More than 20,000 people in NSW who had COVID-19 in January have been reinfected, the first analysis of reinfection data shows.
NSW Health looked at data from 639,430 people infected with COVID for the first time in January when the Omicron wave took off.
The analysis was done by matching the name, and date of birth, of cases.
It showed that within five months, 20,460 people, or 3.2 per cent, had been reinfected.
Reinfection was defined as a positive test four weeks after being released from seven-day isolation, or 36 days after testing positive.
When Sydney woman Maria Colella was first infected in March, despite being fully vaccinated, it hit her hard.
"It was really bad, I had the heavy chest and I just really struggled to walk. I had a really like sore throat," she said.
"It took about six weeks for it to get better."
After two weeks isolating at home, Ms Colella returned a negative PCR test that allowed her to go back to her job in childcare, although she was still suffering the side effects of infection.
"I went to the doctor and she said, 'it sounds like you've got long COVID. It might take a few more weeks (to recover)'," she said.
A week later, in May, she was hit with COVID again.
"I just felt the same symptoms but milder, the shortness of breath, the sore throat, but it wasn't too bad. It only lasted a couple of days. The persistent cough didn't last as long," she said.
"I got over it faster."
Nick Wood, a paediatrician and immunisation expert from the University of Sydney, said in theory, the first exposure to COVID should give some natural immunity that would stop people getting as sick the second time around.
"Your prior immunological exposure, natural infection and vaccine history all probably plays into how you as the individual deal with your second infection," he said.
People who were immune-suppressed or who had ongoing respiratory problems from the first infection would be more impacted with subsequent infections, he said.
"That's all the difficulties in teasing it out how severe, but I think the general, the belief is that the second or third infection are probably less severe than the initial primary infection."
Dr Wood said the BA.4 and BA.5 sub-variants of Omicron were able to evade both vaccine-induced immunity and infection from a previous variant.
"The immunity that they generate is not enough to stop you being infected," he said.
He said that over time, experts hope that as new variants come along, the population is more able to deal with them because of past infections or vaccination.
"So that if you do get sick, you get sicker for less time, and you have less of the virus, so you shed less," he said.
"Hopefully, this will just go the way of being diluted to the effect that it becomes more like a common cold."
Ms Colella is still not feeling 100 per cent after her two doses of COVID.
She isn't able to exercise as much as she did before she caught COVID and she's taking extra precautions to avoid getting infected again.
"I don't go to crowded places like restaurants and I avoid rooms where there's like, full of people, especially at this time of year," she said.
"Maybe when the weather gets warmer and it stops raining and there's less cases, I'll be more relaxed."
The NSW Health report noted that some reinfections may not have been reported, therefore underestimating the reinfection rate.
In addition, some people, particularly those who were immunocompromised, could continue to test positive a long time after their initial infection, resulting in them being included as reinfections.
