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

April 2020

Our support service answer lots of questions about meningitis every day, helping to deliver accurate information.  With news reports talking about the current coronavirus pandemic, one of the things that has puzzled those we support is why different infectious diseases are described in different ways depending on how many people they affected.


On March 11th, the World Health Organization officially called COVID-19 a pandemic. You asked us what a pandemic means and why meningitis isn’t classed as a pandemic. To understand this, let’s first look at their definitions:

According to the World Health Organization, a disease outbreak is when cases of a disease are in excess of what we would normally expect to see. The number of cases that would be classed as an outbreak varies according to what causes the disease and the size and type of previous and existing exposure to the cause. For meningococcal disease in the UK, for example, we unfortunately see around 1,200 cases every year. Around 97% of cases are sporadic and will not lead directly to any further cases. However, if two or more confirmed or probable cases of meningococcal disease occur in the same location within four weeks, and they have a common link, this would be investigated by public health teams, and may well be treated as an outbreak.
An epidemic is defined by WHO 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”.

This is very similar to the definition of an outbreak. In fact, some health organisations have the same definition for outbreak AND epidemic. However, ‘outbreak’ is usually used when diseases happen in a more limited geographic area. If an outbreak of a diseases spreads quickly to more people than experts would expect and moves into a large geographic area, it is often then called an epidemic.

As you will have started to see, there aren’t always distinct rules. Some diseases have to have a specific number of cases in a given county to be defined as an outbreak or epidemic because we have lot of experience with them, but for new disease it takes an element of expert judgement.
A pandemic is defined as a “an epidemic occurring worldwide, or over a very wide area, crossing international boundaries and usually affecting a large number of people”.

Note that the pandemic definition includes the word epidemic, but not vice versa. This tells us that an epidemic can be reclassified as a pandemic once it passes a critical point.
As you can tell, it is difficult to differentiate between an epidemic and a pandemic from their definitions alone. The definition of a pandemic tells us it is an ‘epidemic occurring worldwide, or over a very wide area … affecting a large number of people’. One might contest, then, that a meningitis epidemic (such as those that occur in the ‘meningitis belt’ of sub-Saharan Africa each year) could also be classed as a pandemic. After all, thousands of people in this region – which consists of 26 countries - died every year of MenA meningitis epidemics until a vaccine was introduced in 2010.

So, given that the dictionary definitions of pandemic and epidemic are so similar, why does it matter that WHO have labelled COVID-19 a pandemic – but not meningitis?

Pandemics in politics

Throughout history, the word pandemic has been used to trigger collective global action against a large scale epidemic. In their COVID-19 reclassification announcement, WHO stated that decision to label COVID-19 a pandemic was due to ‘alarming levels of spread and severity’ alongside ‘alarming levels of inaction’.

Labelling COVID-19 a ‘pandemic’ prompts governments around the world to take the epidemic seriously. As WHO puts it, the use of the word pandemic is an ‘alarm bell, loud and clear’.

Why isn’t meningitis also a pandemic?

In the past, meningitis has been classed as a pandemic. As WHO explains, ‘the most recent meningococcal meningitis pandemic began in the mid-1990s. In 1996, almost 190,000 cases were notified to WHO in Burkina Faso, Chad, Mali, Niger, Nigeria and other countries. However, this was rapidly brought under control thanks to a tremendous international effort to develop and implement an effective vaccine.

Vaccination has proven to be an effective tool in combatting the spread of meningitis.
Meningitis outbreaks and epidemics are devastating, but fortunately we do have tools to bring them under control - chiefly vaccines. Because we can take early action to control outbreaks and epidemics, they rarely reach pandemic status. However, with 5 million people affected each year, it is vital that we extend control of meningitis and do more to prevent epidemics and outbreaks which do occur every year. Although no single vaccine exists to prevent meningitis, there are vaccines available against many of its most common forms. These vaccines may not necessarily be widely available to all, but they do exist. Sadly, there is currently no vaccine available to prevent COVID-19, which means that it is much more difficult to bring this virus under control.

Meningitis is a serious disease which devastates lives around the world every single year. Although some parts of the world are affected more than others, it is absolutely a global health issue. At Meningitis Research Foundation, we are working hard to raise awareness of its importance on the worlds stage, and prompt much greater collective action to defeat meningitis – wherever it exists – by 2030.

Robust health systems around the world are vital in making this happen, and robust health systems will not exist without cohesive action against COVID-19. We urge everyone to follow WHO, PHE and CDC guidance in order to help defeat the Coronavirus pandemic as soon as possible.
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The MRF Membership and Support team are here for you for any questions you might have about meningitis and septicaemia and their effects on you or your family and friends.

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About the author

Holly Edwards
Senior Communications Officer

Hi, I'm Holly, and I joined MRF in 2017.

I'm always looking for stories that will help more people understand meningitis and the devastating impact it can have. Working with people who have been affected by meningitis is a great privilege, and I feel very lucky to do what I do.

Tel: 0333 405 6255