Now, Queensland researchers hope blood samples from those who are back on their feet could unlock a way to fight the illness.
Identifying how their immune systems responded would help scientists get a step closer to a T cell immunotherapy, Dr Corey Smith, head of QIMR Berghofer Medical Research Institute's Translational and Human Immunology Group says.
"T cells - which are a type of immune or white blood cell - play a critical role in fighting diseases and infection," Dr Smith said.
"We believe it's likely that in patients who are getting less sick from COVID-19, it's because their T cells are responding well and fighting this virus.
"But we need to test that theory in the laboratory and that's why we need blood samples from recovered patients.
"We will grow their T cells in the lab and screen them against the virus to see if they fight it."
Once scientists understand how patients fought off the virus, they hope to develop immunotherapies against it.