THREE patients who were initially given the coronavirus all-clear have tested positive for the deadly infection – sparking fears of "silent carriers".
The cases include two people in Japan and one in the US who were evacuated from Wuhan, China, where the outbreak started.
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Experts say it raises the risk of people being infected with the disease despite showing no symptoms.
It comes as experts warn the incubation period could be up to 24 days – in rare cases.
Officials had previously warned that infected people can spread the virus for up to two weeks before showing any symptoms.
But new research by Chinese government medical advisers warned their tests show it could be up to ten days longer.
Meanwhile, a leading health expert has warned that the deadly virus could kill 45 million people – if it's not contained.
The 'silent carriers'
One of the latest "silent carrier" cases involved a US citizen who was evacuated from Wuhan and taken to a hospital in San Diego, California, for testing.
He was temporarily discharged after initial testing determined he hadn't contracted the virus, officially named Covid-19.
However, further testing confirmed that the patient had actually did have coronavirus, health chiefs confirmed on Monday.
Meanwhile, two Japanese men who were evacuated from Wuhan last month also tested positive for the virus, despite an initial all-clear.
One of the men, who is in his 40s, was tested on his return to Saitama and told he didn't have the new coronavirus on January 30.
But he became ill with a fever on Saturday before being diagnosed with Covid-19 on Monday – 12 days later.
The other man, in his 50s, had been staying in a hotel since his return from China.
He was tested twice and both came back negative, but a third test on Monday – 12 days later – was positive.
He started to feel unwell last week and tested positive for the virus three days later.
However, scientists claim these instances 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 someone was tested very soon after they had been infected, there might not be enough of the virus to show up on the analysis.
He stressed that "on the balance of probability" these patients were unlikely to be infectious at that time.
What to do if you're worried you've got coronavirus
BRITISH health chiefs have raised the coronavirus risk to the public from low to moderate.
Health professionals are working to contact anyone who has been in close contact with people who have coronavirus.
The majority of those who have been infected with the virus so far have either visited China or been in close contact with someone who has.
But if you are concerned known the signs is one of the best ways to protect yourself from 2019-nCoV.
Symptoms usually include:
- a cough
- a high temperature
- difficulty breathing
In most cases, you won't know whether you have a coronavirus or a different cold-causing virus.
But if a coronavirus infection spreads to the lower respiratory tract, it can cause pneumonia, especially in older people, people with heart disease or people with weakened immune systems.
It is incredibly contagious and is spread through contact with anything the virus is on as well as infected breath, coughs or sneezes.
The best way to prevent catching any form of coronavirus is to practice good hygiene.
If you have cold-like symptoms, you can help protect others by staying home when you are sick and avoiding contact with others.
You should also cover your mouth and nose with a tissue when you cough and sneeze then throw it away and wash your hands.
Cleaning and disinfecting objects and surfaces which you may have touched is also important.
If you have returned from Wuhan in the last 14 days:
- Stay indoors and avoid contact with other people as you would with other flu viruses
- Call NHS 111 to inform them of your recent travel to the city
- your recent travel to the city
If you are in Northern Ireland, call your GP.
Please follow this advice even if you do not have symptoms of the virus.
Meanwhile, leading symptom-checking provider to the NHS Doctorlink has been updated to help identify patients' risk of having coronavirus.
Prof Tedder, a visiting professor in medical virology, told MailOnline: "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."
This is inevitable when you are sampling people shortly after they have become infected
The number of people killed by the new virus has now surpassed 1,110 while infections have reached more than 45,000.
World health bosses have warned Covid-19 poses a bigger threat than terrorism.
WHO director general Tedros Adhanom Ghebreyesus said the epidemic poses a “very grave threat” to the entire globe.
He said: “To be honest, a virus is more powerful in creating political, economic and social upheaval than any terrorist attack.
"A virus can have more powerful consequences than any terrorist action.
“If the world doesn’t want to wake up and consider this enemy virus as Public Enemy number 1, I don’t think we will learn from our lessons.”
It comes as top Hong Kong medical official warned that the deadly virus could infect 60 per cent of the world's population.
Professor Gabriel Leung, chair of public health medicine in the city, also said that even if the death rate sits at one per cent, it could still kill millions.
With the global population currently at more than seven billion, it means the new coronavirus could potentially infect more than four billion.
If one per cent of those people die, it would result in more than 45 million deaths.
However, World Health Organisation bosses have urged scientists to stop "throwing around figures that there is no basis for".
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