A month after he first tested positive to COVID-19, Darryl Glover woke up feeling under the weather.

Mr Glover, from NSW's South Coast, had hoped to visit his wife, who is in care, and got a PCR test just in case.

"I couldn't believe it came back positive," he said.

"I felt nowhere near as unwell as I did the first time - if I hadn't had the positive PCR I would have just thought I'd done too much over the weekend."

We don't yet know how many new cases are re-infections, which is why it remains important to register positive RAT tests, so scientists can learn more about how the virus works.

Professor Trevor Drew, Director of CSIRO's Australian Centre for Disease Preparedness, said Mr Glover's experience was likely to become more common as the pandemic progresses.

"Coronavirus likes to infect the membranes of our nose, mouth and lungs," he explained.

"Our immune system on those surfaces can't be too reactive, because then you get debilitating allergy symptoms to things that are harmless, like pollen.

"So the virus is very rapid in the way it gets into those cells and can bind and get in before the immune molecules in the mucous membranes can find it. An additional trick Omicron seems to have developed that we haven't seen in previous variants is it can go from cell to cell and infect the membrane of the cell next door to fuse.

"It's a bit like a burglar going into a row of terrace houses - it's learnt to break through the wall from one house to another so it doesn't have so come out and be exposed to our immune system."

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This means that while vaccines remain very effective at preventing serious disease and death, and help to reduce the spread of infection, it's difficult for the body to mount a strong first-line defence again COVID, regardless of previous infections or vaccination status.

Professor Drew said there was hope of a more effective vaccine however, and that may not be far away.

"We know from coronavirus infections of animals the best vaccination is a combination of both an intramuscular vaccine, which is what we have currently, and a nasal vaccine," he said. "There are already a number of research projects on how we might improve immunity on cell surfaces with intra-nasal vaccines, but it will be a little while before we manage to deploy those."

Until then, getting vaccinated, getting a booster, social distancing, mask wearing and hand washing are the best way to prevent infection - and remember, getting COVID once is no silver bullet.