Is the prospect of herd immunity still alive in Australia?

The possibility of herd immunity has been much debated since the onset of the pandemic.
 
According to one version of events, the UK based their initial strategy in early 2020 on achieving herd immunity – with catastrophic consequences.
 
With the virus largely suppressed in Australia until relatively recently, the discussion has perhaps not been as lively here so far.
 
When raised, the prospect has often been dismissed as a distant one, likely to be unobtainable without extremely wide vaccination coverage – and maybe not even then.
 
For non-COVID childhood immunisation, the Department of Health (DoH) states the need to aim high.
 
‘Australia’s national aspirational coverage target is 95%,’ it says on its website. ‘Reaching this aspirational target will give us enough herd immunity to stop the spread of measles and other vaccine-preventable diseases.’
 
For COVID-19, that sort of figure was not considered realistic a few months ago, but now vaccination rates have soared beyond expectations. In many places they are higher than 95% in the eligible population.
 
So has that changed the narrative for herd immunity? Could it be within the country’s grasp after all?
 
One expert who thinks so, albeit with caveats, is Associate Professor James Trauer, who heads the Epidemiological Modelling Unit for the School of Public Health and Preventive Medicine at Monash University.
 
Writing in The Conversation earlier this month about the current circulation of the virus in Sydney and Melbourne, he argued the difference in transmission rates is likely to be attributable to ‘structural factors’ such as climate.
 
He wrote that, while COVID outbreaks are inevitable, large cities ‘will likely remain at, or close to, herd immunity through some combination of transmission and vaccination’.
 
Developing the theme, Associate Professor Trauer said that despite the lifting of many restrictions, the transmission rates remained notably low.
 
In New South Wales, Victoria and the ACT, the three areas that have had the most significant presence of COVID-19 in the community so far, the effective reproduction number (Reff) has hovered below or close to one for most of the month so far. All, however, have had slight upticks in recent days.
 
‘New South Wales is probably the best example of a part of Australia that’s close to herd immunity,’ Associate Professor Trauer told newsGP.
 
‘They’ve made a huge difference to the epidemic through vaccination, clearly, and that’s been the major thing that’s turned it around in that state.
 
‘Maybe it’s not quite herd immunity, but it’s pretty close.’
 
He predicts a relatively consistent, but limited spread, in cities where the virus is already present, which he believes will contrast to a ‘patchy’ flaring in areas beyond the big cities that could be vulnerable to more severe outbreaks.
 
‘There could be gaps, but every so often cases will probably seed and cause a sporadic outbreak, which might be a bit more serious than the sort of continuing epidemic in a city like Melbourne,’ Associate Professor Trauer said.
 
Uncertainties ahead
The public health physician also acknowledges the importance of waning immunity and the role that boosters are likely to play in continuing to keep transmission rates down.
 
While the studies so far point to a very significant boost in immunity after a third dose, he says the durability of that protection will be key.
 
‘If vaccine-related protection against infection lasts maybe three months after the second dose, we can’t assume that is the same after the third dose,’ Associate Professor Trauer said.
 
He points to other vaccines, such as for Hepatitis B, where the standard regimen offers solid, long-lasting protection after three doses.
 
‘That’s one of the reasons why I don’t love the term booster because this implies we’re going to have to keep on coming back and have these things every year,’ Associate Professor Trauer said.
 
‘It might just be that three doses is the magic number that we need to get a really good level of protection.’
 
With infection rates now soaring in many parts of Europe, he is also conscious of a potential seasonal impact – an area he thinks is still poorly understood.
 
While he believes structural factors may explain the difference between Sydney and Melbourne, he says they have not been pinpointed precisely yet.
 
‘We don’t necessarily understand all of these things,’ Associate Professor Trauer said.
 
‘It’s hard to know whether it’s climate, whether it is sunlight, temperature, or humidity. We don’t really fully get it.’
 
Variants, children and a counterview
With understanding of the virus evolving all the time, its potential to take unexpected turns, such as the new variant currently emerging from southern Africa, remains a big threat.
 
For UNSW epidemiologist Professor Mary-Louise McLaws, one of the virus’ previous mutations is a reason herd immunity is unlikely to take hold.
 
‘It will be very hard to get [herd immunity] with COVID with Delta,’ she told newsGP.
 
‘And that’s because Delta has now developed a method of ensuring that it can infect children ... the idea of the adults protecting everybody else, it no longer holds.’
 
Professor McLaws contends that even with opening vaccination to 5–11-year-olds, as seems likely in the new year, the prospect of whole community protection will still not be attainable.
 
‘Herd immunity will remain elusive, because then you’ll have the 0–4[-year-olds],’ she said.  
 
‘But I do think that the risk once we start vaccinating the 5–11[-year-olds] will drop, and that will be a very good thing.’
 
Associate Professor Trauer also believes vaccinating this age group will have a public health benefit, but does not support a hard-line approach, saying any mandate would be ‘problematic’.
 
‘The tricky thing is that there is much less individual benefit for these kids,’ he said.
 
‘It’s reasonable to offer them one dose, in my view – particularly if they’ve never been infected before.’
 
‘Done incredibly well’
However the pandemic unfolds from here, Associate Professor Trauer believes there should be greater celebration of what has been achieved in Australia so far.
 
‘We do want to avoid complacency,’ he said. ‘But when we were first looking at the pandemic, we were guessing that we could have had about 100,000 deaths in the country.
 
‘We’ve probably already saved most of those lives, and by continuing to have an effective public health response, particularly with high vaccination coverage, we can be hopeful we’ll save even more.’
 
And even with the potential that a new variant of concern could impact current containment strategies, Associate Professor Trauer remains hopeful that Australia can avoid the high mortality rates seen elsewhere.
 
‘We’re mostly seeing daily death numbers in the single digits across the country, which is so much better than other countries have managed,’ he said.
 
‘Because of the protests, and the statements that people like Craig Kelly and Clive Palmer make, we seem to have lost a bit of the positivity.
 
‘Australia has done incredibly well.’
 
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COVID-19 herd immunity vaccine rollout