University of Western Australia
This article was consistent with advice from the WA Department of Health at the time of publication. The updated advice of the Australian Technical Advisory Group on Immunisation recommends the Pfizer vaccine is the preferred vaccine for adults under the age of 60.
Risk versus benefit modelling by researchers at The University of Western Australia has weighed up the potential of harm from an extremely rare AstraZeneca vaccine blood clot complication against COVID-19 ICU admission from not being vaccinated.
Associate Professor Matthew Linden from UWA’s School of Biomedical Sciences said the modelling had identified the implications for each adult age group of waiting out the months it might take Australia to secure further supplies of an alternative vaccine.
Based on Australia’s recent low risk of COVID exposure and a possible delay of 16 weeks for an alternative vaccine, the results suggested only under 50-year-olds should consider “holding out” for an alternative vaccine. For them, there was no clear benefit from AstraZeneca vaccination when virus transmission was low.
For those aged over 50, at greater risk of death and severe disease from COVID-19, there was a clear benefit in not waiting and being vaccinated with AstraZeneca, even when there was little-to-no community transmission.
However, the benefit of AstraZeneca was found to far outweigh the risk for all age groups in a high COVID exposure scenario – such as at the level seen during the peak of Victoria’s second wave.
“Our modelling suggests that even in the current low exposure environment for COVID in Australia for people over 50 the benefit clearly outweighs the risk and they need to have access to the AstraZeneca vaccine now - it is in their interest to do so."
Associate Professor Matthew Linden
The assessment was based on a model developed by the Winton Centre for Risk at Cambridge University, which takes into consideration the different consequences COVID-19 has for people in their 20s, 30s, 40s, 50s and 60s and the constant “very low risk” of serious vaccine harm from AstraZeneca.
Associate Professor Linden applied the different levels of exposure risk seen in Australia. The findings are based on the population as a whole and do not take into consideration individual risk factors.
“The AstraZeneca vaccine is very effective at preventing severe disease as a result of COVID, preventing the need for hospitalisation and limiting death,” he said.
“Our modelling suggests that even in the current low exposure environment for COVID in Australia for people over 50 the benefit clearly outweighs the risk and they need to have access to the AstraZeneca vaccine now - it is in their interest to do so.
“Death from COVID is common, in Australia it’s about three per cent and the risk of COVID requiring you to have an ICU admission increases dramatically once you cross that 50-year-old threshold.
“COVID itself has been associated with blood clots – blood clots are an extremely high risk for hospitalised COVID patients, while they are an extremely rare side effect of the vaccine.
“In Australia, we have a low exposure rate so the likelihood of you being exposed to COVID over a 16-week period while waiting for an alternative vaccine to become available is quite low, but even at that low level for someone over 50 there is more benefit from being protected by the AstraZeneca now than waiting out that period.
“In our low exposure modelling, the benefits of AstraZeneca only clearly outweighed the risks for people over 50, so it makes sense for them to have the vaccine now and for people under 50 to hold out for an alternative vaccine - unless they are at a higher exposure risk to COVID-19 perhaps due to their profession, or they are at higher risk of death or disability if they were to be infected.”
Associate Professor Linden said the longer the delay in securing an alternative vaccine – beyond the 16-week period used in the modelling – the more the risks and benefits “tipped in favour” of taking the AstraZeneca vaccine.
“It is important to note that when you calculate exposure over a 16-week period and that period is followed by another 16-week period of exposure, the risk of clots do not increase because you are only taking the vaccine once,” he said.
“COVID remains well managed in Australia and well managed in WA in particular. However, we see it regularly re-enters the community even with the world’s best practices to prevent it. As we move to open state and international borders, the risk of exposure increases, and so does the benefit of vaccination.”
Associate Professor Linden said when West Australians were evaluating their own personal risk benefit for taking the AstraZeneca vaccine, they should have a discussion with their GP or specialist.
Media references
Simone Hewett, UWA Media & PR Manager, 08 6488 3229 / 0432 637 716