The WHO said on 11 March there was still time to “change the course” of the virus’ spread and urged countries to take “aggressive action” to combat the disease.
However, the announcement is likely to raise fears that the world is running out of time to prevent the spread of coronavirus.
Dr Mike Ryan, the WHO's emergencies chief, has said he hopes the use of the term can “galvanise the world to fight” as cases rise.
But what exactly is a pandemic and what impact will the WHO’s announcement have on the fight against Covid-19?
What is a pandemic?
In the simplest sense, a pandemic is an outbreak of a disease which occurs over a wide geographic area and affects an exceptionally high proportion of the population.
It differs from an epidemic, which refers to the rapid spread of infectious disease in a given population, because it concerns outbreaks of disease which have spread around the world, usually to multiple continents.
The term concerns a disease’s geographic spread and does not comment on the severity of an outbreak - a pandemic is not necessarily more deadly than an epidemic, it has just spread more widely.
How does the WHO decide when to declare one?
A pandemic is generally called when sustained community-level spreading has been recorded in multiple parts of the world.
Although coronavirus cases have been reported around the world since February, many of those cases involved people who had flown to countries from virus-hit regions of China, where the virus was first identified.
The pandemic declaration suggests spreading within multiple countries for a significant amount of time is likely.
However, there is no strict rule to define when a pandemic should be declared and previous pandemics have had varying degrees of severity.
In 2003, Sars affected 26 countries and had a fatality rate of about 10 per cent, but it was not labelled as a pandemic as it was contained quickly.
In 2009, H1N1 influenza (also known as swine flu) was declared a pandemic, even though its fatality rate was below 1 per cent.
With Covid-19, the WHO said: “We are deeply concerned both by the alarming levels of spread and severity, and by the alarming levels of inaction.
“We have therefore made the assessment that Covid-19 can be characterised as a pandemic.”
Why has it only been declared now?
Although Covid-19 cases have been reported around the world for weeks, the WHO has resisted calls to declare a pandemic until now.
That is partly because the declaration of a pandemic can trigger mass panic, which could hinder public health efforts to control the virus.
On Wednesday, the WHO’s director-general warned the word could be dangerous when misused.
“Pandemic is not a word to use lightly or carelessly,” Dr Tedros Adhanom Ghebreyesus said.
“It is a word that, if misused, can cause unreasonable fear, or unjustified acceptance that the fight is over, leading to unnecessary suffering and death.”
How does this effect the response to Covid-19?
The declaration of a pandemic does not change the WHO’s assessment of the threat posed by Covid-19, nor does it change the advice recommended to countries battling outbreaks.
However, the organisation’s statement on Wednesday did suggest a sense of frustration at some countries not taking the virus seriously enough.
“We have called every day for countries to take urgent and aggressive action,” Dr Tedros said.
“We have rung the alarm bell loud and clear.”
The declaration could mean governments around the world intensify their efforts to slow the spread of the virus and introduce stricter "social distancing" measures, such as banning large gatherings.
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