One in six patients with COVID-19 discharged from a Chinese hospital have retested positive to the virus, researchers have found in a study that further muddies the issue of immunity.

Their study shows that despite being under strict quarantine conditions, 69 out of 414 patients believed recovered from COVID-19 later tested positive — and 16 required readmission to hospital.

The joint Chinese-US study, published preprint without peer review, followed consecutive patients treated at a hospital in Shenzhen, China, between January and April, who were discharged after their symptoms had resolved and they had received two negative RT-PCR tests. 

They were then subjected to quarantine at a designated centre for two weeks after recovery, during which PCR testing was conducted every 3-5 days, followed by a further two weeks in home quarantine with weekly testing.