The basics: What we know — and don’t know — about the virus spreading in China and beyond

It’s been only three weeks since Chinese authorities alerted the World Health Organization that it had an outbreak of unusual pneumonia cases. An astonishing amount of information has been learned in the days since about the outbreak, caused by a new virus that jumped from animals into people. But a frustrating amount of information remains still beyond our grasp.

Scientists and public health officials in China and beyond are scrambling to try to learn more about the virus, which has already spread out from the Chinese city of Wuhan, where the outbreak appears to have started, to other parts of China and several other countries, including the United States.

We all have more questions than answers. But here’s some of what we’ve learned about this new disease threat.

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Where did the virus come from?

Health officials in China tracked many of the initial cases to a large seafood market in Wuhan, a city in central China, that also sold wild animals as meat. The leader of an expert committee working on the outbreak has said some type of game is the most likely source.

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The virus, provisionally called 2019-nCoV, is a coronavirus — from the same family as the viruses that caused SARS and MERS. These viruses are generally believed to originate in bats, but they can spread to other animals before making the leap to people. The 2002-2003 SARS outbreak, for example, was traced back to palm civets and other small mammals, while MERS jumps from camels to people. Once people are infected with a virus, it’s possible they can transmit it to other people — though how easily it can spread among people depends on a number of factors.

So how easily can this virus spread?

Experts don’t yet know the mode of transmission — or how easily transmission can happen. Many of the cases detected initially were tied directly to the market in Wuhan. But some of the subsequent infections were clearly examples of person-to-person spread.

Health officials are confident that there is at least limited person-to-person spread. But the World Health Organization has also raised the possibility that there could be ongoing, sustained transmission — meaning that the virus could transmit easily from one person to the next and then onward, like the flu.

What are the symptoms of this viral infection? How dangerous is it?

China alerted the WHO on Dec. 31 to an unusual burst of cases of pneumonia — an inflammation of the lungs, accompanied by cough and phlegm, that can be caused by a variety of pathogens. As health officials have learned more, they’ve seen a broader array of symptoms, according to the WHO. Overall, health authorities say, symptoms include fever, cough, and shortness of breath.

A number of patients have been treated at hospitals and released within a few days. For others, the complications have been much more severe. As of Wednesday, 17 people with the infection in China had died.

It’s possible that some people have such mild symptoms they’re not seeking medical care or that health authorities have missed some of these mild cases. That makes it hard to know how dangerous the virus is to most people, and it remains very early in the outbreak.

The WHO said this week that of 278 confirmed cases in China, 51 people are severely ill, with 12 in critical condition.

“A major concern is the range of severity of symptoms this virus is causing,” Jeremy Farrar, the director of the Wellcome Trust, said in a statement Tuesday. “It is clear some people are being affected and are infectious while experiencing only very mild symptoms or possibly without experiencing symptoms at all (asymptomatic). This may be masking the true numbers infected and the extent of person to person transmission. It is a matter of urgency to work this out.”

One common thread among most of the fatal cases is that the patients had other health problems. The first death was a 61-year-old man who also had abdominal tumors and chronic liver disease. Two deaths reported Tuesday — the fifth and sixth of the outbreak — were a 66-year-old man with COPD, hypertension, and other conditions, and a 48-year-old woman who had diabetes and who had had a stroke.

Why has the number of cases spiked so quickly?

There’s no disputing the case count has risen sharply. As of Friday, local health officials in Wuhan had confirmed 45 cases, with three identified “exported” cases in Thailand and Japan. By Wednesday, there were nearly 450 confirmed cases in China.

There are a few possible explanations for the spike. One is that the virus is simply spreading quickly on its own. But when you’re looking for something, you’re also more likely to spot it. As awareness about the virus increased, and health authorities throughout China and in other countries started keeping an eye out, they started investigating cases of respiratory infections more rigorously. People with symptoms may have been more likely to seek out medical care. Plus, researchers have publicly released the genetic sequences of virus they’ve collected, which allows for conclusive diagnosis.

There’s also the political factor. Chinese President Xi Jinping said Monday that the outbreak “must be taken seriously” and that “party committees, governments, and relevant departments at all levels should put people’s lives and health first.” This was seen as a signal from the country’s leader that officials should be transparent about the scope of the outbreak as opposed to restricting information.

