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The word “pandemic” is not a comforting one.

And yet it is increasingly being bandied about in connection with the spread of the novel coronavirus that emerged from China.


U.S. health officials have been signaling for nearly two weeks now that a coronavirus pandemic may be on the horizon. While stressing that the virus presents only a “low risk” to Americans right now, the director of the National Institute of Allergy and Infectious Diseases, Dr. Anthony Fauci, acknowledged Tuesday that that might not remain the case for long.

“Is there a risk that this is going to turn into a global pandemic? Absolutely, yes,” Fauci said.

But what is a pandemic? And if efforts like the quarantining of returning travelers cannot stop spread of the virus, what will? We’re peering into the unknown here, but given the ease with which this coronavirus seems to transmit from person to person, the world is likely to see much broader international spread of the virus, now known as SARS-CoV-2. (The disease the virus causes has been named Covid-19.)


Here’s what you need to know about pandemics.

What is a pandemic?

A disease doesn’t have to infect all of the globe to be a pandemic. Lots of people, over broad swaths of territory, will suffice.

The World Health Organization defines pandemic as the worldwide spread of a new disease. The Centers for Disease Control and Prevention offers a definition that is a bit more elastic, describing it as a disease that spreads across regions.

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The word is most commonly used in the context of influenza. Flu pandemics occur when new flu strains emerge from nature — say a swine flu virus or a flu strain circulating among chickens — and start infecting people. But other diseases can and have been declared pandemics.

It’s important to know that calling a disease outbreak a pandemic doesn’t connote severity. The term relates solely to the amount of ground the outbreak covers.

There can be mild pandemics, such as the 2009 H1N1 flu pandemic; it killed roughly as many people as regular flu, though many were younger than those who succumb to seasonal flu.

At the opposite end of the severity spectrum from the 2009 pandemic was the 1918 Spanish flu. The poster child for a bad pandemic, it killed between 50 million and 100 million around the globe in 1918-1919.

Other flu pandemics from the era of modern virology — 1957, 1968 — looked a lot more like 2009 than 1918.

When was the last pandemic? When was the last pandemic caused by a coronavirus?

The last pandemic was the aforementioned 2009 H1N1 outbreak. As for when or whether there has been a coronavirus pandemic before, that’s not known.

There are four coronaviruses that are endemic in people, meaning they circulate in an ongoing manner. You likely don’t know their names because they mostly cause colds; they’re among the multitude of viruses that swarm around us during cold and flu season. (They’re known as OC43, NL63, 229E, and HKU1.)

They were all formerly animal viruses but crossed over to infect people before the existence of viruses was recognized. As a result, there is no record of how rapidly they spread when they first started infecting people and whether they caused severe disease before settling down to become causes of the common cold.

Two other animal coronaviruses have crossed into people in the past two decades, SARS and MERS. SARS spread to a number of countries around the world but the virus was snuffed out after about 8,000 cases and nearly 800 deaths. MERS, which still jumps to people from camels, has mainly infected people on the Arabian Peninsula. Neither is considered to have caused a pandemic.

A new type of coronavirus is responsible for the outbreak of respiratory illnesses that began in Wuhan, China December 2019. While experts are still unclear how exactly these viruses are transmitted, coronaviruses such as those that caused the SARS and MERS outbreaks in years past offer clues. Hyacinth Empinado and Alex Hogan/STAT

Who would declare a pandemic?

An official declaration would come from the WHO, though the agency may be reluctant to unleash this term anytime soon.

The WHO was harshly criticized when the 2009 flu pandemic turned out to be much less severe than people had feared. Rather than feeling relieved the pandemic wasn’t causing large numbers of deaths, people felt aggrieved they’d been scared over something they later concluded was far less scary than expected. And governments that had contacts to buy pandemic vaccine — contracts that were triggered by the WHO’s declaration — were left on the hook for vaccine many people didn’t want.

In other words, the term can be a loaded one. But if this virus is discovered to be spreading in an unstoppable fashion in countries on several continents, infectious disease experts will use the P word — whether the WHO officially declares a pandemic or not.

Should the outbreak be declared a pandemic now?

There currently isn’t evidence that the virus is spreading out of control in many places outside of China. That said, it’s currently challenging to know how much transmission is going on outside of China because a lot of infections appear to be quite mild and thus easy to miss.

Until recently only two laboratories in Africa — one in Senegal, the other in South Africa — were able to test for the new coronavirus, known provisionally as SARS-CoV-2, which was previously known as 2019-nCoV. Late last week the WHO shipped testing reagents to labs in a number of African countries. So far no African countries have reported cases; but given the strong links between China and many African countries, they may find cases soon.

But aren’t there cases in a lot of places?

Roughly 24 countries have diagnosed infections, and the overall numbers are growing. Most of the cases still are in tourists from China or people who had recently been in China. But increasingly countries are reporting cases of “secondary spread” — that is, cases in which a person who hadn’t traveled to China contracted the virus from someone who had. Some of those secondary cases are spreading the virus as well.

