Hong Kong boy's mysterious Covid-19 infection keeps experts guessing

Hong Kong went on a roller-coaster ride on Thursday (May 20) that started with the city within touching distance of achieving the milestone of a month with zero untraceable Covid-19 infections – and the long-awaited border reopening in sight.

But it all ended with the clock being reset after a four-year-old boy’s untraceable infection, previously thought to be a false positive, was confirmed by an infectious disease expert.

The development left behind a trail of unresolved questions.

“It is possible this case could remain a mystery forever,” one of the experts in the investigation said.

At the heart of the puzzle was how different test results were reached, culminating in the infection being confirmed.

Professor Yuen Kwok Yung, chair of infectious diseases in the University of Hong Kong’s department of microbiology, whose laboratory delivered the final verdict on the case, believed the discrepancies could be down to methodologies and equipment.

[[nid:521927]]

The boy’s infection was first detected by private contractor KingMed Diagnostics on Friday after he developed a fever, sore throat and a runny nose and was admitted to Prince of Wales Hospital in Sha Tin. The Department of Health confirmed the case the following day.

Questions, however, were soon raised after the boy’s symptoms quickly subsided and he produced no antibodies to the coronavirus, while none of his 34 close contacts, including family members, classmates and kindergarten staff, tested positive for the virus.

Further PCR and serology tests done by the Hospital Authority after admission yielded negative results. Serology tests are used to detect antibodies while a polymerase chain reaction (PCR) test is carried out to detect the presence of the virus.

On Wednesday evening, Professor Leo Poon Lit Man of HKU’s school of public health revealed that his team, which specialises in analysing genome sequences of Covid-19 patients’ specimens, also found no virus traces in the PCR test conducted for the boy.

But Yuen, who is also a government coronavirus adviser, dropped his bombshell on Thursday evening after touring the KingMed and health department labs.

He revealed he found no defects in the screening procedures at either, and his own facility at HKU found positive responses in several positions within the viral genome collected from the boy.

[[nid:528299]]

Addressing the mystery, Yuen explained that multiplex diagnostic methods, commonly used for genome-sequencing purposes, which placed 20 to 30 PCR specimens in one tube, were less sensitive than the monoplex ones he used.

Samples with an extremely low viral load could go undetected by a multiplex operation, he said.

Poon told the Post he agreed his method could have a lower sensitivity in detecting an infection, as that was not the primary aim of the test.

He said genome sequencing for the boy was conducted to check whether the virus he carried was a locally prevalent strain or a variant from overseas, or other forms not reported before, and to see whether the case was linked to previous clusters such as ones involving the Ursus Fitness gym or K11 Musea shopping centre.

“The aim of [genome sequencing] can help uncover any unknown transmission chain, but the sensitivity [in identifying an infection] would be lower,” he said.

But Poon maintained it was rare, based on past experience, for no virus traces to be detected when conducting genome sequencing for someone who had truly been infected.

Another mystery was the lack of antibodies in the boy. Yuen speculated the child caught the virus in late January when a cluster broke out in Block 13 of Pak Tin Estate in Shek Kip Mei, even though he lived on a different floor in a flat facing a different direction than the infected family.

Yuen suspected the boy was asymptomatic in January and therefore developed no antibodies, citing overseas experience that showed 19 to 20 per cent of Covid-19 patients did not produce antibodies if they had no symptoms.

[[nid:528823]]

“It is quite common that those patients could continue to shed remnant virus in their nasal area for more than two months, while some can still shed after six months,” he said, referring to the positive PCR result based on samples taken from the boy’s nasal region.

But why did the boy have a fever when he was taken to hospital? Yuen suggested it could be the result of a co-infection with another virus, or even bacteria, which also produced rashes on his skin, a condition he said was rare for Covid-19 patients.

The viral interference, where one infection suppressed the viral reproduction from another, would mean the boy’s Covid-19 viral load was low, especially if the other infection was more acute, Yuen said.

Poon, however, pointed out that the shedding of virus months after the initial infection usually happened to patients with severe symptoms rather than those with mild conditions experienced by the child.

“I can only say this might be a very special case,” he said, adding more tests would be needed to confirm this theory.

On the possibility raised by Yuen, concerning co-infection of two viruses, Poon agreed that such a condition could exist, but again said further tests would be needed to find out.

[[nid:524930]]

Some other tests that could be done could include a thorough antibody test, and those looking for different types of viruses to prove whether he had other infections.

It was possible the case could remain a mystery forever, Poon said.

As the day of twists and turns drew to a close, Yuen admitted there were still questions outstanding, and he could not say for sure how the boy got infected.

“But before we have found the definitive answer, we still need to treat him as a confirmed case,” he said. “We have to be on the safe side.”

This article was first published in South China Morning Post.