The first new disease outbreak in the social media era has been defined by the rapid spread of panic and uncertainty. Here’s what you should worry about — and what you shouldn’t.

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Update: This story was originally published with a different headline comparing the risk of the coronavirus in the US at the time to the flu. The headline, and the story, was based on information available in late January. Because the story is still being widely shared, we have decided to change the headline in order to reflect our current understanding of the pandemic. Here is the most recent BuzzFeed News coverage on the coronavirus.

WASHINGTON — The world of 24-hour news and global stock markets has met its match in the newly identified Chinese coronavirus, which moves at the speed of biology and defies hot takes on how much of a threat it poses to humanity.

“This outbreak is unrolling right in front of our eyes,” CDC’s Nancy Messonnier, director of the National Center for Immunization and Respiratory Diseases, said in a Monday briefing on the 2019-nCoV virus. “Scientists all over the world are looking at the available data to analyze it to come with information that would be helpful.” In less than a month, a report of dozens of people infected with a newly identified virus in Wuhan, China, after visiting a seafood market has turned into an outbreak affecting more than 7,800 people, causing over 170 deaths so far. The Chinese government has quarantined 50 million people in central China, banning travel following a small number of air travel-related cases, all nonfatal, which have spread to more than a dozen countries. The World Health Organization has announced it will reconvene a panel to recommend whether to declare the coronavirus outbreak a global emergency, a decision expected on Thursday. WHO is assembling an international team to send to China to help investigate cases there.

In the US, there are now six confirmed cases. An additional 110 people are under observation, according to Messonnier. The “great majority” are travelers from Wuhan or people who had close contact with them. “The general risk to the [US] public is low at this time,” she stressed. Still, the first new disease outbreak in the social media era has been defined by panic and uncertainty. An onslaught of sensational disinformation has spread like wildfire on platforms like Twitter and WhatsApp, along with overt propaganda and censorship from Chinese state media, making the actual risks of the fast-moving outbreak difficult to grasp. In some ways, the uncertainty is built into the new disease, as scientists and health officials scramble to understand it. Even without conspiracy peddlers on YouTube, these early weeks have seen plenty of genuine confusion as case numbers — trickling in from Chinese state media reports — have continued to climb. And health officials worldwide have grappled with identifying the spread of a new disease with what an early scientific report has called “non-specific” symptoms — coughing, fever, and pneumonia. Right now, the virus has more questions than answers. Here is what we know so far — and what we don’t. The coronavirus is infectious — but that shouldn't cause panic. Health experts measure the potential infectiousness of a disease by a metric called R0, or “R naught.” This is an estimate of the average number of people infected by each new case in a completely susceptible population with no efforts made — such as quarantine, hygiene, or hospitalization — to stop its spread.

Friendly reminder about #nCoV2019 transmissibility estimates from the past day: The basic reproduction number (R_0) is an *average*. An R_0 of 2 doesn't necessarily mean that every case will infect 2 other people. In fact, here are 3 (non-exhaustive) scenarios in which R_0 = 2.

But the R0 is also a snapshot in time: The measure changes as scientists get more data about the spread of the disease, which has led to fear and uncertainty online about the high initial estimates. Over the weekend, a number of estimates emerged, ranging from 1.3 to 3.8. Harvard health economist Eric Feigl-Ding called the 3.8 value “thermonuclear pandemic level bad” in a tweet that triggered thousands of panicked shares on the platform, followed by widespread criticism from scientists. Such declarations were “absolutely premature and hyperbolic,” epidemiologist Maimuna Majumder of Harvard Medical School and Boston Children’s Hospital, an author on one of the preliminary R0 estimate papers, told BuzzFeed News. Even the high number isn’t so terrifying, Messonnier said. “As a comparison, the R0 for measles is between 12 and 18,” she said. She noted that R0 is a moving target during an outbreak, with the goal of moving it below a measure of 1 as stronger actions are taken to quickly identify existing cases and prevent new ones. Each case would then lead to fewer people with the illness over time, snuffing out its spread. That’s what happened with SARS — which has an R0 of 3.0, according to the World Health Organization, but an “effective” infectiousness of less than 1 under quarantine measures that only started five months into its 2002 outbreak. SARS infected more than 8,000 people in two years, killing 774.

