The mysterious virus causing dozens of people to fall sick with pneumonia in China has now popped up in a woman in Thailand — the first case of international spread since the outbreak was announced on December 31.

The woman — a 61-year-old from China — had traveled to Bangkok from Wuhan, the mainland Chinese city of 19 million that’s currently the center of the outbreak, according to the World Health Organization (WHO) and the Bangkok Post. Sixteen people who sat near the woman on her flight have so far tested negative for the virus, and the woman is now recovering at the Bamrasnaradura Infectious Diseases Institute in Nonthaburi province, just outside of Bangkok.

But what’s most significant about this case: the woman never visited the food market that’s been linked to the outbreak, according to the Thai Ministry of Public Health’s Department of Disease Control. If that’s correct, it could mean the new virus has the potential to spread from person to person, instead of just from the animal carrying the virus to humans, as health authorities have been suggesting. It also means the virus may have already moved well beyond Wuhan.

A representative of the WHO confirmed that suspicion on Wednesday. “It is possible that there is limited human-to-human transmission, potentially among families,” Maria Van Kerkhove, acting head of WHO’s emerging diseases unit, told Reuters. She added that right now, most of the spread in the outbreak doesn’t appear to be through this route.

Instead, the outbreak has been linked to a local food market in Wuhan, the capital of Hubei, one of China’s most populous provinces. Health officials in Wuhan have reported that the patients with the virus so far were “mainly business staff and purchasers” at the Huanan Seafood Wholesale Market, where vendors peddle seafood as well as animals such as birds and rabbits. This suggested that the new virus had spread to humans from one of the animals there.

With the first case in Thailand, WHO warned about the possibility of broader spread and said it was considering organizing an emergency committee meeting. If such a meeting is called, a panel of independent experts would discuss whether the outbreak is dire enough to constitute a “public health emergency of international concern,” a rare designation the WHO gives to diseases that pose a global threat.

China is sharing information about the outbreak fast — a contrast to SARS

Because health authorities are dealing with a new virus, the picture of this outbreak — and its seriousness — is shifting rapidly.

The outbreak in Wuhan was only declared on December 31. By January 9, the state broadcaster China Central Television reported that 15 of the people who had become ill tested positive for the new virus. By January 11, scientists in China shared the genetic sequence of the new virus and the WHO applauded China’s efforts. “WHO is reassured of the quality of the ongoing investigations and the response measures implemented in Wuhan, and the commitment to share information regularly,” the agency said in a statement.

This was also a contrast to the SARS outbreak of 2003, when China was heavily criticized for withholding information about the outbreak for too long.

But other aspects of the outbreak bear a startling resemblance to SARS, which also involved a then-new virus when it was discovered in 2003. The virus jumped from civet cats — a food delicacy in China — to humans, and went on to spread to two dozen countries. It eventually killed 774 people and infected more than 8,000.

The one patient death “marks this virus as a significant concern”

To date, authorities in Wuhan have reported 41 patients with the novel coronavirus infection. Of the 41, seven are severely ill, and two have been discharged from the hospital. One patient — a 61-year-old man with underlying health problems, including chronic liver disease — died on January 9. The man went into the hospital with respiratory failure and severe pneumonia. He had bought food from the Wuhan market regularly and tested positive for the new virus.

The leading hypothesis is that the disease is the result of a new coronavirus, a member of the same family of viruses as SARS.

Coronaviruses attack the respiratory system and can target the cells deep within the lungs. “There are tons of coronaviruses,” said Vincent Munster, an emerging viral diseases researcher at the National Institute of Allergy and Infectious Diseases.

Most infect mammals, including bats. Two of the six that are known to infect humans, SARS and MERS, can cause severe pneumonia and even death. The rest lead to milder symptoms, like a common cold.

In the current outbreak, the main symptoms reported are fever followed by difficulty breathing, the WHO said. In chest X-rays, patients appear to have lesions in both lungs. According to the Bangkok Post, the patient in Thailand had neither fever nor respiratory symptoms, and might be discharged in a few days.

But the death of one patient “marks this virus as a significant concern,” said Peter Daszak, the president of EcoHealth Alliance, a US global health research organization working in China.

Big questions remain

Researchers will need to confirm that this new virus is without a doubt the driver of the outbreak, said Marion Koopmans, who studies emerging infectious diseases as director of the department of virology at Erasmus Medical Center in Rotterdam. They’ll then need to figure out the range of illness the virus causes in the confirmed cases.

“If [the new virus] is indeed the likely cause, and seeded at the market, a key question is how transmissible this is,” Koopmans said. Viruses that spill over from animals may not be very transmissible “if they sit deep in the lungs and are not shed easily,” she added. That would mean, Munster said, “the epidemic potential of this virus might still be limited.”

But the Thailand case suggests the potential for human-to-human spread. And it’s possible, Daszak said, that the virus was incubating in patients for a couple of weeks, and that the secondary cases — from people who were directly exposed to the animal carrying the virus — are only now going to start popping up.

If that’s the case, “the scope of this outbreak expands massively,” he added. “We already have 40-plus cases in China and one is traveling. I don’t understand why there are so many cases if there’s no human-to-human transmission.”

Health officials would also need to find out which animal is spreading the virus to humans — the “natural reservoir” of the virus — and how the virus made the jump, and then make sure that animal is contained.

“We don’t know where the virus came from,” Daszak said. “We don’t know the geographic origin of the wildlife reservoir. And it’s now known to be lethal.”

The WHO is not recommending any measures for travelers and advising against travel or trade restrictions on China, even on the cusp of China’s Lunar New Year holiday, during which hundreds of millions of people are expected to travel. The market that’s been linked to the outbreak has been closed for disinfection, and health officials are following 739 close contacts of the patients, including 419 medical staff, to see if they develop symptoms. Health officials in Wuhan say there have been no new cases there since January 3.

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