At the beginning of 2021, Australia agreed to purchase 51 million doses of a COVID-19 vaccine made by US biotech company Novavax.
The vaccines had been slated to arrive mid-year, but manufacturing delays and supply shortages have slowed the company's progress.
It's now expected the Novavax jab will play an important role in Australia's booster program.
So what's the evidence for its effectiveness and safety? And when is it expected to arrive?
How does Novavax work?
Novavax is what's known as a "protein subunit vaccine" — instead of delivering the entire virus, it delivers just one part of it.
"Its major component is the spike protein found on the surface of the SARS-CoV-2 virus," University of Sydney microbiologist Jamie Triccas says.
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Like other COVID-19 vaccines, the idea is that by exposing your immune system to the spike protein, your body learns to recognise it, and can then fight off the real virus down the track.
But the Novavax jab works a little differently to mRNA vaccines (like Pfizer and Moderna) and viral vector vaccines (like AstraZeneca).
Instead of delivering genetic instructions to the body to create its own harmless spike proteins, Novavax delivers the actual spike protein itself.
To ensure the body generates a strong immune response, the vaccine also contains, like many vaccines, an adjuvant, which helps to wake up the immune system and trigger a response.
"Novavax has its own adjuvant called Matrix-M, which has a particular formation that tells the immune system there is a vaccine present and it needs to respond," Professor Triccas says.
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Matrix-M is based on a natural product called saponin, which is an extract from the Chilean soapbark tree.
There are some reports of people preferring to wait to be vaccinated with Novavax because of the way it's formulated.
But health authorities have repeatedly warned against waiting for a particular vaccine, as unvaccinated people are far more likely to die of COVID-19.
Murdoch University immunologist Cassandra Berry echoed those warnings.
"It's much safer to get vaccinated now … than to run the gauntlet and wait till the last minute," Professor Berry says.
"It could be quite a few months away until Novavax is approved and we have our supply."
What do we know about its safety and effectiveness?
Preliminary data recently published from a large phase 3 clinical trial showedx the vaccine was 90.4 per cent effective at preventing infection, and 100 per cent effective against severe disease.
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The trial involved 30,000 participants from the US and Mexico, who each received two doses of Novavax three weeks apart. The participants were then compared to a placebo group of almost 10,000 people.
As expected, some mild, short-term side effects were noted in the trial participants who received the vaccine, including tenderness at the injection site, headache, aches and pains and fatigue.
Serious adverse events were "rare", and occurred at a similar rate across the vaccine and placebo groups.
Researchers also monitored for very rare side effects — notably blood clotting and myocarditis — which had been seen in a small number of recipients of other COVID-19 vaccines. They found no incidence of either.
The results of the clinical trial back up similar findings from a UK study released in January, which found similar efficacy and safety.
In this other phase 3 trial, which included more than 15,000 people, the Novavax vaccine was 96 per cent effective against the original coronavirus strain, and 86 per cent effective against the Alpha variant.
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While both clinical trials took place before the Delta variant had fully emerged, Novavax says preliminary data from a booster trial suggests the vaccine is highly effective against the now-dominant variant.
"Most of the vaccines have stood up pretty well against Delta … and I see no reason why Novavax would be any different," Professor Triccas says.
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When will Novavax be available?
To date, the vaccine has not yet been approved for use in any country.
The federal government says it expects Novavax vaccines to begin arriving in Australia in late 2021.
But a recent report from US media outlet Politico — which cites anonymous sources — suggests manufacturing problems mean the company is still facing "significant hurdles" when it comes to meeting drug regulators' quality standards.
Novavax's production targets have been repeatedly pushed back throughout the year because of issues with access to raw materials and equipment needed to manufacture the vaccine.
Novavax announced on October 20 that it expected to complete its regulatory submissions in the UK, Europe, Canada, Australia and New Zealand "within the next couple of weeks".
Yesterday, federal Health Minister Greg Hunt confirmed he had spoken with the Australian representative for Novavax.
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"We are expecting Novavax to submit their application for their vaccine to [the TGA] in the coming weeks, if not earlier," Mr Hunt said.
But it's still not clear if Novavax supplies are on track to reach Australia this year, and the Health Department did not respond specifically to a question about this from the ABC.
It's also unclear if the vaccine will be produced in Australia, but Novavax has selected a Sydney-based start-up called Biocelect to roll out the jab in Australia and New Zealand.
How will Novavax be used?
In some countries, especially those yet to receive a meaningful number of COVID-19 vaccines, Novavax — if approved — is likely to be used for primary COVID-19 vaccinations.
Unlike mRNA vaccines, the Novavax vaccine doesn't require ultra-low temperatures and can be stored in fridges for up to three months.
But in Australia, where more than 87 per cent of people over 16 have received at least one dose of a COVID-19 vaccine, and 75.5 per cent are fully vaccinated, it's likely Novavax will mainly be used as booster shots.
Professor Berry says the use of a Novavax jab following a Pfizer, Moderna or AstraZeneca vaccine could significantly help to boost individuals' protection.
"I would like to see a bit more mixing of the vaccine brands and platforms, because that really stimulates your immune system," she said.[Click through to send us your questions about COVID-19]