Following growing concerns about COVID-19 reinfection, medical experts came forward to explain that a second positive test after recovery doesn't necessarily mean reinfection.

Beijing reported a COVID-19 case on January 24 involving a patient that recovered from COVID-19 last August, but then tested positive again.

On the same day, Fushan in Shanxi Province also reported an asymptomatic case involving a patient that had been discharged on January 5, following a positive test last December on a return from overseas. This citizen tested positive for the second time during mass testing on January 23 and received a new round of treatment. 

Other places in Zhejiang and Hebei Provinces have reported similar cases of people testing positive for a second time. 

Some reasons explain such cases, for example, false test results, reactivation of the virus, and reinfection, said Wu Zunyou, chief expert of epidemiology at the Chinese Center for Disease Control and Prevention.

Wu said false results can be cleared by multiple tests, and reinfection is as similar and contagious as a first-time infection. As for reactivation of the virus, he said it is unclear. 

"Tested positive for the second time means the virus in the body is replicating at a low level," said Chen Zhihai, deputy chief of the Infection Department at Beijing Ditan Hospital, adding that "the cases are not much infectious." 

He also said that, so far, there are no cases of people who tested positive again having infected others, as they carry low levels of the virus. "Current research shows recovered patients will not carry the virus through a lifetime."

There are similar cases reported overseas while the situations vary. Experts are still trying to understand the situation. 

Over 470 South Koreans tested positive again after being discharged last April, Wall Street Journal reported last May. Although local health officials are uncertain of why it happens, they found the newly detected cases weren't infectious. 

An expert panel said the reemergence of positive cases is probably linked to tests picking up fragments of the dead virus, which remain in a patient's body after recovery. 

A study released in November showed 32 out of 176 recovered patients in Italy tested positive again, yet only one patient tested with replicating virus. However, it's impossible to identify if it is a recurrent infection or reinfection as more analysis, such as whole-genome sequencing, are needed. 

The U.S. has also seen its first COVID-19 case contracted with coronavirus twice with a second infection "more severe", reported last October. But in this case, scientists identified the patients caught the coronavirus on two separate occasions, rather than the original infection relapsed after becoming dormant. 

With positive cases exceeding 100 million around the world, the increasing retesting positive cases may lead to tougher measures to combat COVID-19. Shanghai has already rolled out followed-up visits on recovered COVID-19 patients. 

Wu Jinglei, chief of Shanghai Municipal Health Commission, said all recovered patients will have a 14-day quarantine after being discharged to avoid possible re-positive cases.