The latest COVID-19 outbreak appears to be largely sweeping through Australians who have not yet had the disease.
Only one in five people who reported an infection in December were known to have had COVID-19 previously, according to the ACT government.
While data is unavailable for most of the country, the numbers from the ACT suggest that past exposure to the disease provides some protection against the prevailing strains.
This contrasts with widely publicised concerns that newer COVID-19 subvariants are highly effective at evading the body's immune defences.
The ACT is one of few jurisdictions that reports its reinfection rate, though its outbreaks largely mirrored those in New South Wales and Victoria last year.
About 46 per cent of Australians had already had COVID-19 by June last year, according to an analysis of antibodies in blood samples.
That proportion is almost certainly far higher today. This, combined with the ACT data, indicates a previous infection is associated with avoiding infection during this latest surge.
However, any protective effect that exists now is unlikely to last, as immunity wanes and the disease evolves.
Infectious diseases specialist Sanjaya Senanayake, an associate professor at the Australian National University, said the ACT data reflected what was happening overseas.
Singapore, for example, was also reporting that about 20 per cent of known new cases were reinfections.
Dr Senanayake said this relatively small number of reinfections showed that immunity — whether from vaccines or past exposure — was working for most people.
"It's hard to differentiate, in a place like Australia, between purely vaccine-induced immunity and infection-induced immunity, because many of us have both been COVID-19-infected and have had [several] vaccines," he said.
"This is hybrid immunity we're seeing.
"And what it tells us is that, even though in laboratory settings … these new subvariants have the potential to evade the immune system, this hybrid immunity is providing good protection."
From pandemic to endemic disease
Having had COVID-19 before will not protect everyone from reinfection this summer — Prime Minister Anthony Albanese, who got the disease a second time last month, is one high-profile exception among many.
Older and/or chronically unwell people are more likely to be reinfected.
So are health workers and other people who are exposed to coronavirus on a regular basis.
Dr Senanayake said the virus had changed significantly over the past three years and would continue to change.
But he said this progression appeared to mostly positive.
"We as the hosts have also changed, because of this hybrid immunity," he said.
"That is going to lead to a situation where this virus will become endemic."
In September last year, World Health Organization director-general Tedros Adhanom Ghebreyesus declared that the pandemic's "end is in sight", but warned countries to maintain efforts to control outbreaks and vaccinate their populations.
Dr Senanayake agreed that the endemic period — when COVID-19 "is part of our normal lives, just like flu is part of our normal lives" — was approaching, but was not quite here.
He said endemic COVID-19 would still pose a life-threatening risk to some, but most people would likely experience very mild or asymptomatic infections.
"That's reassuring because that means that, even if they become infected, it doesn't impact on their personal health, they don't end up in hospital," he said.
"We know from data in places like Portugal, they found that even infection with the very early COVID-19 variants provide you with some protection against the later [Omicron variants].
"So with antivirals in place to protect our most vulnerable and better vaccines available, and this hybrid immunity, I think we're heading in the right direction with COVID-19."
