COVID-19 patients who tested positive again after being cured are less likely to be infectious, but they also need to be isolated to minimize risks, experts said following recent reports of such cases.

A 68-year-old woman in Jingzhou, Hubei province tested positive for the novel coronavirus on Sunday, several months after being cured, the city's COVID-19 control and prevention headquarters said on Wednesday in a notice. She was first diagnosed with the disease on Feb 8, it said.

The patient has been receiving treatment again under isolation and all her contacts were traced and tested negative for the virus, the notice said, adding there is no evidence yet to prove the isolated case could spread the virus.

In an attempt to reduce transmission of the virus, the headquarters on Friday released another notice banning drugstores from selling drugs to people with a fever.

All those with a fever should voluntarily go to fever clinics for treatment, where they can have free nucleic acid and antibody tests for the virus, the notice said.

In Shanghai, the municipal health authority on Thursday also reported a similar case.

The man, from Jilin province, who was cured after being diagnosed with COVID-19 in April, tested positive for the novel coronavirus again on Monday.

He doesn't have any symptoms and is receiving treatment under isolation. Shanghai's health authority has traced 23 close contacts of the patient and put them under isolation, and all of them tested negative for the virus, it said.

Previously, the man was cured and discharged from hospital in June after testing negative for the virus, and was released from isolation on July 9 after testing negative on four occasions, according to a statement released on Wednesday by Yanbian Korean autonomous prefecture, in Jilin province, where the man lives. All of his 178 contacts, including family members, tested negative for the virus and are under isolation, the statement said.

The exact cause for recurrence of the novel coronavirus on cured COVID-19 patients remains unclear, but experts said such phenomenon is not rare and caution is needed.

Jiang Rongmeng, a senior expert in infectious diseases at Beijing Ditan Hospital, said with more patients cured and leaving hospital, patients testing positive again have been identified in many places, and in some places as many as 15 percent of cured patients could test positive again.

One possibility for cured patients testing positive for the virus again is that they may have always carried the virus, even if they are considered cured and discharged from hospital, he said.

"They may have tested negative for the virus before leaving hospital, because they may still have carried a little amount of virus, which could not be tested, or improper testing methods gave them a false negative," Jiang said.

Wang Guiqiang, head of Peking University First Hospital's infectious disease department, said in an earlier interview that in some cases testing kits are not sensitive enough or the virus is concealed deep in the lungs, resulting in inaccurate results.

Some patients, especially seniors and those with chronic diseases, may have weaker immune systems, which could result in them remaining positive for the virus for a long period even with treatment, he said.

As a new virus, the novel coronavirus still poses a number of questions to humans, and more research into the virus is needed, Jiang said.

"We have not found a clear relationship between incidences of cured patients testing positive again and their ages, illness and immunity," he said.