Months into the battle against COVID-19, scientists are still struggling to understand the new virus that has sickened over 422,000 people worldwide as of Wednesday morning. In particular, numerous reports of recovered patients testing positive for the virus again have raised more questions about how this pathogen behaves, and who is at risk.

In early March, a patient in Wuhan, the central Chinese city where COVID-19 was first detected, died of the virus five days after he was deemed recovered and discharged from the hospital. Meanwhile, 5-10% of discharged COVID-19 patients in Hubei province, where Wuhan is located, tested positive in later checkups, according to preliminary data from several quarantine facilities.

Other provinces including Sichuan, Jiangsu, Hainan, and Guangdong have reported similar cases of reinfection. Health authorities in southern China’s Guangdong province said in late February that 14% of recovered patients evaluated tested positive again after being discharged.

Around the world, recovered patients in Hong Kong, South Korea, and Japan have also tested positive again days to weeks after being discharged.

It remains unclear why patients would test positive again, but experts say inaccurate tests could lead to patients being discharged before fully recovering.

Who is greenlighted to leave the hospital?

According to the COVID-19 treatment guidelines set by China’s national health authority, patients can be discharged from the hospital if they have two consecutive negative nucleic acid tests, as well as improvements in clinical symptoms such as fever and cough.

However, many factors may affect the accuracy of a test, such as the reliability of its manufacturer and how, where, and when the sample is collected.

“We don’t even know whether this (nucleic acid) test is the best way to detect infections,” Shen Yinzhong, an infectious disease expert at the Shanghai Public Health Clinical Center, tells Sixth Tone. “All laboratory tests have limitations, so we (as doctors) wouldn’t rely solely on test results when making a diagnosis. Instead, we consider patients’ medical histories and clinical symptoms in addition to those tests for a comprehensive evaluation.”

Experts further say that a negative test does not necessarily mean the patient is virus-free, nor does a positive test mean the patient is still sick.

“The RT-PCR test (nucleic acid test) is not a test of cure,” Philip Tierno Jr., a pathologist at New York University, tells Sixth Tone. “It will be positive to any of the viral RNA remnant that remains at the site, even though that viral matter is dead.”

Are people who test positive after recovering still infectious?

To date, there are no known cases of discharged patients infecting others, but it’s too early to say categorically that they aren’t contagious.

“It could be that there’s no live virus, or it could be that they were no longer infectious,” Shen says. “It’s also possible that these patients followed our suggestions to rest at home and thus did not spread the virus.”

China’s health authority has recommended that all COVID-19 patients quarantine themselves at home for an additional two weeks after being discharged.

Can a fully recovered person be reinfected?

Although there is no evidence to date that might completely rule out this possibility, experts suggest reinfection is unlikely, especially over the short term after recovery.

The human body can develop long-lasting immunity against some pathogens like the chickenpox virus, but immunity against other viruses can be short-lived, lasting only a few months.

“From the preliminary data, our immunity against the coronavirus is likely to be short-term,” Shen says. “But for how short, we don’t know.”

Shen added that in theory, a patient could also become reinfected if the immunity they develop is too weak to offer full protection.

But as the outbreak subsides in China, the infection risk for healthy people who don’t have immunity against the virus is relatively low. Therefore, it’s unlikely that patients who are discharged after appearing healthy will contract the virus again, Shen says.

Could antibody tests be a better discharge criterion?

Following reports of positive tests among cleared patients, some hospitals and quarantine sites in China have adopted antibody tests as an additional discharge requirement. These tests detect specific defenses generated by the immune system when staving off infection.

A hospital staff member told The Paper, Sixth Tone’s sister publication, that the additional test is intended to “ensure 100% recovery of the patients.”

But Shen says the antibody test may not be absolutely necessary, although it could be used to supplement other evidence. “Individuals produce antibodies at different rates,” he says, adding that he considers patients’ symptoms and general feelings to be more important criteria.

“We are still in the early stages of understanding the virus, but that’s OK,” Shen says. “There is no need to panic. When we find a problem, we look for a solution scientifically, calmly, and rationally.”

Editor: David Paulk.

(Header image: A medical worker collects a man’s throat sample for a COVID-19 nucleic acid test at a school in Wuhan, Hubei province, March 10, 2020. Xiong Qi/Xinhua)