Dealers at the Xinfadi market have their information registered on Friday. Beijing authorities shut down the market after new COVID-19 cases linked the market. Photo: Li Hao/GT

Public fear over the mysterious COVID-19 infection case in North China's Tianjin was dispelled after the source of his infection was identified. But experts warned that the novel coronavirus threatens epidemiological investigations.

Based on the epidemiological history and genome sequencing results, the case is a human-to-human transmission, according to the Tianjin Centers for Disease Control and Prevention (Tianjin CDC) on Monday. 

A 22-year-old man working at Tianjin's Conrad hotel was reported to have been infected with COVID-19 on June 17. A preliminary investigation showed the confirmed patient, a 22-year-old man, contracted the virus from a chef who also works in the hotel, and the chef tested IgM (Immunoglobulin M) positive, Zhang Ying, deputy head of the Tianjin CDC, said on Monday.

People who test serum IgM positive normally have gained viral antibodies, according to virologists.

Zhang said the chef received negative results from three nucleic acid tests but still infected others. It took an IgM test to show that the chef had already been infected with the novel coronavirus.

Dubbed as a "Sherlock Holmes," Zhang from the Tianjin CDC said, "In the past, we rarely used serology to sort out the source of infection, since big data analysis can find clues. But this time, we found nothing." 

The chef had been to Beijing many times in recent months, and had been to several Beijing restaurants and other amusement venues, but did not show any symptoms. 

The official report did not publish how the chef contracted the virus or if he had any links to the recent cluster infections in Beijng's Xinfadi market. 

The chef tested IgM positive, meaning he was infected with COVID-19, but as he showed no symptoms, therefore he should be considered as an asymptomatic patient, Yang Zhanqiu, deputy director of the pathogen biology department at Wuhan University, explained to the Global Times on Monday.

The chef was very contagious in the early stages of infection, Yang said, noting that it was necessary to conduct an epidemiological investigation into him. 

Results of nucleic acid tests could be inaccurate as the virus strain reportedly from Europe is different from the previous domestic virus strain, Yang said. 

Previously, the mysterious patient who was confirmed to be infected with COVID-19 on June 17 said he had no history of travel in the 14 days prior to the onset of his illness, and no contact history with any confirmed or suspected cases, leading to public fears over his unknown source of infection. 

The patient had been washing dishes in the hotel since May 30, and occasionally cleaned frozen seafood. 

Experts have come up with various theories for how the patient contracted the disease, saying he could have been infected by anyone entering China with the virus, by someone coming from high-risk areas in Beijing, handling contaminated frozen food or being in an infected environment. 

According to the Tianjin CDC, the genome sequencing of the virus sample from the mysterious COVID-19 patient in Tianjin is the same as that from the clustered infections in Beijing's Xinfadi market. Xinfadi market is where the latest outbreak was detected in the capital city, Beijing.

Chinese health officials had said that the strain of virus in Beijing's Xinfadi wholesale food market has the European origin, but "is older than the virus currently spreading in Europe." 

Wu Hao, head of the Fangzhuang community health service center, said the Beijing scenario could happen anywhere in China, and no confirmed cases doesn't mean the virus does not exist. 

Tianjin has traced 9,966 people with connections to the Xinfadi market as of June 20, all of whom have tested negative for COVID-19. The samples taken from local seafood markets also tested negative as of June 20.