Herd immunity likely to be ‘made harder’ by variants

Jolyon Attwooll

6/07/2021 5:22:52 PM

New modelling has underlined the increasing scale of Australia’s vaccination challenge as it aims to open up to the world.

Graphic representing herd immunity
New COVID strains like the Delta variant of concern will make it more difficult for Australia to reach herd immunity.

Research carried out at James Cook University in Queensland suggests around 85% of Australia’s population will need to be vaccinated for herd immunity to occur.
Pressures on vaccine availability, the current vaccination rate and the prospect of increasingly infectious strains all have implications for the trigger points guiding the four-phase plan announced by Prime Minister Scott Morrison after a National Cabinet meeting last week.
Emma McBryde, Professor of Infectious Diseases Epidemiology and Modelling at JCU’s Australian Institute of Tropical Health and Medicine (AITHM) told the Australian Financial Review the plan appears to be based on achieving herd immunity.
However, Professor McBryde said that could be harder to achieve as both the Pfizer and AstraZeneca vaccines appear to be less effective against the more infectious Delta variant.
The modelling also suggests coverage directed at the most infectious ages could reach herd immunity with a vaccination rate of 75% of the population. However, this would require nearly 100% uptake for those aged 20–60 years old, a prospect deemed unrealistic.
‘I’m concerned that [the four-phase plan] is highly contingent on things that might not be possible,’ Professor McBryde said.  
The strategy put forward by National Cabinet hinges on vaccination levels being reached in the population, with the phases to be calculated by modelling carried out by the Doherty Institute – due to be confirmed this month.
Defining herd immunity
Dr Sean Stevens, a member of the RACGP’s COVID Working Group, agrees that variants are likely to have an impact.
‘It is one of the first principles of medicine that any infectious organism is going to mutate over time,’ he told newsGP. ‘You would expect it is going to make it more difficult to achieve herd immunity.
‘I think herd immunity is important. The question is, what do you mean by herd immunity? You could have 80% of the population vaccinated but if they are vaccinated with a vaccine that doesn’t protect from the circulating strain, is that herd immunity?’
This week the Israeli health ministry released preliminary results of a study indicating Pfizer’s efficacy against the Delta variant reduced compared to the protection it offered against the Alpha strain.
The study suggests its efficacy in preventing infections could be as low as 64%, although the reliability of the data has been queried, with Pfizer saying it is ‘yet to be fully assessed’.
The Israeli health ministry has also said that Pfizer is 93% effective at preventing hospitalisations and severe symptoms with the Delta variant.
A recent study published in The Lancet also indicates the Pfizer vaccine has diminished impact on the Delta vaccine when compared to strains identified previously.
‘Both the Oxford/AstraZeneca and Pfizer/BioNTech COVID-19 vaccines were effective in reducing the risk of SARS-CoV-2 infection and COVID-19 hospitalisation in people with the Delta VOC [variant of concern], but these effects on infection appeared to be diminished when compared to those with the Alpha VOC, the study authors wrote.
‘We had insufficient numbers of hospital admissions to compare between vaccines in this respect. The Oxford/AstraZeneca vaccine appeared less effective than the Pfizer/BioNTech vaccine in preventing SARS-CoV-2 infection in those with the Delta VOC.’
The authors said any assessment of the vaccines’ effectiveness should be interpreted cautiously ‘given the observational nature of these data’. 
Choice in September/ October?
In the meantime, Lieutenant General John Frewen, who heads the Federal Government’s COVID-19 taskforce, said the under-40s will have to wait at least two months before being able to access vaccinations other than AstraZeneca.
‘The forecasts we have got at the moment start to see real upticks in mRNA vaccines from September/October,’ he said on Tuesday, adding that his is ‘confident’ there will be a greater range of vaccines available to the under-40s from then.
However, he told reporters that a ‘war-gaming’ exercise carried out between the taskforce and states and territories has considered the potential impact of further restrictions on the supply chain.
There is also an agreement to focus more on parts of the rollout working most effectively, Lieutenant General Frewen said.  
‘One of the outcomes of the today is a commitment that we will need an enhanced way to assess progress of the rollout as we go, and then to be able to decide which of the pathways is working best and how we might be able to dynamically redistribute the vaccine as we go,’ he said.
Rollout rates required
While vaccination rates have increased over the past weeks in Australia, recent calculations suggest this may need to accelerate significantly if the Federal Government is to reach its target of offering everyone the chance to have a vaccine by the end of 2021.
Strategy consultants Provocate have said a total of 48.1 million vaccinations will need to be delivered by the end of the year to reach 85% coverage with two doses.
Their calculations suggest that there needs to be an average of 221,359 jabs in arms every day from now on for the targets to be reached – substantially more than the current seven-day average of approximately 130,000 doses per day.
The consultants believe stock limitations could mean the vaccination rate may need to rise even higher to 300,000 daily vaccinations from October to meet the targets – more than double the 113,000 average they have calculated for June.
As of Tuesday, there have been 8.4 million doses administered around the country, with 146,718 further injections carried out on Monday – below the record levels seen last week (the highest number was achieved on Thursday 1 July when 161,390 jabs were administered across the country).
A total of 71.74% of those over the age of 70 have now received at least one vaccine dose.
Slightly more than 31% of the eligible population has had one dose or more, with the full vaccination rate for eligible adults now standing at 9.43%.
More than half of vaccine doses administered in Australia to date have been delivered through primary care.
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