SINGAPORE - JANUARY 30: Passengers are seen wearing a protective mask on a train ride on January 30, 2020 in Singapore. Singapore has confirmed ten cases of the deadly coronavirus, which emerged last month in the city of Wuhan in China. The virus has since spread to 21 countries with 170 deaths reported as of January 30. (Photo by Suhaimi Abdullah/Getty Images)

The World Health Organization (WHO) has declared China’s deadly coronavirus outbreak a “public health emergency of international concern” (PHEIC) after meeting for a second time.

A committee discussed the issue last week, deciding it was “too early” given the virus’ “restrictive and binary nature”.

The death toll hit 170 on Thursday, while Chinese authorities confirmed 7,711 cases across every region of the mainland country, up from 4,500 on Tuesday.

READ MORE: China's coronavirus is 'spreading quicker than Sars'

Since starting in the city of Wuhan, capital of Hubei province, the previously unknown strain has spread to at least 15 other countries, including the US, Australia and France.

A PHEIC has been declared after people who have never visited China fell ill, catching the virus from a traveller who recently returned from the Asian country.

Only five conditions have ever been declared a PHEIC, the highest alarm the WHO can sound.

These include the Zika virus and Ebola, which killed more than 13,000 people across two outbreaks.

The new strain - 2019-nCoV - triggers flu-like symptoms, including breathlessness and fever. In the most severe cases, victims succumb to pneumonia.

Passengers coming from China wearing masks to prevent a new coronavirus are checked by Saudi Health Ministry employees upon their arrival at King Khalid International Airport, in Riyadh, Saudi Arabia January 29, 2020. REUTERS/Ahmed Yosri

“Over the past few weeks, we have witnessed the emergence of a previously unknown pathogen, which has escalated into an unprecedented outbreak, and which has been met by an unprecedented response,” WHO’s director-general Dr Tedros Adhanom Ghebreyesus said.

“There are now 98 cases in 18 countries outside China, including eight cases of human-to-human transmission in four countries: Germany, Japan, Vietnam and the United States of America.

“So far we have not seen any deaths outside China, for which we must all be grateful. Although these numbers are still relatively small compared to the number of cases in, we must all act together now to limit further spread.

“We don’t know what sort of damage this virus could do if it were to spread in a country with a weaker health system.

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“For all of these reasons, I am declaring a public health emergency of international concern over the global outbreak of 2019-nCoV.”

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Dr Adhanom Ghebreyesus previously said the virus was “an emergency in China, but it has not yet become a global health emergency”.

“There is no evidence of human-to-human transmission outside China, but that doesn’t mean it won’t happen”, he said at the time.

Cases have since arisen in other countries among locals who have not visited China.

“The main reason for this declaration is not because of what is happening in China, but because of what is happening in other countries,” Dr Adhanom Ghebreyesus said.

“To the people of China and to all of those around the world who have been affected by this outbreak, we want you to know that the world stands with you.

“In total, there are now 7,834 confirmed cases, including 7,736 in China, representing almost 99% of all reported cases worldwide.

“One hundred and seventy people have lost their lives to this outbreak, all of them in China.

“We must remember that these are people, not numbers.”

The WHO’s decision has been welcomed by the medical community, but some worry it may come too late.

“This was to be expected, especially given the ramping up of infections in China, increasing numbers of other countries reporting cases of coronavirus infection, and the real threat that this infection poses to global health, especially in places that lack adequate surveillance and infrastructure,” Professor Jonathan Ball, from the University of Nottingham said.

“We need to gain a better understanding of how the virus is behaving if we have any chance of bringing this outbreak to a close and a concerted and increased international effort in the best way to achieve this; I just hope that it is not too late.”

READ MORE: Race is on to develop a vaccine against China's coronavirus

Professor Devi Sridhar, from the University of Edinburgh, called it “long overdue”.

“As Dr Tedros said, the real worry is the virus spreading in countries with weak health systems which are not prepared for containing outbreaks,” she said.

“The benefit of now having the PHEIC is cohesion about the seriousness of this virus, and ensuring there is acceleration in a vaccine, therapeutics and diagnostics.”

Some experts reassured the public not to be unduly alarmed.

“In some respects the PHEIC declaration changes little: the UK and many other countries have already started their preparations to introduce appropriate public measures should they have an epidemic,” Professor Mark Woolhouse, from the University of Edinburgh, said.

“The PHEIC does, however, emphasise the need for all countries to do their best to contain the virus.”

Dr Michael Head, from the University of Southampton, said it was “likely” a “symbolic and political gesture”.

“It is hard to see what extra resource could be applied to addressing the outbreak in China that has not already been available”, he said.

What does a public health emergency mean?

A PHEIC is defined as a “an extraordinary event” that “constitutes a public health risk through the international spread of disease”.

