aged 16 to 29 to early in October 2021.

Immunising younger people, reducing transmission is critical to maximising the gains of a whole population program,” McVernon said.

Professor McVernon noted that extensive scientific research was still being undertaken to better understand the Delta strain, but endorsed the effectiveness of both the Pfizer and AstraZeneca doses as effective vaccines to help control spread in the community.

“We are still gathering evidence around the world about vaccine effectiveness against this strain, and in a whole range of population settings that are quite different from ours, where different proportions of the population are vaccinated and different measures have been in place and case loads from the beginning are very different,” McVernon said. 

“But, basically, what we are delighted to see and what really supports the use of both of the vaccines that we have in Australia is that both the Pfizer and AstraZeneca doses are highly effective at reducing severe disease outcomes following two doses.”

Assumptions limit modelling parameters

Professor Nikolai Petrovsky from Flinders University, who is also a research director at Vaxine Pty Ltd (which is currently developing a COVID-19 vaccine), offered the reminder that model predictions were only as good as their assumptions. In his view, because vaccine effects on transmission were difficult to measure, the modelling should be considered as no better than ‘an educated guess’.

“Any modelling result needs to be treated with a healthy degree of scepticism. A key issue is the data this model is relying upon to predict the size of any impact of vaccination on virus transmission,” Petrovsky said. 

He added that a sensitivity analysis of the modelling would be useful, which could show the degree to which results varied if the value for ‘vaccine effect on transmissibility’ was the same as the value for ‘vaccine effect on virus infection’.

“This model is likely to be highly sensitive to the input value of this vaccine transmission-reducing effect, which seems high and is based on just a single study result,” Petrovsky said.

“It would be usual in such situations to do a sensitivity analysis where the value of this key input is varied to explore to what degree this could affect the modelling result. It would be good to see this done here.”

Citing modelling produced by respected institutions from the London School of Hygiene and other groups in early 2020 which turned out to be wrong, Professor Petrovsky warned that the governments have previously made ‘disastrous public health decisions’ based on models that were flawed.

He also cautioned that before policy decisions were made based on the Doherty Institute modelling, that careful attention should be paid to how effective vaccines were against the Delta strain of the virus. 

Recent COVID-19 outbreaks have occurred in populations with high vaccination coverage approximating those predicted by the model.  

“With delta strain achieving exceptionally high transmission rates even amongst the vaccinated, the model and its assumptions need to be closely examined, before such data is relied upon to make any policy decisions,” Petrovsky said.


Doherty Institute modelling for COVID Delta variant made public

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