Health authorities are urging Australians who haven't yet received their COVID-19 booster vaccine to roll up their sleeves, with coronavirus case numbers expected to rise in the coming weeks.
The increasingly dominant Omicron BA.2 subvariant, thought to be more transmissible than its widely circulating cousin BA.1, has experts predicting that COVID-19 cases could jump significantly.
On Thursday, NSW clocked up more than 20,000 cases alone.
"People need to understand that while the community has gone to sleep on the virus, the virus hasn't gone to sleep on the community," NSW Health Minister Brad Hazzard said last week.
"The virus is still out there and it can wreak havoc if we don't go and get our boosters fast."
Since the federal booster program got underway last November, uptake of third doses has lagged.
Department of Health data shows 66 per cent of Australians over 16 have received more than two doses of a COVID-19 vaccine.
"That's compared to 95 per cent of the population who've got two doses," said epidemiologist and biostatistician Adrian Esterman of the University of South Australia.
"So [booster rates] are way, way down."
Boosters improve protection, especially among older people
While booster vaccines are, in most cases, not mandatory, they're recommended for everyone aged 16 years and over to help maintain immunity against COVID-19.
That's because COVID-19 vaccines steadily lose their effectiveness over time, most notably against infection and symptomatic disease.
"With two doses, you still get some protection against serious illness, but you have virtually zero protection against infection with Omicron," Professor Esterman said.
Infectious diseases physician Peter Collignon said the benefit of booster vaccines was greatest in older people, who faced the highest risk of serious complications from COVID-19.
According to data obtained by Nine Newspapers, about 900,000 Australians aged 60 and over are yet to have a booster.
"Vaccines markedly decrease the risks … three doses, if you're older, give you about a 20-fold reduction in your chance of dying," said Professor Collignon of the Australian National University.
He said Australians only had to look at the situation unfolding in Hong Kong, where case numbers and deaths have risen rapidly in older people, to see the impact of vaccines.
"Hong Kong had a reasonable uptake of COVID vaccines in 30- and 40-year-olds, but in their elderly, they had poor uptake — about 30 per cent, even just of two doses," he said.
"They're now seeing some of the highest death rates in the world."
Professor Collignon said Australia should be aiming for 95 per cent booster coverage in people over 70, although suggested the same should apply to people over 50.
What about waning immunity? And the BA.2 variant?
The effectiveness of COVID-19 boosters, like that of the primary vaccine doses, appears to wane over time, most notably against infection.
"The third dose brings you really good protection against both infection and serious disease, but the protection against infection basically halves in a couple of months," Professor Esterman said.
Against more serious disease, a report published by the CDC in February found that booster effectiveness declined, but less significantly.
Data shows that protection against emergency department visits went from 87 per cent in the first two months after a third dose, to 66 per cent after four months.
Vaccine effectiveness against hospitalisation went from 91 per cent in the two months after a third dose to 78 per cent by the fourth month.
One limitation of this data is that it wasn't broken down by age, and the researchers couldn't distinguish between a booster dose or a third dose given to an immunocompromised person (as part of their primary series). This means booster shots may appear less effective than they actually are.
Professor Collignon said ongoing studies were needed to measure the longevity of the third dose, and that it was important to look at more than just the antibodies it triggers.
"Just looking at antibody levels doesn't necessarily correlate with how good protection is, because white cells and T-cells are probably more important in stopping people getting seriously ill … and that's much harder to measure," he said.
As for whether the BA.2 subvariant is better able to evade our immune defenses (induced by vaccines or infection), early research suggests this isn't the case.
Preliminary UK data shows vaccine effectiveness against symptomatic disease is similar for BA.1 and BA.2, and according to the World Health Organization, initial studies indicate infection with BA.1 provides "strong protection against reinfection with BA.2".
Tackling low booster uptake
Infectious disease social scientist Holly Seale said in order to boost the sluggish uptake of third doses in some groups, it was important to understand why people weren't coming forward.
She said a "shift in perception" around COVID-19 risks, coupled with a drop in cases and reduced public health measures, meant some people were likely feeling less compelled to seek out vaccination.
"People might think, 'What's the rationale for getting a booster if everyone [they know] seems to be getting through COVID OK?" Dr Seale said.
State-based data shows booster rates are as low as 40 per cent among some younger cohorts.
Professor Esterman said it was understandable that young people had been slower to seek out third doses, given they faced a much lower risk of complications from COVID-19.
But he said the risk of serious illness wasn't zero, and that there was also the possibility of long COVID to consider.
"To say I don't care if I get infected is taking a bit of a risk," he said.
Dr Seale added that booster rates may be low in some instances because "we've got a large number of people who are getting COVID-19", and therefore delaying vaccination.
But she added that an increase in public health campaigns could help to tackle low uptake, especially in areas where limited community education or outreach meant the message about boosters wasn't getting through.
"We need to keep refreshing those messages," she said.
Will some of us need fourth doses?
Earlier this week, Pfizer announced that it had sought emergency authorisation from the US Food and Drug Administration for a second COVID-19 booster for adults aged 65 and older.
Some countries have already moved to recommend a second booster for vulnerable groups, including people with immunodeficiencies, people aged over 70 (or in Israel, over 60), healthcare workers, and aged care residents and employees.
In Australia, fourth doses are currently only recommended for people who are immunocompromised, who tend to produce a less robust response to vaccines.
But last week, federal Health Minister Greg Hunt said the government's vaccine advisory body, ATAGI, was also considering recommending a fourth dose for Australians over 65.
Professor Esterman said he expected fourth-dose eligibility to be expanded, especially ahead of winter when both COVID and flu infections are forecast to rise.
"What's happening is a lot of elderly and vulnerable people who got their booster dose in November or December would now have little protection against infection, and really need a fourth dose — let alone a third one," he said.
"People who are severely immunocompromised are a very small, select group of people.
"There's a much wider group of people who are elderly, frail, or who have underlying health conditions that put them at risk."
Professor Collignon said fourth doses might prove to be necessary for older people, but that there wasn't enough data yet to suggest they were needed — or likely beneficial — for the wider population.