It also “potentially requires a coordinated international response” to stem the spread of infection.

Under the International Health Regulations act of 2005, all 196 member states of the WHO are legally bound to help control the disease.

By definition, the health scare is “serious, unusual or unexpected”, “carries implications for public health beyond the affected state’s border” and “may require international action”.

A declaration is made by the WHO’s director-general, who is advised by its emergency committee.

The committee is made up of experts in disease control, viruses, vaccine development and how infections spread.

It gives “temporary recommendations” on the health measures required to prevent or reduce the disease spreading internationally, while “avoiding unnecessary interference with trade and travel”.

Recommendations automatically expire after three months.

The committee therefore meets at least every 12 weeks to assess the situation and review whether new temporary recommendations are required or the declaration can be terminated.

“There is no reason for measures that unnecessarily interfere with international travel and trade,” Dr Adhanom Ghebreyesus said on Thursday.

The medic added “we must accelerate” the development of vaccines and other treatments, while clamping down on rumours.

“The only way we will defeat this outbreak is for all countries to work together in a spirit of solidarity and cooperation,” Dr Adhanom Ghebreyesus said.

“We are all in this together and we can only stop it together.”

What measures have been introduced so far?

Wuhan - with its 11m residents - has effectively gone into “lockdown”, with planes, trains, buses, subways and ferries all suspended.

People who live in Hubei have also reportedly been told to work from home.

Countries all over the world have advised travellers to avoid any non-essential trips to China.

Several international airlines have suspended flights in and out of the country, the second-largest economy in the world.

Companies like Google, Ikea and Starbucks - which have a presence in China - have temporarily “shut up shop”.

READ MORE: The chart that shows the rapid rise in China's coronavirus cases

Locals have reportedly been told to avoid crowds and wear masks.

Libraries, museums and theatres cancelled exhibitions, while cafés and cinemas have closed.

The Chinese Football Association announced it is postponing all games scheduled for this season.

Countries around the world are screening travellers for signs of infection.

Critics argue, however, it can take days for symptoms to develop, allowing newly-infected passengers to be missed, Yahoo UK reported.

People wearing face masks carry their luggage as they walk outside Beijing Railway Station as the country is hit by an outbreak of the new coronavirus, in Beijing, China January 30, 2020. REUTERS/Carlos Garcia Rawlins

What other infections were ‘public health emergencies’?

The swine flu pandemic of 2009 was the first PHEIC.

It is now known to have killed around 284,500 people, about 15 times more than confirmed at the time, Reuters reported.

This was followed by the Ebola outbreak in the west African countries Sierra Leone, Guinea and Liberia.

Between 2013 and 2016, at least 11,300 people died.

In 2014, the WHO declared the resurgence of polio a PHEIC.

Pakistan made up more than a fifth of global cases, rising from 58 in 2012 to 93 the following year.

READ MORE: How did China's coronavirus outbreak start?

Zika, which causes babies to be born with abnormally small heads, was declared a PHEIC in 2016.

Starting in Brazil the year before, it spread to more than 60 countries and territories.

By the time the emergency was announced, around 2,300 babies had been born with the tell-tale microcephaly, which can cause developmental problems.

Ebola was once again declared a PHEIC when it struck the Democratic Republic of Congo in July last year.

By 14 January 2019, 3,406 cases had arisen, including 2,236 deaths.

Coronaviruses are a class of viruses that cause everything from the common cold to the severe acute respiratory syndrome (Sars) epidemic.

The new strain likely originated at a live animal and seafood market in Wuhan that “conducted illegal transactions of wild animals”, the BBC reported.

Constantly evolving viruses can make the “jump” over to humans.

Most of those who initially fell ill worked at, or visited, the market.

China's National Health Commission confirmed the virus can spread person-to-person, via sneezing, coughing or shaking contaminated hands.

Fatal pneumonia comes about when a respiratory infection causes the alveoli (air sacs) in the lungs to become inflamed and filled with fluid or pus, according to the American Lung Association.

The lungs then struggle to draw in air, resulting in reduced oxygen in the bloodstream.

“Without treatment the end is inevitable,” said the charity Médecins Sans Frontières.

“Deaths occurs because of asphyxiation.”

Sars made headlines in the early 2000s after 774 people died across dozens of countries, mainly in Asia.

Genetic analyses reveal 2019-nCoV is more closely related to Sars than any other coronavirus.

The BBC notes, however, the death toll of the new strain “remains far lower”.

The US Centers for Disease Control and Prevention (CDC) has warned there is no specific treatment for coronaviruses.

If the infection triggers pneumonia, doctors work to combat the complication.

When a virus is to blame – like 2019-nCoV – pneumonia may be treated via “antiviral medication”, according to the American Lung Association.

Professor Peter Horby from the University of Oxford claims, however, there is “no effective anti-viral”, with treatment being “supportive”.