The COVID-19 pandemic won't become an endemic for a long time, viral immunologist Nathan Bartlett says

"We're still very much trying to understand how this virus will behave in the long term," said Associate Professor Bartlett, of HMRI and the University of Newcastle.

"At the moment we're still in a game of tug-of-war with this virus, as we work towards a high level of sustained immunity in the population."

He said it may take several years for immunity to "slowly but surely be established and maintained".

"If or when it becomes an endemic virus is still very unclear."

He said the push for immunity in the population was essentially a war with the virus, amid battles for the number of cases and severe disease to slowly fall.

"You eventually get to an equilibrium where COVID-19 would almost always be a mild infection, where everyone has some level of immunity and it's more like the flu by that stage.

"We're still a long way from there, unfortunately. There's a long way to go."

Dr Bartlett said an endemic meant "a relatively predictable level of infection within a given geographical area".

For example, we know when flu season peaks.

"Some seasons are worse than others, depending on how the virus changes. But we all have some level of persistent immunity to flu, which is boosted by the flu vaccine."

The flu vaccine is updated annually to give the best possible protection, which is a particularly important measure for the elderly and those with chronic diseases.

There were many endemic respiratory viruses - including the other coronaviruses - that "we all get infected with early in life and repeatedly throughout life".

For a virus to become truly endemic, infection starts at birth.

"All respiratory viruses infect people as infants and children. They get lots of infections, but children's immune systems are such that they don't usually get seriously ill with these respiratory viruses. COVID is a great example of that.

"Kids get lots of respiratory viruses, but they won't get super ill except in very rare cases - a lot less rarely than older adults, certainly."

When children get infected with endemic viruses, the immune system "establishes a level of baseline immunity".

"Of course, that doesn't happen when a new virus introduces itself into a population. The majority of the population are adults.

"And so now you have an adult immune system faced with a new virus. That's a very different equation in terms of the sort of disease profile that carries.

"This is why the elderly and even middle-aged adults are very vulnerable to COVID."

Nevertheless, he said COVID can cause serious illness and death in some younger people "because we haven't established any level of immunity from birth because the virus wasn't endemic".

"If this virus is still circulating for years to come, babies will be exposed to it from birth and carry that immune protection into adulthood and indeed it will be a true endemic virus.

"It won't kill you, you'll probably get infected multiple times as a child and develop a level of immunity. So when you do reach adulthood, it's not going to whack you around as much."

Globally, governments have managed the pandemic as a balance between protecting people from the virus and protecting economies, amid struggles between individual rights and the common good.

Asked if he thought we'd been too accepting in NSW of harsh restrictions, Dr Bartlett said: "We had no choice".

"We had a window of opportunity in early 2021 to get everyone vaccinated before winter. Obviously that didn't happen."

As Delta spread in Sydney and elsewhere, NSW cities and regions entered various levels of lockdown.

"It really couldn't go any other way because how else could you protect everyone until there was a high level of vaccine-induced immune protection?"

And while COVID posed a serious public health risk, lockdown came with rises in unemployment and isolation that also posed health risks.

The questions for governments became about how much risk should be taken.

"Risk equations can go on and on. I totally get the frustration around this," Dr Bartlett said.

"But it is largely counterproductive and doesn't get you anywhere because the risk moves as the virus and levels of infection change. We can have an outbreak and you're in a whole different scenario."

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