Take the case of a British businessman who got infected at a conference in Singapore in late January. He later traveled to the French Alps where he transmitted the virus to five family members on a ski trip. Later he flew home to Britain and spent some time in his local pub before realizing he was infected with the virus.

There’s also a cluster of now 14 cases in Germany stemming from a business trip a woman from China made to her company’s headquarters in Bavaria. She infected at least a couple of colleagues, who infected others. Several family members of infected colleagues have themselves tested positive for the virus.

WHO Director-General Tedros Adhanom Ghebreyesus warned this week that these types of cases could ignite more widespread transmission.

“The detection of this small number of cases could be the spark that becomes a bigger fire. But for now it’s only a spark,” said Tedros, as he is known. “Our objective remains containment. We call on all countries to use the window that we have to prevent a bigger fire.”

When will we know if that bigger fire has been ignited?

It’s worth watching Singapore. The city-state, which has a very strong public health system, was among the places hit during the 2003 SARS outbreak and has since then substantially beefed up its outbreak response capacity.

As of Tuesday, Singapore had diagnosed 47 cases, with an increasing number of those infections in people who had been infected in the city. Health authorities there have tried to trace how each case became infected. They are starting to see infections in people who aren’t part of known clusters of cases — which the government acknowledges could be a sign that the virus is spreading undetected in parts of the city.

In an extraordinarily candid address to his country, Prime Minister Lee Hsien Loong acknowledged the coronavirus “is probably already circulating in our own population.” The address was posted on Facebook on Saturday.

“If the numbers keep growing, at some point we will have to reconsider our strategy. If the virus is widespread, it’s futile to try to trace every contact,” Lee said. “We are not at that point yet. It may or may not happen. But we are thinking ahead and anticipating the next few steps.”

Infectious diseases epidemiologist Marc Lipsitch predicted that if Singapore is starting to find cases that can’t be connected to known transmission chains, other places will start seeing them, too.

Lipsitch, a professor at Harvard’s T.H. Chan School of Public Health, thinks the next couple of weeks will tell the tale. But he believes the world is in the early stages of a coronavirus pandemic.

“My picture of the future is essentially it’s like 2009 flu but more severe. The big question is how much more severe,” he told STAT.

The Covid-19 outbreak in China is quickly spreading worldwide, sparking quick calculations on how deadly this new disease is. One measure is called a case fatality rate. While the formula is simple, it’s difficult to get a precise answer. Hyacinth Empinado/STAT

How bad could this be?

The WHO says data from China suggests about 82% of confirmed cases have only mild infection, about 15% are severe enough to require hospital care, and about 3% need intensive care.

Preliminary data suggest roughly 2% of people who tested positive for the virus have died. While lower than the SARS 10% fatality rate, that number is not at all reassuring. But it’s also too soon to draw conclusions.

To calculate the fatality rate you have to have a good idea of how many people have actually been infected and how many have died. Because so many people likely have mild infections and haven’t been counted, it’s impossible at this stage to know the fatality rate for sure.

“If we’ve very fortunate, there are tons of mild cases being missed and maybe it’s 0.4% or something … which is big,” said Lipsitch. “But that would be a lot of mild cases missed.”

Four deaths per 1,000 people infected would amount to four times as many deaths as influenza normally causes in an average year. Given that this is a new virus to which most people will have no immunity, it’s conceivable the attack rate — the number of people infected — will be higher than the attack rate for seasonal flu. In the U.S., the CDC estimates that about 8% of people will contract influenza in any non-pandemic year.

The damage could be worse in low-income countries that don’t have the capacity to support a lot of severely ill pneumonia patients who need mechanical ventilation.

“This disease may appear relatively mild in the context of a sophisticated health system. That may not be the case should this disease reach a system that is not as capable as that of China,” said Dr. Mike Ryan, WHO’s health emergencies director.

Transmission of viruses that cause colds and flu typically peters out in the Northern Hemisphere with the arrival of spring. President Trump said this week that the virus might fade by April. Might the advent of warmer temperatures stymie spread of this new virus?

Though this coronavirus isn’t related to influenza, it is acting a lot like a new flu virus. One of the characteristics of pandemic flu viruses is that they are able to circulate out of season; there are so many people susceptible that the virus can keep transmitting.

In 2009, during the last flu pandemic, the fact that there was a new flu virus circulating was only realized in late April — around the time when flu typically subsides in this part of the world. Transmission abated somewhat over the summer, but the big wave of infections in North America began early in the fall. By the time vaccine started to become available in November, the outbreak had peaked.

The temperatures this week in Singapore have been in the mid-to-high 80s. That suggests higher temperatures may slow, but probably won’t stop transmission of the new virus, Lipsitch said.

“I think the fact that Singapore is seeing cases transmit is also kind of evidence that it’s not a matter of yes or no,” he said. “It’s a matter of degree.”

Andrew Joseph contributed reporting.

    • My understanding is that it’s extremely contagious, spreading fast (including via carriers who don’t know they have it) and has the potential to kill hundreds of thousands, if not more, in the U.S. alone. Taking aggressive preventive measures up front is the only way to stem the tide … if it’s not too late already.

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