London - Three patients have tested positive for coronavirus after initially being given the all-clear – raising the prospect of "silent carriers".
The cases – reported on Tuesday in the US and Japan – raise the prospect that people can be infected with the disease while believing they are healthy.
It also calls into question the accuracy of the test.
However, scientists claim such occurrences are "common" when patients have not yet entered the "diagnostic window" – when infections are large enough to be detected.
Professor Richard Tedder, an expert in viruses from Imperial College London, explained that if individuals were tested very soon after they have become infected, there may not be enough of the virus in the body to show up on the analysis.
He stressed that "on the balance of probability" these patients were unlikely to be infectious at that time.
One case involved a Japanese man in his 50s who had fled the Chinese city of Wuhan on an evacuation flight on January 29. He was tested twice and both came back negative, but a third test on Monday – 12 days later – was positive. He has been isolated in his hotel room since his return from China.
The second man, who is in his 40s, returned from Wuhan on January 30 and initially tested negative but was diagnosed with the virus on Monday. He is also understood to have been in isolation. Meanwhile, in San Diego in California, a woman who had been evacuated from Wuhan to a military quarantine centre tested positive after developing a cough.
She had previously been given the all clear and had been allowed to live alongside others who were also in isolation on the base.
Professor Tedder, a visiting professor in medical virology, said: "This is inevitable when you are sampling people shortly after they have become infected. This is common to all infections – a so-called diagnostic window.
"I don’t think we should be unduly worried by these cases."