Since the new coronavirus strain Covid-19 emerged at the end of last year, experts have wondered how the outbreak may play out.
The infection has spread well beyond its epicentre in the Chinese city of Wuhan into more than 30 countries.
Confirmed cases exceeded 81,200 on Wednesday, of which 78,064 are in mainland China, according to John Hopkins University data.
The death toll is more than 2,700.
While plateauing incidences have left some optimistic it may have “peaked” in China, infections are arising thousands of miles away.
South Korea, second to China in patient numbers, has more than 1,200 cases and 12 deaths.
The UK has tested over 6,700 suspected patients, of whom 13 came back positive.
While it may sound bleak, the World Health Organization (WHO) stressed it is not yet a “pandemic”, with the term “not fitting the facts”.
Nevertheless, a “Covid-19 Reasonable Worst Case Scenario” report put together by the UK government’s National Security Communications Team warns that up to 80% of Brits could become infected.
Seen by The Sun, the report adds “2%-to-3% of symptomatic cases will result in a fatality”.
A government spokesperson said “this does not mean we expect it to happen”.
What could a worst-case scenario look like for coronavirus Covid-19?
While experts are not disputing the “up to 80%” figure, one stresses this would be a “worse-case scenario”.
“When preparing for an outbreak we take the most dramatic figure, which the government prepares for,” said Professor David Heymann from the London School of Hygiene and Tropical Medicine.
“This is an epidemiological model based on the basic reproduction number and will change daily as new information comes in.”
The basic reproduction number is the number of people an infected person goes on to infect.
A reproduction number of two would mean every patient is expected to pass the virus to two others.
Experts previously said the number is variable and declines with “interventions”.
With the virus virtually unheard of just two months ago, no one can say for sure how the outbreak may play out.
“End-of-the-epidemic calculations, whether for the total number infected, the total number requiring hospitalisation or the total number of deaths, are projecting far into the future,” Professor Christl Donnelly, from Imperial College London, told Yahoo UK.
“In the meantime, it is prudent for the government to consider reasonable worst-case scenarios in their forward planning.
“There are many things we still do not know.
“We don’t know how, if at all, seasonality will affect transmission of this virus and we don’t fully understand susceptibility and transmission in different demographic groups, schoolchildren for example.”
Another expert questioned how “helpful” the “up to 80%” prediction is, urging people to maintain a sense of “perspective”.
“Personally I think these numbers are a gross exaggeration and not helpful,” Professor Mark Harris, from the University of Leeds, told Yahoo UK.
“If we look at the situation in China, although the numbers are still going up, the rate of increase has slowed dramatically and has been stable for the past week.”
A change in how the virus is diagnosed caused cases to spike over night, from 44,700 on 12 February to 59,800 the following day.
They have since been rising more slowly.
Cases in China came in at 78,100 on Wednesday, up from 77,700 the day before.
“Out of a population of 1.4 billion, [China has had] 78,000 cases and 2,700 deaths”, said Professor Harris.
“[The] population of [the] UK is currently 67 million, so at the same rate as China we might be looking at 3,700 cases and 129 deaths.”
Professor Harris compared this to the 26,000 people who are thought to have died from flu in the UK in 2017/18.
“We need to put this virus into perspective,” he said.
“Yes, we should be concerned, but we should apply appropriate containment measures to break the chain of transmission from person-to-person.”
The government report is likely based on Covid-19’s reproduction number, which has also been up for debate.
Scientists from the London School of Hygiene & Tropical Medicine (LSHTM) estimated the number fluctuated between 1.5 and 4.5 before travel restrictions were introduced in Wuhan, stressing “substantial uncertainty”.
This led them to predict the outbreak may peak in “mid-to-late February”.
Speaking at the time, Dr Robin Thompson from the University of Oxford, said: “One proviso must be forecasting the peak of any outbreak is challenging, and so there is significant uncertainty in estimates of both the timing of the peak and the total number of cases that will occur.”
What is Covid-19?
Covid-19 is one of seven strains of the coronavirus class that are known to infect humans.
Others range from the mild common cold to severe acute respiratory syndrome (Sars), an outbreak of which killed 774 people in 2004.
Most of the people who initially became unwell from Covid-19 worked at, or visited, a seafood and live animal market.
The only known method of transmission is face-to-face via infected droplets that have been coughed or sneezed out.
Symptoms tend to be flu-like, such as fever, cough and breathlessness.
In the most severe cases, victims develop pneumonia.
This comes about when a respiratory infection causes the alveoli (air sacs) in the lungs to become inflamed and filled with fluid or pus.
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.”
While no one can say for sure where the virus came from, bats seem most likely.
The nocturnal creatures are thought to have been behind Sars and fellow coronavirus Middle Eastern respiratory syndrome (Mers), which killed 858 people in its 2012 outbreak.
Scientists from Peking University in Beijing suggested snakes may have been the “intermediate host” for Covid-19.
A team from South China Agricultural University later found it could have “jumped” from bats to humans via pangolins.
Covid-19 has no specific treatment, with care being “supportive” while a patient’s immune system works to fight off the virus.
To prevent infection, the NHS recommends regular hand-washing and avoiding those with suspect symptoms.