A Doherty Institute study, published in the Nature Immunology journal on Wednesday, has shown the body's T cells provide long-lasting memory against the virus following vaccination or infection from COVID-19.
T cells play a crucial role in supporting the development of the body's B cell response, which helps produce the antibodies that recognise COVID-19 and stop infection.
Using new technology called tetramers, researchers tracked T cell responses in people who had recovered from COVID-19 over 15 months and found a sustained level of these cells was capable of recognising the SARS-CoV-2 spike protein.
"Even though some parts of the immune response wane, we can now see that T cells recognising the virus are quite stable over time," paper author Jennifer Juno said.
"After more than a year, they were still roughly 10-fold higher than someone who had never been exposed to the spike protein through infection or vaccination.
"Vaccination boosted the levels of these T cells to be up to 30 times higher than they were before."
Researchers also found a third COVID-19 vaccination dose did an "incredible job" of reactivating T cells and bringing their levels back up again, she said.
The Doherty Institute is now studying how T cells react when vaccinated people catch COVID-19, to understand whether they are reactivated in the same way.
It comes as international travellers could soon have their pre-departure testing requirements removed after the prime minister flagged a further easing of COVID-19 restrictions.
Travellers currently need to show a negative test result in order to board a flight to Australia even if they are fully vaccinated.
Prime Minister Scott Morrison says the removal of pre-departure testing is an important milestone for Australia.
Health Minister Greg Hunt will be making an announcement on the matter soon.
"While we're never complacent about the challenges COVID can present and new variants, we're watching all of those closely as we keep looking through that windscreen," Mr Morrison told reporters in Cairns on Tuesday.
"Hopefully we can continue to see COVID in the rear vision mirror."
But a spokesman for Mr Hunt said continues to receive health advice before making further changes to Australia's biosecurity measures.
The current biosecurity declaration is due to lapse on April 17, aligning with international cruise ships returning to Australian waters for the first time since March 2020.
LATEST 24-HOUR COVID-19 DATA FROM ACROSS AUSTRALIA:
NSW: 24,115 cases, five deaths, 1162 in hospital
Vic: 10,471 cases, 11 deaths, 243 in hospital
Tas: 1825 cases, no deaths, 32 in hospital
Qld: 10,476 cases, seven deaths, 252 in hospital
ACT: 1314 cases, no deaths, 42 in hospital
WA: 8429 cases, one death, 194 in hospital
SA: 4594 cases, two deaths, 161 in hospital
NT: 328 cases, no deaths, 18 in hospital
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