• On Friday, South Korea reported that 91 coronavirus patients being considered for discharge tested positive for the virus again. 
• The director of the Korea Centers for Disease Control and Prevention said the virus may have been “reactivated” rather than the patients being re-infected. 
• It remains unclear why the patients tested positive after initially testing negative for COVID-19 — WHO announced Saturday that it would be investigating the reports. 
•  “As COVID-19 is a new disease, we need more epidemiological data to draw any conclusions,” WHO told Reuters. 
• Visit Business Insider’s homepage for more stories.

The World Health Organization (WHO) announced Saturday that it is investigating reports that coronavirus patients who initially tested negative tested positive for the virus days later. 

“We are aware of these reports of individuals who have tested negative for COVID-19 using PCR (polymerase chain reaction) testing and then after some days testing positive again,” a WHO spokesperson told Reuters.

On Friday, South Korea reported that 91 coronavirus patients they believed to have recovered from the disease tested positive for the virus again.

Guidelines from WHO on clinical management recommended that a clinically recovered COVID-19 patient should test negative for the virus twice, with tests conducted at least 24 hours apart, before being discharged from the hospital.  

 The COVID-19 patients in South Korea were being considered for discharge after testing negative for the disease — however, tests administered later showed positive results.

South Korean health officials said they would be launching epidemiological investigations to determine what was behind the trend.

Jeong Eun-kyeong, the director of the Korea Centers for Disease Control and Prevention, said in a news briefing that the virus may have “reactivated” in the patients, as opposed to the patients being re-infected again, Bloomberg reported. 

“While we are putting more weight on reactivation as the possible cause, we are conducting a comprehensive study on this,” Jeong said Monday. “There have been many cases when a patient during treatment will test negative one day and positive another.”

The number of patients who were believed to have been cleared of the coronavirus but later tested positive jumped from 51 on Monday to 91 on Friday. Shortly after, WHO announced it would also be looking into the recent COVID-19 trend in South Korea as well. 

“We are closely liaising with our clinical experts and working hard to get more information on those individual cases. It is important to make sure that when samples are collected for testing on suspected patients, procedures are followed properly,” a spokesperson for WHO told Reuters in a statement.

According to WHO, current studies show that patients with mild COVID-19 experience a period of about two weeks between the onset of symptoms and clinical recovery. But it remains unclear why these patients are testing positive after they were believed to have recovered from COVID-19.

“As COVID-19 is a new disease, we need more epidemiological data to draw any conclusions,” the statement added. 

More than 390,000 people worldwide have recovered from the coronavirus, according to data collected by John Hopkins. 

Infectious disease experts, including Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Disease, believe that “people who recover [from COVID-19] are really protected against re-infection.”

People who have been infected develop antibodies that can “probably fight off the coronavirus if they encounter it again,” making them temporarily immune to the coronavirus, according to Business Insider’s Morgan McFall-Johnsen. However, it’s unclear how long the protection lasts, she added. 

Recently, Dr. Anthony Fauci said people who recover from the coronavirus will likely be immune should a second wave of infection spread in the early fall. But preliminary studies about coronavirus immunity show that not all recovered patients develop the antibodies needed to protect ourselves from the virus.