How likely is it that the numbers we’re seeing reflect the actual number of infections?



Not very. With most infectious diseases, there’s a spectrum of illness. With some diseases, some infections are so mild the person may have no discernible symptoms or may feel just a little off. With some diseases, the other end of the spectrum can be critical illness and death.

Infectious disease experts sometimes use the analogy of an iceberg, with the really sick cases represented by the part of the iceberg that’s above the water. The people who are asymptomatic or so mildly sick they don’t seek medical attention are the part of the iceberg that doesn’t come into view. It’s not yet clear with this new disease how much of the disease we’re seeing. Until that picture comes into focus, it’s hard to put the number of deaths being reported into any useful context.

Seventeen deaths out of nearly 450 cases is a fairly concerning number. If there are actually 10 times or 100 times more cases the decimal on any rough calculation of a fatality rate slides to a less alarming position. While it’s impossible to know with any degree of certainty how big this outbreak is at the moment, infectious disease epidemiologists are using mathematical models to try to come up with estimates. One group, led by Neil Ferguson of Imperial College London, estimated Friday there had been at least 1,723 cases in Wuhan alone by Jan. 12. Ferguson’s team has re-run the model, plugging in the latest available data. The new estimate, published Wednesday, suggests there were 4,000 cases in Wuhan by Jan. 18.

How is this like SARS? How is it different?

There are obvious similarities and sharp distinctions. Let’s start with the former.



This outbreak is being caused by a coronavirus, from the same family as the virus that caused SARS. In fact, genetically this virus is similar-ish to SARS — its genetic sequence varies from the SARS epidemic virus by about 20%.

Both outbreaks started in China. SARS jumped to people from small wild mammals sold as a delicacy in what are known as wet markets, where live animals can be bought and butchered on the spot. It will surprise exactly no one if this virus jumped to people in the same way.

Another similarity: This outbreak is evolving super quickly, just as SARS did 17 years ago.

But the differences between this outbreak and SARS are profound. Chinese authorities tried to cover up SARS for months before exported cases finally alerted the world to what was going on.

This time they have been far more forthcoming, alerting the WHO, isolating a virus from a patient, sharing genetic sequences so that laboratories could design tests for the new virus, sharing information on case increases daily. This is night and day, experts say.

They also appear to have discovered that a new virus had emerged really rapidly — the result, likely, of the major investment the country made in infectious diseases science after SARS.

They discovered it fast? How fast?

There’s no way to be certain when this coronavirus started infecting people. But by studying the available genetic sequences — 14 have been placed into international databases to date — some informed guesses can be made.

Andrew Rambaut, a professor of molecular evolution at the University of Edinburgh’s Institute of Evolutionary Biology, has compared the sequences. Rambaut told STAT there is very little variation in them, suggesting they haven’t been spreading in people for very long.

By looking at the rate at which coronaviruses mutate — their evolutionary clock — Rambaut has estimated when these viruses started infecting people.

The sequences suggest China caught this outbreak almost immediately.

“The sequence evidence suggests that the virus first cropped up in humans very, very shortly before the outbreak was identified,” Rambaut said, pointing to early December. The Chinese reported the outbreak to the WHO on Dec. 31.

That suggests a remarkably finely tuned surveillance system, he said — all the more impressive because these unusual pneumonia cases were occurring during cold and flu season, when pneumonia is pretty common.

What does it mean there’s now a case in the U.S.?

Health authorities suggested it was only a matter of time before a case would appear in the U.S., and on Tuesday, they announced that a man in his 30s in Washington state who had traveled from Wuhan had the virus. Wuhan is a travel hub, with about 10% of the 60,000 people who travel from the city to the U.S. every year come in January because of the Lunar New Year celebrations, according to the CDC.

But health authorities say that, for now, the risk to the broader American public is low. That’s based on what’s known now — but the situation is evolving very rapidly. It’s important for everyone to remember that in outbreaks, information evolves.

CDC officials said they were anticipating additional cases in the U.S., likely from other travelers. All passengers coming to the country from Wuhan will now land at one of five airports — San Francisco, Los Angeles, New York’s JFK, Atlanta, and Chicago’s O’Hare — where officials are surveying passengers about their health and screening them for symptoms. The screening at the airports is meant not only to try to spot possible cases, but also serve as a reminder that people coming from Wuhan who may develop symptoms later should seek medical care.