Coronavirus: Outbreak, epidemic, pandemic - what's the difference?

The coronavirus crisis has brought with it an ever-expanding glossary of terms, many of which had previously been little-called upon - or downright non-existent - elements of our daily vocabulary.

‘Social-distancing’, ‘self-isolating’, 'contact-tracing', ‘asymptomatic’… We’ve had to get to grips with a whole host of new or unfamiliar words and phrases.

'Outbreak', 'epidemic', 'pandemic': three words of similar, yet differing meaning

Not only that, but we're having to deal with a subject area that seems to be replete with closely-related terms whose meanings either overlap or differ only marginally.

For example, ‘outbreak’, ‘epidemic’ and ‘pandemic’ have become stalwarts of our everyday lexicon in the months since coronavirus first emerged - yet although their meanings are not the same, there has been a degree of interchangeability about the way they have been used.

So what is the difference between 'outbreak', 'epidemic' and 'pandemic'? Well, it basically comes down to scale: ‘outbreak’ is the smallest; ‘pandemic’ is the largest.

From 'outbreak' to 'epidemic'

The lexicographers Merriam Webster define an ‘outbreak’ as a “sudden rise in the incidence of a disease” that is usually “confined to a localized area or a specific group of people”.

An ‘outbreak’ then graduates on to ‘epidemic’ status once it has multiplied into several ‘outbreaks’ and is no longer so limited in geographical scope. “An ‘outbreak’ can become an ‘epidemic’ if the spread becomes more severe, infecting more people over a wider area,” Merriam Webster say.

The World Health Organization (WHO), meanwhile, considers ‘epidemic’ as a spread that is regional in reach, defining the term as the “occurrence in a community or region of cases of an illness, specific health-related behaviour, or other health-related events clearly in excess of normal expectancy.”

'Pandemic': the "worldwide spread of a new disease"

The word ‘pandemic’ then comes into play when an illness starts to go global.

The WHO describes ‘pandemic’ as the “worldwide spread of a new disease”, with the Centers for Disease Control and Prevention (CDC) agreeing that it “refers to an epidemic that has spread over several countries or continents, usually affecting a large number of people”.

This definition certainly applies to Covid-19, which was officially declared as a pandemic by the WHO on 11 March, and has now racked up over 600,000 confirmed cases in more than 175 countries.

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