Photo/Illutration The new coronavirus that causes COVID-19 (The U.S. National Institute of Allergy and Infectious Diseases via The Asahi Shimbun)

Health experts remain puzzled over why COVID-19 patients who have recovered from the disease are again testing positive for novel coronavirus infections.

Such cases are rare. But they do raise questions about possible limits on testing accuracy and highlight the need for more studies on the virus that has spread around the world.

At least 35 people in Japan, ranging in age from their 20s to their 90s, have tested positive twice for the coronavirus, according to reports in The Asahi Shimbun and interviews with local government officials.

One was a woman in her 20s living in Nagoya who had developed a cough and fever of 38.5 degrees in late February. She was initially diagnosed as having pneumonia but was subsequently confirmed to be infected by the novel coronavirus and hospitalized for 10 days.

About a month after she was released from the hospital, she began coughing again and took another test for the coronavirus, resulting in the second positive result.

Three additional individuals in Nagoya tested positive a second time after symptoms arose following their release from hospital.

Among all 35 cases in Japan, 32 patients showed symptoms again before testing positive a second time. Three showed no symptoms, but their second tests still came back positive.

In 27 of the cases, details have been compiled about how they were confirmed infected a second time. Symptoms appeared on average 8.9 days after they were released from hospital, with the time ranging from one to 31 days.

A research team at the National Institute of Infectious Diseases is looking into the matter as experts try to pinpoint some of the characteristics of the novel coronavirus.

The first confirmed report of a second positive test result in Japan came in late February regarding an individual in Osaka.

In March, the health ministry acknowledged cases of second infections but described them as “rare examples.” However, ministry officials have urged COVID-19 patients to check their health for at least four more weeks after leaving hospitals.

Chief Cabinet Secretary Yoshihide Suga at his May 7 news conference acknowledged there were cases of second positive results.

Such cases have also been reported abroad.

In late February, officials of the Guangdong provincial government in China announced that 14 percent of patients released from hospitals after receiving treatment for COVID-19 again tested positive for the disease.

In South Korea, 3.3 percent of the 10,000 or so individuals confirmed with COVID-19 and later released had produced a second positive test as of May 6.

South Korean media reports indicated that some of the cases may have been false positives created by the detection of dead genetic material of the virus in the patient.

But experts are still unsure what causes the second positive results.

One reason may be that the virus remains in the patient’s body.

The polymerase chain reaction (PCR) test that Japan uses may not detect the novel coronavirus if the amount in the patient is too small. Although that might lead to a negative result, it does not necessarily mean the virus is completely out of the patient’s system.

“While there might be the possibility of the virus (remaining in the patient’s body) reactivating, it should be considered a rare occurrence,” said Keita Morikane, an associate professor at Yamagata University specializing in infection control.

But Tetsuya Mizutani, who heads the Research and Education Center for Prevention of Global Infectious Diseases of Animals at the Tokyo University of Agriculture and Technology, said it was too early to say if those cases were a reactivation of the virus within the body or a second infection from the outside.

“There will be a need to analyze the immune condition of individual patients and the volume of the virus when a second positive test result is obtained,” Mizutani said.

(This article was compiled from reports by Kazuya Goto, Shunsuke Kimura and Akiyoshi Abe.)

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