Two members of a World Health Organization team investigating the origins of the coronavirus did not board a flight to China after testing positive for Covid-19 antibodies, the UN agency said on Thursday. The two scientists were negative for the standard nucleic acid test used to diagnose active infection, but tested positive for IgM antibodies, which may point to recent infection. China requires passengers flying from Singapore to take a nucleic acid test and an IgM antibody test up to two days before boarding China-bound flights. WHO guidance, however, says antibody tests should not be used to diagnose acute Covid-19 infection, as antibodies develop a few weeks after infection. The WHO said the two scientists were still in Singapore to be retested for IgM antibodies and another type, IgG antibodies. Those are thought to be longer lasting than IgM and suggest a person was previously infected. Asked about the two team members at a regular press briefing, Chinese foreign ministry spokesman Zhao Lijian said “relevant epidemic prevention and control requirements and regulations will be strictly enforced”, according to a Reuters report. The announcement came after the international team of 13 scientists arrived in the central Chinese city of Wuhan after flying together from Singapore on Thursday morning . “All team members had multiple negative PCR and antibody tests for Covid-19 in their home countries prior to travelling,” the WHO tweeted. The experts who continued on to Wuhan would begin their work immediately while they observed a two-week quarantine for international travellers, the WHO said. Leong Hoe Nam, a Singapore-based infectious disease specialist, said the IgM antibody test was “fraught with false positives”. “IgM stays positive for a long time and [this means that] it could be a false positive,” he said, adding that this was why many countries only required travellers to present a negative PCR test. Leong said antibodies would typically be present some 14 to 21 days after the onset of illness and this likely indicated that the WHO scientists had a past infection. “Scientists believe that the virus is unable to transmit infections after 10 to 14 days of illness and an antibody test that turns positive after 14 to 21 days should thus suggest non-infectivity,” he said. “The traveller shouldn’t be labelled otherwise.” The 10 international scientists taking part in the mission include public health experts, animal health specialists and virus hunters from Japan, Qatar, Germany, Vietnam, Russia, Australia, Denmark, the Netherlands, Britain and the United States. Scientists seek the invisible in Wuhan investigation into deadly pathogen In addition, the 15-person delegation meant to fly to Wuhan from Singapore includes experts from the WHO and the World Organization for Animal Health (OIE), a WHO spokesman said. Two scientists from the Food and Agriculture Organization of the United Nations (FAO) will also participate remotely. A resolution passed by the WHO’s governing body in May called for the inquiry into the origins of the coronavirus, believed to come from an animal. The resolution stipulated that experts from both the OIE and FAO should work with the WHO to identify the source of the virus and how it began spreading among humans. In a confusing false start last week, two members en route to the country were stopped by what the WHO described as “visa clearance” issues . Beijing later said both sides were in close communication on the specific dates and arrangements for the team. On Monday, China’s National Health Commission confirmed the WHO team would arrive on Thursday and work with Chinese scientists during the trip. Additional reporting by Dewey Sim