Epidemic Vs. Pandemic: What Exactly Is the Difference?

They're both pretty terrifying—here's what to know about each.

Alex Sandoval

The worldwide coronavirus outbreak has officially been declared a pandemic, the World Health Organization (WHO) announced March 11 during a media briefing. According to WHO Director-General Tedros Adhanom Ghebreyesus, PhD, the decision to classify COVID-19 as a pandemic was not easy but resulted from concern for "the alarming levels of spread and severity, and by the alarming levels of inaction."

That word—pandemic—is enough to induce widespread panic, and with good reason: According to the WHO, a pandemic is the worldwide spread of a new disease. "A pandemic is when an epidemic spreads between countries," says David Jones, MD, PhD, a professor of the culture of medicine at Harvard University. In the case of COVID-19 specifically, the WHO said that it's the first pandemic caused by a coronavirus.

In the grand scheme of things, a pandemic is the highest possible level of disease, or a measure of how many people have gotten sick from a particular disease and how far it has spread—but before a common illness reaches pandemic proportions, it has to exceed a few other levels, according to the Centers for Disease Control and Prevention (CDC):

  • Sporadic: When a disease occurs infrequently and irregularly.
  • Endemic: A constant presence and/or usual prevalence of a disease or infection within a geographic area. (Hyperendemic is a situation in which there are persistent, high levels of disease occurrence.)
  • Epidemic: A sudden increase in the number of cases of a disease—more than what's typically expected for the population in that area.
  • Pandemic: An epidemic that has spread over several countries or continents, affecting a large number of people.

As far as epidemics go, they're considered such on a case-by-case basis, says Dr. Jones. Two examples come to mind: HIV and typhoid fever (stay with us here). HIV is clearly a huge problem, but, says Dr. Jones, it's not necessarily an epidemic in the US right now. "Since there have been 50,000 new cases of HIV each year in the US, and this number has been pretty stable for decades, there’s not really an HIV epidemic in the US — because we are getting the ‘expected’ number of cases,” he says.

Typhoid fever, on the other hand, sickened 51 people in Long Island in 1906. It's an extremely small number, comparatively speaking, but at the time and in that specific area, 51 cases of typhoid fever was a dramatic enough spike to be considered an epidemic.

Sometimes, an epidemic stays contained to a specific area—but when it extends into other countries or continents, and epidemic turns into a full-blown pandemic. That was the case in 2009 when the WHO declared the swine flu (caused by the H1N1 flu virus) a "Public Health Emergency of International Concern"—aka a pandemic.

When an epidemic crosses over into pandemic, the biggest difference is that more governments are involved in attempting to prevent the progression of the disease and, potentially, treat the people who have it. According to the CDC, the FDA may begin issuing “emergency use authorizations” (EUAs) during a pandemic, which allows doctors to use medications outside of the FDA-approved use. During the swine flu pandemic, the FDA issued EUAs for two antiviral drugs to attempt to prevent the flu in young children and to treat patients who’ve had symptoms for more than two days (which Tamiflu is not prescribed for).

Still, the edges of these definitions are a bit blurred. Technically, an epidemic only refers to infectious diseases, but it's also been applied to non-diseases, like gun violence and opioids. Similarly, what constitutes a pandemic is also a bit unclear. If we were to stick stringently to the definition, every illness that crosses the border from one country into another is a pandemic.

Consider the current epidemic of coronavirus, a respiratory illness that began in China before cases appeared in the US, Thailand, and South Korea. Technically, that’s a pandemic, says Gerald Keusch, MD, a professor of medicine and international health at Boston University. But it wasn't initially declared one. While something could have crossed borders to technically become a pandemic, if only a few people in other countries have reported being sick it doesn’t have the same urgency or fear factor as other illnesses that have been declared an international emergency.

And that fear factor seems to truly be the connotation that words like “epidemic” and “pandemic” strike. When the news declares gun violence, the opioid crisis, or this year’s influenza as an epidemic, it’s supposed to tell us that there’s a sense of urgency to fix the problem.

The information in this story is accurate as of press time. However, as the situation surrounding COVID-19 continues to evolve, it's possible that some data have changed since publication. While Health is trying to keep our stories as up-to-date as possible, we also encourage readers to stay informed on news and recommendations for their own communities by using the CDC, WHO, and their local public health department as resources.

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