Pandemic, epidemic and outbreak - what makes them different?

On Wednesday night, the World Health Organization (WHO) has declared coronavirus (COVID -19) as a pandemic.

As the coronavirus spreads, the words "outbreak", "epidemic" and "pandemic" are being heard more often than before, and at times mixed up in confusion. The difference between the three scenarios of disease spread is a matter of scale.

American epidemiologist Rebecca SB Fischer recently wrote an article in The Conversation about the three terms amid the confusion and panic in people around the world. In succinct words, Fischer has delineated the differences and similarities between the three. 

Outbreaks – Small but unusual

Epidemiologists track diseases over time and geography – learning to predict how many cases of an illness should normally happen within a defined period of time, place and population.

"An outbreak is a noticeable, often small, increase over the expected number of cases," wrote Fischer.

According to Fischer, it is normal if one or two sick kids suffering from diarrhoea at a daycare in a typical week. However, if 15 children in a daycare come down with diarrhoea all at once, that is an outbreak.

Outbreaks will be more noticeable when a new disease emerges as the anticipated number of illnesses caused by that disease was zero.

Fischer used the cluster of pneumonia cases that sprung up unexpectedly among market-goers in Wuhan as another example.

Public health officials now know the spike in pneumonia cases there constituted an outbreak of a new type of coronavirus, named SARS-CoV-2.

Fischer suggests that as soon as an outbreak is detected, local health authorities must start an investigation to determine exactly who is affected and how many have the disease, as the information can be used to figure out how best to contain the outbreak and prevent additional illness.

Epidemic - Bigger and spreading

To quote Fischer – "An epidemic is an outbreak over a larger geographic area."

When people in places outside of Wuhan began testing positive for infection with SARS-CoV-2 (also known as Covid-19), the outbreak was spreading – a likely sign that containment efforts were insufficient or came too late.

Fischer says that given no treatment or vaccine is yet available, insufficient containment effort was not unexpected. However, widespread cases of Covid-19 across China meant that the Wuhan outbreak had grown to an epidemic.

Covid-19 was first noticed in Wuhan, China, but quickly spread across the globe.

Pandemic - International and out of control

Epidemiologists classify a situation as a pandemic only once the disease is sustained in some of the newly affected regions through local transmission.

"Once an epidemic spreads to multiple countries or regions of the world, it is considered a pandemic," Fischer wrote.

In her example, she said that a traveler with Covid-19 returning to the US from China doesn't make it a pandemic; however, once they (carrier) infect a few family members or friends, there is some debate.

If new local outbreaks ensue, epidemiologists will agree that efforts to control global spread have failed and refer to the emerging situation as a pandemic.

More political than medical

The broader concerns of governments or international health organisations is different that the concerns of epidemiologists, who are principally concerned with preventing disease.

In its history, the WHO has declared only two pandemics till now. First, for the influenza in 1918 and for influenza H1N1 in 2009.

"For weeks, epidemiologists like me have been calling the coronavirus a pandemic. From an epidemiological perspective, the WHO's declaration is overdue," – Fischer wrote.

A pandemic is the highest level of global health emergency – signifying widespread outbreaks affecting multiple regions of the world.

The American epidemiologist adds: "The formal declaration of Covid-19 or any other infectious disease as pandemic tells governments, agencies and aid organisations worldwide to shift efforts from containment to mitigation. It has economic, political and societal impacts on a global scale, and the WHO takes extreme care when making this determination."

Fischer remarks that the formal declaration should not incite fear or panic in people, but simply make them more careful.

"It doesn't mean the virus has become more infectious or more deadly, nor that your personal risk of getting the disease is greater. And it doesn't mean that efforts to fight Covid-19 are being abandoned. But it is an historical event," Fischer concluded.