Australia's COVID-19 vaccine booster program is well underway, with more than 5 million people now triple jabbed.
In December, the federal government's vaccine advisory body slashed the recommended interval between second and third doses to four months, in a bid to stem the rising tide of Omicron infections.
On Wednesday, New South Wales, Victoria, South Australia and the ACT went one step further, bringing third doses forward to three months.
In other parts of the country, the booster interval will be shortened to three months from the end of January.
Here's what you need to know about rolling up your sleeve again.
Who should get a booster?
Booster vaccines are available to everyone aged 18 years and over who received their primary course of COVID-19 vaccines — dose one and two — at least three or four months ago (depending on your state or territory).
This includes pregnant women and severely immunocompromised people who received a third COVID-19 vaccine as part of their primary course.
While booster doses are, in most cases, not mandatory, they are recommended to maintain immunity against COVID-19.
According to the Australian Technical Advisory Group on Immunisation (ATAGI), a timely booster dose is especially important for:
- people with risk factors for severe disease (such as those aged 60 years and over, those with underlying medical conditions, those in aged/disability care, and Aboriginal and Torres Strait Islander people)
- people with increased risk of exposure to SARS-CoV-2 (such as healthcare workers)
Why are boosters recommended?
Research shows that COVID-19 vaccines steadily lose their effectiveness over time, most notably against infection and symptomatic disease.
This is because a few weeks after vaccination, your antibody levels begin to drop, meaning your first line of immune defence is weaker, said Jennifer Juno, a senior research fellow at The Doherty Institute.
In addition to tackling waning immunity, boosting antibody levels is important in the face of the rapidly-spreading Omicron variant.
Mutations on the variant's spike protein mean Omicron is better able to evade detection by the immune system, RMIT University vaccine researcher Kylie Quinn said.
"The emergence of Delta and now Omicron have made it more and more difficult for our immune systems to recognise the virus and keep up with it," Dr Quinn said.
Preliminary research from the UK suggests that two doses of the AstraZeneca vaccine provide almost no protection against Omicron infection or mild illness four to six months after vaccination.
For people who received two doses of Pfizer, that protection falls to about 35 per cent.
"What the booster is doing is increasing the amount of antibody that's in circulation, which is hopefully going to prevent infections," Dr Quinn said.
The good news is that protection against severe disease isn't nearly as affected.
Several studies have shown that six months after vaccination, two doses of the AstraZeneca and Pfizer vaccines provide 79 and 90 per cent protection respectively against hospitalisation and death.
Still, experts say reducing infections is important, both to protect the most vulnerable and lessen pressure on health services, and because we don't yet know the long-term impacts of infection with Omicron.
How effective are booster shots?
So far, research shows that people vaccinated with a third COVID-19 shot have a stronger immune response to Omicron than those vaccinated with just two doses.
According to the latest UK data, vaccine effectiveness against symptomatic disease rose to 65-75 per cent two to four weeks after a booster vaccine with either Pfizer or Moderna.
Against hospitalisation, a third dose (of either) boosted protection to 92 per cent on average.
Dr Quinn said third doses helped to both increase antibody levels and improve the protection offered by T-cells, another important component of the immune system linked to the prevention of severe disease.
"If you get a high enough level of antibodies, it sort of wakes up your memory immune cells, which can promote their survival and lead to more durable responses over time," Dr Quinn said.
"It can also improve the quality of our immune response by making the types of antibodies we produce better."
But the UK data also suggests third doses may not offer long-term protection against infection or transmission.
Ten weeks after a booster dose, protection against symptomatic disease dropped to 45-50 per cent. Protection against hospitalisation remained higher at 83 per cent.
While Israel and several European countries have begun to roll out fourth vaccine doses in response to concerns about waning immunity, Dr Quinn said more data was needed to understand the durability of current booster shots.
Should I get a different vaccine to my previous ones?
In Australia, both Pfizer and Moderna COVID-19 vaccines are recommended as booster shots, regardless of which vaccine you had for your first two doses.
(You can also opt for the AstraZeneca vaccine if you can't have an mRNA vaccine, or previously had AstraZeneca — although it's not preferred.)
Dr Quinn says some research has indicated that "mixing and matching" vaccines — getting a booster that's different to your first and second doses — could give your immune system a bigger boost.
"Definitely, for folks who have received AstraZeneca [for their primary doses] and then receive either of the mRNA vaccines, that's a really good combination," she said.
But does one mRNA vaccine have a slight edge over the other?
And should people who received Pfizer or Moderna in their primary course seek out the alternative for their booster?
"There isn't necessarily a huge difference between these two booster vaccines," Dr Juno said.
UK researchers recently measured the immune response of nearly 3,000 people who received one of seven COVID-19 booster vaccines (or a control jab) two to three months after their second dose of either AstraZeneca or Pfizer.
They found that people boosted with Pfizer following two doses of AstraZeneca had antibody levels nearly 25 times higher than controls. For those who received a Moderna booster (following AstraZeneca), antibody levels increased 32-fold.
Among those who had Pfizer for their first two shots, a third Pfizer dose saw antibody levels rise more than eightfold. For those who received a Moderna booster, antibody levels increased 11-fold.
The researchers cautioned against comparing the performance of boosters as people started with different antibody levels (e.g. those who had received Pfizer for their first two doses tended to have higher antibody levels to begin with).
It's also important to note that the Moderna booster being rolled out in Australia and elsewhere is half the dosage of the original Moderna vaccine (used in the primary vaccine course, and in this study).
Dr Quinn and Dr Juno said other evidence suggested the Moderna booster might still give a slightly stronger immune response, but that the difference was modest — and likely smaller than what was reported in the UK study.
"Generally, either of these mRNA vaccines are going to give you a really good immune response for your booster," Dr Juno said.
"I would be more inclined to get whichever vaccine is more easily available to me right now, so that I can get protection as quickly as possible, rather than being overly concerned about whether it's Moderna or Pfizer."
Editor's note 19/01/21: Additional information about the dosage difference between Moderna's primary vaccine and booster vaccine was added after publication to reflect the limitations of The Lancet study, especially when applying the findings in an Australian context.