The coronavirus spreads person-to-person — but exactly how it spreads is still being worked out. The virus is a newly discovered member of the coronavirus family that includes the viruses that caused the SARS and MERS outbreaks. Human coronaviruses first jump from animals to people — from bats during SARS and camels in MERS — and then mutate to spread person-to-person. The animal that the new coronavirus originated in is still in dispute; one early scientific paper concluded it came from bats, while another argued that it closely resembled a virus that infects snakes. It’s important to know this origin to help stop future animal-to-human virus outbreaks. Another dispute in the 2019-nCoV outbreak is over when it becomes infectious in the course of an illness, with some Chinese officials suggesting it is early, when no symptoms are evident. That matters because the disease is thought to have a 2- to 14-day incubation period when a potentially infectious person could be unknowingly spreading the disease. China has suffered a widespread shortage of medical face masks, which experts say are only somewhat effective at stopping the spread of respiratory illnesses. Chinese Premier Li Keqiang also reportedly pledged to provide hospitals in Wuhan with 20,000 pairs of safety goggles to prevent exposure through the eyes; this could benefit health care workers who are exposed to coughing patients in the worst stages of the disease. For people in the US, CDC suggests washing your hands frequently; not putting your fingers in your mouth, eyes, or nose; and avoiding sick people — standard advice during flu season. Masks would not be necessary for anyone in the US, Health and Human Services Secretary Alex Azar said on Tuesday. SARS, the closest known genetic relative to the new coronavirus, only becomes infectious late in the illness, when it is lodged in the lungs and triggers coughing, releasing germs. The flu, by contrast, affects the nose and throat early in an infection and is often spread by sneezing. In Monday’s CDC briefing, Messonnier pushed back against claims about early infectiousness, reporting “no evidence” for such a spread. She cited travelers without symptoms entering the US without infecting other people on their planes and secondary cases seen primarily in relatives and medical personnel.

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The coronavirus can mutate — but so far that hasn’t led to a supervirus. These viruses are “messy” in their reproduction, coronavirus expert Stanley Perlman of the University of Iowa told BuzzFeed News, because their genetic material is RNA. That means they lack proofreading enzymes that plants and animals have in their cells, which use DNA as their genetic material; as the cells divide, the enzymes police mutations. Viruses, instead, are barely alive — they're mostly just packages of reproductive genes surrounded by a protein shell. That makes such RNA viruses prone to mutations such as the one that facilitated the jump of the coronavirus from something at a seafood market in Wuhan to people in the first place. It also raises legitimate fears about a mutation making the virus more infectious as the outbreak spreads. However, CDC has compared the genetic map of the virus that infected the first two US patients to the one from the first documented Chinese patient in Wuhan. No differences between the two were seen. This suggests that the virus has not mutated, Messonnier said. The risk of getting infected is high in China — but people in the US should be way more worried about the flu. The great majority of cases are in China, where the risk of catching the illness is high, according to WHO. "It's clear the outbreak is spreading rapidly in China," said Azar. More than 50% of the new cases in China are outside Wuhan's Hubei province, he noted, spreading to 30 Chinese provinces. One preliminary estimate by an international scientific team, suggests that nearly 200,000 cases may be diagnosed in China by early February. Outside of China, cases are not spiking. They have turned up in 18 countries and appear to be concentrated among people who were in Wuhan before the travel ban started or had close contact with people who were there. This led a WHO emergency panel to decline to declare the outbreak an international emergency last week. The deaths reported by China are largely among the elderly or people with underlying health conditions, putting them at high risk for pneumonia. A commentary in the Lancet medical journal estimated a 2.9% mortality risk from the virus, compared to 10% for SARS, but added that number is likely to decrease as more mild cases are documented. At the same time, “there is no room for complacency,” said that report, noting the 1918 Spanish flu killed around 50 million people worldwide with a mortality rate of less than 5%. To put the risk in more context, the current US flu season has killed 54 infants so far, according to CDC. And in the first two weeks of 2020, the flu has killed more than 5,000 people in the US, mostly through associated pneumonia. Others have suggested imports from China could carry a risk of transmission overseas. But coronavirus particles die within a few hours outside a host cell, according to Messonnier. So there is little risk of commerce from China spreading the outbreak.

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The widespread quarantine could stop the spread of the disease — but may harm people in China. Quarantines worked with SARS once Chinese officials quit hiding the extent of the country’s 2002 outbreak. That is the route China is going with now, quickly instituting a massive travel ban on Wuhan that later expanded to include much of Hubei province. But some experts have criticized China’s response to the new coronavirus as excessive and likely ineffective. “When done properly, limiting population movement can help ease the speed by which a disease spreads,” Rebecca Katz of the Center for Global Health Science and Security at Georgetown University, said in a statement on the unprecedented travel ban. “Broadly-applied interventions such as travel bans can cause public panic, impede individual rights, lead to secondary effects like shortages of food, and may not be effective at containing a virus if it has already spread outside of the epicenter, as nCoV-2019 has done,” Katz said. “Social distancing” measures, such as preventing people from riding buses or trains together, are better tools than blanket travel bans, she said.

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