What is the difference between an epidemic and a pandemic?

An epidemic is the rapid spread of an infectious disease, usually in a particular area over a short period of time. Epidemiologists look for these additional key features

  • a high number of infections in relation to the expected number. Endemic diseases (like influenza) return every year but usually at a low and predictable rate.
  • spread accelerated by person-to-person transmission
  • a rapidly increasing morbidity rate (proportion of the population with disease) 
  • a population that extends beyond shared accommodation (not a cruise ship, for example, where the world outbreak would be used.)

For endemic diseases an epidemic can be clearly marked on a statistical chart - with a sudden rising curve.  Here the data from hospital visits in the US suggests a possible influenza epidemic in the winter of 2007/8

 (credit: modification of work by Centers for Disease Control and Prevention from 'The language of epidemiology')

With a new pathogen, it is more difficult to identify the start of an epidemic. First is necessary to establish the nature of the disease. Key questions include:

  • What symptoms do the infected present?

  • How does it spread? 

  • What characteristics does it share with other known pathogens

What is an outbreak?

The word outbreak is almost synonymous with epidemic in this context - but generally refers to specific location. 

In December 2019, hospital doctors in Wuhan began reporting a cluster of patients with "pneumonia of unclear cause". On January 9 China confirmed the outbreak of a new coronavirus and published its genome. This established a link to Sars and Mers viruses.

Scientists were now able to develop tests for the virus but they were now dealing with a rapidly escalating epidemic in Wuhan. The infection rate had shot up across the city and throughout Hubei Province. And new cases were beginning reported in other locations:

These were the reported cases on Jan 24 2020. The Chinese authorities have now conceded that the true infection rate was least double that indicated.

What is a pandemic?

A pandemic is the word used when an epidemic spreads to multiple regions or worldwide. This has been a controversial topic in this crisis. The WHO (World Health Organisation) did not declare Covid-19 a pandemic until March 12, 2020. By this point, there were major outbreaks is multiple countries.

A pandemic often involves a new pathogen or pathogen strain. This is because a disease will spread more quickly when it is attacking a host with little or no immunity to it. Over time the virulence of a disease will decrease as a result of what has become known as 'herd immunity'.

The most effective way to achieve this immunity is through the widespread application of a vaccine. That is why so much intensive scientific effort is being applied to this task.

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