If your last interaction with COVID-19 — either by vaccination or infection — feels like a distant memory, it might be time to book in for a booster shot.
As of this week, all Australian adults who haven't had a COVID-19 booster or a confirmed case of the virus in the past six months are eligible for an additional jab.
And that's not all that's changed: an updated Omicron-specific booster is also set to be rolled out early next month.
The Australian Technical Advisory Group on Immunisation (ATAGI) has advised that Pfizer's bivalent BA.4/5 booster can be used in all people aged 12 years and over.
Here's what you need to know.
What makes this booster different?
As SARS-CoV-2 has spread around the world, the virus evolved in ways that allow it to transmit more easily and better evade pre-existing immunity generated by vaccination and prior infection.
In a bid to keep up with this viral evolution, newer COVID-19 "bivalent" vaccines have been developed to target both the original strain of the virus and newer, more immune-evasive variants.
Pfizer's latest bivalent vaccine is the first booster to be approved in Australia that specifically targets Omicron subvariants BA.4 and BA.5.
Both rose to prominence in 2022, overtaking previous subvariants like BA.1 and BA.2.
The latest jab is the third bivalent vaccine approved for use in Australia, all of which are Omicron-specific.
The first bivalent option was Moderna's (Spikevax) original/BA.1 vaccine which was made available in October last year, shortly followed by Pfizer's original/BA.1 vaccine in December.
Pfizer's new BA.4/5 bivalent vaccine differs from its previous formulation by replacing mRNA of the BA.1 Omicron subvariant with mRNA that encodes the BA.4/5 spike protein instead.
A fourth bivalent vaccine — from Moderna, also targeting BA.4 and BA.5 — was approved by the Therapeutic Goods Administration (TGA) last week and is undergoing consideration by ATAGI.
How much more effective is it?
Compared to Pfizer's original COVID-19 vaccine, studies in laboratory settings suggest the new bivalent booster offers better protection against Omicron BA.4 and BA.5, said epidemiologist Terry Nolan.
"Antibody response studies have been done — in the test tube — to show what the boost is, and if it neutralises the virus," said Professor Nolan, head of the vaccine and immunisation research group at the Doherty Institute.
"There's good evidence [the new bivalent vaccines] produce very good antibodies against their targets, BA.4/5, and the original ancestral strain."
Early studies also found BA.4/5 vaccines induce "higher neutralising activity" against other emerging Omicron subvariants — including BQ.1 and XBB — than the earlier bivalent and original vaccines.
This is important because although the BA.4 and BA.5 continue to circulate in Australia, other Omicron subvariants are becoming increasingly dominant.
"At the moment, the closest we can get to the virus is with the bivalent BA.4/5 vaccine," Professor Nolan said.
As for whether findings in the lab translate into meaningful differences in clinical protection, early data — according to ATAGI — suggests bivalent vaccines have "a small advantage" over the original vaccines when it comes to preventing severe disease.
One study in the US found vaccine effectiveness against hospitalisation or death with a bivalent BA.4/5 booster (either Pfizer or Moderna) was 61.8 per cent, compared to 24.9 per cent effectiveness from an original booster.
Another (preprint) study in Nordic countries found vaccine effectiveness against hospitalisation with a second booster of a BA.4/5 bivalent vaccine was 80 per cent, compared with a second booster of an original vaccine at 65 per cent effectiveness.
Evidence from "extensive use" of both Pfizer and Moderna's BA.4/5 bivalent boosters in the US and Europe in recent months has shown they both provide "clear reductions in hospitalisation and death", according to the TGA.
Is it worth waiting for the new booster?
While all COVID-19 vaccines are expected to provide protection, Omicron-specific mRNA vaccines are the preferred booster option in Australia.
These include Moderna's bivalent BA.1 vaccine, Pfizer's bivalent BA.1 vaccine, and Pfizer's new BA.4/5 jab — which will be available from March 6.
Given the "imminent availability" of the BA.4/5 booster, Professor Nolan said he believed it was potentially worth holding on for.
But Catherine Bennett, chair in epidemiology at Deakin University, said most people shouldn't wait.
The introduction of bivalent boosters was a "step change" in vaccine protection, she said, while the addition of BA.4/5 vaccines was more like an "incremental change".
"I'm going on a work trip to India and would much rather be boosted [than wait]," Professor Bennett said.
"Waning immunity is the bigger difference here.
"If people are at low risk or not at the six-month mark, they can wait for BA.4/5."
Does everyone need a booster?
According to ATAGI, healthy people aged between 18 and 64 years (and those aged between 5 and 17 at risk of severe illness) should consider a 2023 booster.
But the vaccine advisory body explicitly recommends that anyone aged 65 years and over, as well as younger adults who have medical comorbidities, disability or complex health needs, has an additional shot ahead of winter.
That's because the increase in protection from a booster is most beneficial for people at higher risk of severe COVID-19, Professor Nolan said.
"That has always been and should remain the target: to prevent older people from getting severe illness and ending up in hospital."
An additional booster dose may still be beneficial for younger adults, but the benefit is "less clear", Professor Bennett said.
A recent CDC study found people aged 18-49 who received a bivalent booster were about 50 per cent less likely to have a symptomatic infection compared to people who received two to four doses of the original vaccine.
But younger people have a lower risk of severe disease, and — for those under 40, especially men — a slightly increased risk of heart inflammation (known as myocarditis) following vaccination.
It's important to note this risk is rare, appears to be lower after COVID-19 boosters compared to primary vaccines, and that long COVID is also associated with several cardiovascular complications.
Professor Bennett said the choice ultimately came down to individual circumstances, and that younger adults may opt for another dose to reduce their chances of becoming unwell or passing COVID-19 on to others.
"If they've come through summer and had a long infection-free period, they might think: I'll have a dose now ... and avoid another infection."
