WHO Director-General Tedros Adhanom Ghebreyesus (center) announced on March 11, 2020 that the new coronavirus outbreak can now be characterized as a pandemic.

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In a press conference Wednesday morning, WHO Director General Tedros Adhanom Ghebreyesus said what has been increasingly obvious for weeks: The novel coronavirus that has swept the world, infecting more than 120,000 people in more than 100 countries, is a pandemic.

“WHO has been assessing this outbreak around the clock and we’re deeply concerned both by the alarming levels of spread and severity and the alarming levels of inaction. We have therefore made the assessment that Covid-19 can be characterized by “pandemic,” Tedros said, going on to argue that the designation did not mean the situation was hopeless: “All countries can still change the course of this pandemic.”

The news came after identified cases doubled in the US in the space of just two days, Chancellor Angela Merkel in Germany said that 60 to 70 percent of Germans could become infected, and Italy locked down its entire population and warned the world that they were running out of ICU capacity — while experts warned many other countries were on track for large outbreaks and health care capacity issues.

The rapid spread of the virus in more countries likely contributed to the WHO’s decision to declare the disease a pandemic. For weeks, the organization had held off, observing that 90 percent of all cases were in mainland China and that most countries had reported only a few cases.

But in recent days, that’s become increasingly untrue. There are now more than 40,000 cases outside mainland China, 33 percent of all cases. There is substantial community transmission in countries including Iran, Italy, and the United States. The declaration that it is a pandemic doesn’t change any facts on the ground, but it reflects the WHO pulling out all the stops in their effort to get the world to respond to the situation with the seriousness it deserves.

What is a pandemic?

To define a pandemic, we need to first define a few other terms.

First, when a disease infects a lot of people in one area in a short period of time, that’s an outbreak. For example, the US has experienced a series of measles outbreaks over the past few years, with 2019 the worst year for measles since 1992.

The measles outbreaks were quite different from a global public health perspective than the coronavirus outbreak. Measles was introduced by travelers from other countries and spread in populations with low vaccination rates, instead of jumping from animals. Since there is a vaccine, and since some people have had exposure and immunity, the measles outbreaks — while a serious public health crisis — didn’t present a risk on the scale of the coronavirus outbreak.

The novel coronavirus crisis began with an outbreak of 41 identified cases in Wuhan, China, in December that began spreading rapidly in January.

Doctors at West China Hospital of Sichuan University use 5G technology to diagnose coronavirus patients remotely on February 27, 2020.

Liu Zhongjun/China News Service via Getty Images

That brings us to a term you might have seen thrown around: epidemic. The World Health Organization defines an epidemic as “the occurrence in a community or region of cases of an illness ... clearly in excess of normal expectancy.” The CDC defines it as “an increase, often sudden, in the number of cases of a disease above what is normally expected” in a region. You may have heard the term used to refer to the “opioid epidemic,” or other instances of a long-existing hazard becoming rapidly more deadly or more widespread.

When it comes to defining a pandemic, things get a little more complicated.

According to A Dictionary of Epidemiology, the standard reference for epidemiologists, a pandemic is “an epidemic occurring worldwide, or over a very wide area, crossing international boundaries and usually affecting a large number of people.”

A “pandemic” disease sounds much scarier than a mere “outbreak.” But it’s important to remember that “pandemic” refers to how many parts of the world are dealing with an elevated rate of the disease — and, in theory, says nothing about how serious the disease is.

In 2009, the H1N1 influenza was deemed a pandemic. One in five people worldwide caught the disease. But it was not very deadly, with an overall mortality rate estimated at 0.02 percent, so the societal disruption was limited and the loss of lives, while tragic, didn’t overwhelm medical systems.

Past coronaviruses have been dangerous, but they have not gone pandemic around the globe. The 2002-’03 SARS outbreak — primarily in China and Hong Kong, though with cases around the world — had about a 10 percent mortality rate. But as the disease was swiftly contained, it was never a pandemic, and no cases have been reported anywhere in the world since 2004.

Similarly, the even deadlier MERS coronavirus — with an approximately 35 percent mortality rate — was not very contagious and circulated only slowly. MERS originated in Saudi Arabia, and nearly all transmission has been there, though a traveler from the Middle East spurred a 2015 outbreak in South Korea. It has not been entirely eliminated, but it has never been a pandemic.

Sometimes, though, diseases have both a high mortality rate and enough transmissibility to become a pandemic. That’s the worst-case scenario for public health. The best example is the 1918-’19 influenza pandemic, which — aided by the troop movements of World War I — traveled all around the world and struck an estimated 500 million people.

Red Cross workers on duty in St. Louis, Missouri, during the Spanish influenza epidemic, 1918.

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At the time, that represented 25 percent of the population of the world. There are varying estimates for its lethality, but it was likely lower than that of SARS or MERS and may even have been as low as a few percentage points. That’s still high enough to have had a devastating effect on society. The death toll is estimated to have been 40 million to 50 million, though some estimates are higher.

When public health experts talk about their fear of another pandemic, they have in mind the possibility of something like the 1918-’19 influenza. That said, it’s worth noting that even a “mild” pandemic like H1N1 in 2009 is estimated to have caused up to 575,000 deaths, because when a disease infects the whole world the death toll will be very high even if the disease is fairly mild.

Why it matters that something is called a pandemic

So if the distinction between a pandemic and an epidemic isn’t how severe they are or how scared we should be, what is the distinction? Why bother categorizing diseases by how many countries are affected?

One answer is that pandemics must be battled, at the international level, significantly differently than epidemics.

When one region of the world experiences an epidemic, the rest of the world is on the sidelines. They may shut their borders to the affected region, or send help, or both. They may start making preparations in case the disease affects their own country. When coronavirus broke out in Wuhan, many of China’s neighbors sent masks. In a now-retracted Lancet paper, health workers in Wuhan asked if they’d consider sending volunteer nurses and doctors, too.

A man waits in front of the consultations center dedicated to the coronavirus in Mulhouse, France, on March 9, 2020.

Sebastien Bozon/AFP via Getty Images

When there’s a pandemic, there are essentially no sidelines. To some extent, it no longer makes as much sense for countries to shut their borders against the affected countries, as the disease is present everywhere.

Public health officials might recommend shifting to social distancing measures — which reduce spread within a country — instead of screening efforts that try to keep the disease out of the country at all. Countries still must share medical knowledge and expertise with one another, but directing supplies to affected areas is more complicated when affected areas are all over the globe. A world fighting a pandemic is not focused on containment to a distant elsewhere but on harm-reduction at home.