The coronavirus outbreak has been declared a global public health emergency by the World Health Organisation – just hours before an evacuation flight to bring British nationals back to the UK is due to leave the Chinese city of Wuhan.

It is the sixth time the World Health Organisation (WHO) has issued the emergency declaration in just over a decade, following outbreaks of zika, ebola (twice), swine flu and polio.

More than 170 people have died in China and around 7,700 have been infected, with cases detected in countries including the US, Japan and South Korea.

In response to the WHO announcement, the four chief medical officers of the UK have increased the risk level of coronavirus from low to moderate, adding they “do not think the risk to individuals in the UK has changed” but that the government should “plan for all eventualities”.

In Britain, an evacuation flight to bring UK nationals back to the UK from the Chinese city at the centre of the coronavirus outbreak will leave shortly.

Speaking at a press conference in Geneva, Switzerland, director general of the WHO Tedros Adhanom said: “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.

“As I have said repeatedly since my return from Beijing, the Chinese government is to be congratulated for the extraordinary measures it has taken to contain the outbreak, despite the severe social and economic impact those measures are having on the Chinese people.

“We would have seen many more cases outside China by now – and probably deaths – if it were not for the government’s efforts, and the progress they have made to protect their own people and the people of the world.”

Last week, WHO said it was “too early” to declare an international public health emergency but on Thursday said action was needed to help countries to prepare for the possibility of it spreading further.

The new virus has now infected more people in China than fell ill during the 2002-2003 severe acute respiratory syndrome (Sars) outbreak. The number of cases has jumped to 7,711, surpassing the 5,327 people diagnosed with Sars.

A letter from the chief medical officers said: “We have been working in close collaboration with international colleagues and the World Health Organisation to monitor the situation in China and around the world.

“In light of the increasing number of cases in China and using existing and widely tested models, the four UK chief medical officers consider it prudent for our governments to escalate planning and preparation in case of a more widespread outbreak.

“For that reason, we are advising an increase of the UK risk level from low to moderate. This does not mean we think the risk to individuals in the UK has changed at this stage, but that government should plan for all eventualities.

“As we have previously said, it is likely there will be individual cases and we are confident in the ability of the NHS and HSC in Northern Ireland to manage these in a way that protects the public and provides high quality care.”

The officers are Prof Chris Whitty for England, Dr Frank Atherton for Wales, Dr Catherine Calderwood for Scotland and Dr Michael McBride for Northern Ireland.

Swine flu

The first declaration happened in April 2009, with H1N1, or swine flu, spreading around the world and killing more than 200,000 people.

Britain recorded 214 deaths, while 20 people died from swine flu in Ireland.

Initially travellers returning from Mexico, where the virus originated, were met by authorities at the airport and hygiene measures were recommended by the Health Protection Agency.

Polio

Increased wild polio numbers in Pakistan, Afghanistan and Nigeria led to an emergency being declared in May 2014, a remarkable step given that the disease was thought to have been almost eradicated.

There have been no domestic cases of polio in the UK since 1982.

Ebola

The deadly disease broke out in Guinea, Liberia and Sierra Leone, with an emergency declared from August 2014 to March 2016. Almost 30,000 people were infected and more than 11,000 died.

The UK made a significant contribution after the outbreak by working with the Wellcome Trust on an experimental vaccine which the European Medicines Agency approved to treat Ebola in November last year.

A vaccination programme is under way in the Democratic Republic of Congo’s (DRC) North Kivu province.

A second emergency was declared, after months of hesitation, in July after it was confirmed that Ebola had spread to Goma, the capital of North Kivu, where DRC’s international airport is located.

The government said at the time that the Britain was “providing expertise and support” to help the WHO “in a very insecure region”.

Zika

The life-threatening virus was first identified in a monkey in Uganda in 1975.

Its spread by mosquitoes in the Americas led the WHO to declare it an emergency from February to November 2016.

The first emergency from a mosquito-borne disease is thought to have been responsible for 2,000 possible cases of birth defects and 29 infant deaths in Brazil.

The UK largely escaped the virus, with just two travel-associated cases confirmed in 2018.