(Reuters) - International alarm over the coronavirus that emerged in Wuhan, China in December is driven by its rapid spread and the fact that infectious disease experts cannot yet know how deadly or contagious it is.
Within weeks, the virus has infected nearly 8,000 people in China and killed 170.
Close to 100 cases have been confirmed in other countries, from Japan to the United States. On Thursday, the World Health Organization declared the outbreak a global emergency.
Statistics from China indicate that just over 2% of people known to be infected with the virus have died, suggesting that it may be less deadly than the coronaviruses behind Severe Acute Respiratory Syndrome (SARS) and Middle East Respiratory Syndrome (MERS).
But disease experts caution that it will take several more weeks to be confident of how the new virus behaves given how quickly it has spread and the fact that a reliable diagnostic test has only recently been introduced.
“Not everybody is being seen, not everybody is being tested,” Dr. William Schaffner, professor of preventive medicine and infectious diseases at Vanderbilt University Medical Center in Nashville, told Reuters.
“All the experts, myself included, tell the public that there is much we don’t know about this virus and we are learning as we go along. That is not so reassuring.”
Some experts question whether the new virus shares similarities with seasonal flu, which has a low mortality rate but infects so many people that more than half a million may die from it each year, according to global health estimates.
GRAPHIC: Tracking the novel coronavirus here
The latest statistics indicate a fatality rate of about 2.2%, but the actual rate may be higher or lower as there are likely more unconfirmed cases, he said.
The SARS virus killed about 10% of all infected individuals, while the MERS outbreak identified in 2012 had a fatality rate of around 35%.
In emerging infectious disease outbreaks, the most serious cases are identified first. Coronavirus infections can range from mild cold-like symptoms to severe cases that cause pneumonia, acute respiratory illness and death.
About 20% of confirmed cases in the China coronavirus outbreak are classified as severe, similar to SARS and MERS, Schaffner said.
GRAPHIC: Online package of China virus news here
The previously unknown strain is believed to have originated late last year from illegally traded wildlife at an animal market in Wuhan.
Within weeks it appeared capable of being transmitted from one person to another via droplets when an infected person breathes out, coughs or sneezes. It can also spread via contaminated surfaces such as door handles or railings.
“The rapidity of this outbreak is startling and certainly much more rapid than SARS,” said Jonathan Ball, a professor of molecular virology at Britain’s University of Nottingham.
The incubation period is estimated between one and 14 days, and there have been anecdotal accounts of “symptomless spreading” by someone who is infected but unaware of it.
Neil Ferguson, an infectious disease specialist at Imperial College London, says that is too early to know if so-called “super-spreader” events seen with MERS and SARS is happening with the new coronavirus.
Some 60 million people are under virtual lockdown in China’s Hubei province – of which Wuhan is the capital. Australia, South Korea, Singapore, New Zealand and Indonesia are quarantining evacuees from China for at least two weeks.
Many global airlines have suspended or scaled back direct flights to China’s major cities.
Experts say China has been more forthcoming with information than it was during its 2003 SARS outbreak, but there are still concerns about how much data they are sharing.
Some also questioned the effectiveness of the quarantine attempt on millions of people.
“That kind of thing can backfire. It can make it harder to get resources in,” said Dr. Amesh Adalja, from the Johns Hopkins University Center for Health Security.
Reporting by Deena Beasley in Los Angeles and Kate Kelland in London; Editing by Michele Gershberg and Lisa Shumaker