The UK based its entire strategy around herd immunity. Leading US expert Anthony Fauci says some immunity is very likely after getting the disease. But at the same time, reports are piling up of reinfected cases. So what’s going on?

“We haven’t formally proved it, but it is strongly likely that that’s the case,” Fauci said. “Because if this acts like any other virus, once you recover, you won’t get reinfected.”

But in the case of a Japanese man, this wasn’t the case.

The man in his 70s first tested positive for the virus on February 14 while he was onboard the Diamond Princess — then under quarantine off Yokohama. He left a medical facility in Tokyo on March 2, after he was confirmed negative. He returned to his home using public transportation.

Ten days later, he developed a fever and felt sick.

He went to the hospital and one day later the man was confirmed to be infected again.

He isn’t the only one with this type of story. Mr. Wang, a resident of Xuzhou, in Jiangsu province, had a month-long fight with the coronavirus. When he emerged victorious, he was greeted by residents in his building with a cake and flowers.

“With strong neighborly feelings, we welcome you home” — a banner on the building read.

They had a group photo and were featured on the news. But three days later, Wang felt sick and tested positive. The entire building is now in quarantine, and his whereabouts and overall state are unknown.

Reports of reinfections have been coming in for over a month in China, but such reports are marred by inconsistencies and are not considered to be reliable — in part, due to the imperfect tests and concerns about China manipulating data.

But similar reports are coming in from Japan and South Korea.

In Guangdong, officials responsible for the coronavirus response announced that 14% of declared recoveries in the province had later retested positive. But this province accounts for a low number of cases. In total, the official reinfection rate is much lower, at around 0.2%, and it’s often accompanied by milder symptoms — it’s rarely symptomatic at all. But in the case of a 36-year-old man in Wuhan, it was fatal: the virus killed him 5 days after he was released from a hospital, testing negative.

“If you get an infection, your immune system is revved up against that virus,” Keiji Fukuda, director of Hong Kong University’s School of Public Health, told the Los Angeles Times. “To get reinfected again when you’re in that situation would be quite unusual unless your immune system was not functioning right.”

A possible explanation could be that patients carry dormant viral fragments that flag the test, and trigger a positive response. Another possibility is that the infection wasn’t fully eliminated, but still remained inside the body at low levels, and then re-emerges. Or simply, the tests might not work well.

But the reinfection rate remains a key question mark worthy of further investigation.