What’s the Difference Between a Pandemic and an Epidemic?

A lab technician begins semi-automated testing for COVID-19 at a lab in Lake Success, New York. Photo: Andrew Theodorakis/Getty Images

On Wednesday, the World Health Organization declared that the outbreak of the COVID-19 novel coronavirus has officially reached the level of a pandemic. “We have never before seen a pandemic sparked by a coronavirus,” WHO director-general Tedros Adhanom Ghebreyesus said Wednesday. “And we have never before seen a pandemic that can be controlled at the same time.”

Though the term carries some fearsome connotations, its use is somewhat symbolic — designed more to help health agencies raise concern among the public than to designate any difference between the outbreak’s status on Tuesday and that on Wednesday. “Describing the situation as a pandemic does not change WHO’s assessment of the threat posed by this coronavirus,” Ghebreyesus said. “It doesn’t change what WHO is doing, and it doesn’t change what countries should do.” The designation has no legal meaning, and doesn’t involve any new measures being deployed to stop the spread of the virus.

The last pandemic was caused by the H1N1 flu in 2009 — the same strain as the Spanish flu in 1918 — which killed hundreds of thousands of people across the world. To date, there have been over 120,000 cases of COVID-19 worldwide, resulting in over 4,300 deaths. “In the days and weeks ahead, we expect to see the number of cases, the number of deaths, and the number of affected countries climb even higher,” Tedros said.

Prior to the Wednesday declaration, the WHO classified the risk of coronavirus spread as “very high,” one level below that of pandemic.
The term more commonly used was epidemic, defined by epidemiologist Rebecca S.B. Fischer as “an outbreak over a larger geographic area.” Fischer writes:

When people in places outside of Wuhan began testing positive for infection with SARS-CoV-2 (which causes the disease known as COVID-19), epidemiologists knew the outbreak was spreading, a likely sign that containment efforts were insufficient or came too late. This was not unexpected, given that no treatment or vaccine is yet available. But widespread cases of COVID-19 across China meant that the Wuhan outbreak had grown to an epidemic.

Meanwhile, Fischer writes that epidemiologists use the term “pandemic” to describe an “epidemic spread[ing] to multiple countries or regions of the world” where the disease is then “sustained in some of the newly affected regions through local transmission,” as the coronavirus has in states including Washington, California, and New York. “A formal declaration of COVID-19 or any other infectious disease as pandemic tells governments, agencies, and aid organizations worldwide to shift efforts from containment to mitigation,” Fischer states.

Dr. Michael Ryan, the head of the WHO’s emergencies program, said that the declaration is meant “to galvanize the world to fight.” On Wednesday, he recommended countries hire more contact-tracers, who track down those who have been exposed to people who have tested positive for the coronavirus, and to test and isolate anyone who was infected.

On Wednesday, WHO Director Tedros suggested that some countries were not taking the threat of COVID-19 seriously enough, though in question period, Dr. Ryan declined to name them. “The WHO does not criticize its member states in public,” he said. “You know who you are.”

Considering the Trump administration’s deeply inadequate response to the crisis thus far, it’s hard to imagine that the United States wasn’t on Ryan’s list:

What’s the Difference Between a Pandemic and an Epidemic?