Some long COVID symptoms may not last quite as long as first feared, according a study of Australians infected early in the pandemic.

Immunological symptoms that were present eight months after contracting the virus had largely resolved for most patients by the two-year mark, the joint study by the Kirby Institute at UNSW Sydney and St Vincent's Hospital Sydney found.

"For the majority of samples we analysed in the laboratory, the biomarkers previously indicating abnormal immune function have resolved," said lead author Chansavath Phetsouphanh from the Kirby Institute.

The study followed a group of people who contracted COVID-19 during Australia's first wave, as well as a control group, getting participants to self-report health information and analysing blood samples.

In a global first in January 2022, the study demonstrated that long COVID symptoms were consistent with biomarkers showing a sustained inflammatory response at eight months.

Nearly 18 months later, "significant improvements" were found in the blood tests of the infected participants and there were no longer any observable differences between the groups, Dr Phetsouphanh said.

That was backed up by the participants' self-reported data, with 62 per cent reporting improvements in quality of life.

While this was cause for optimism, it still left more than a third who did not report improvements, lead investigator Gail Matthews warned.

"This is likely explained by the reality that patients may have a range of underlying causes for their long COVID symptoms, not all of which are driven by immunological abnormalities and some of which are likely to persist even when the immunological environment has largely returned to normal," Professor Matthews said.

It was also worth noting that these results derived from participants who had not been vaccinated and had been exposed to a "mild or moderate" early strain of COVID-19, Kirby Institute director Anthony Kelleher said.

"Immunology is a complex science, and it is impossible to say for certain that outcomes in our unvaccinated clinical cohort will be true for vaccinated people or for people who may have been infected with a different strain of COVID-19," Professor Kelleher said.

"What we do know is that for most people with long COVID, both their symptoms and their biomarkers improve significantly over time, and this is a cause for optimism."