Some of the world’s top experts tracking the spread of the coronavirus predict that in a matter of weeks much of Europe could be facing a surge in cases similar to the one that has locked down Italy, overwhelmed its hospitals in the north and brought the country of 60 million to a standstill.

Mathematical models developed by epidemiologists to track the virus show a sharp trajectory of infections in Spain, Germany, France and Britain. Spain, which declared a state of emergency on Friday, showed particularly concerning exponential growth, experts said.

The modelers in Europe say a similar arc is likely in the United States, but anticipating the spread is made more difficult by the lack of widespread testing. US officials are working with 50 academic modeling groups, but no forecasts have been released by the Centers for Disease Control and Prevention or the White House coronavirus task force. One model, for the American Hospital Association, projects a worst-case scenario of hundreds of thousands of deaths.

“Italy is about two weeks ahead of Britain and the rest of Europe,” said Francois Balloux, director of the Genetics Institute at University College London.

Observations by epidemiologists show that if unchecked, the number of infections double approximately every five days, with infected individuals, on average, capable of passing the virus on to an average of about 2.5 people.

“What we are seeing is not rocket science,” professor Balloux said. “You have two cases, then four, then eight, then 16.”

Epidemiologists say that decisive action is required to change the rate of infection and “flatten the curve.” That reality appeared to sink in for some political leaders in the United States and Europe this week, as countries closed schools, encouraged working from home, banned large gatherings and imposed new travel restrictions. French Health Ministry Director General Jérôme Salomon said France must prepare itself for “the Italian scenario.”

Still, some experts worry that governments aren’t doing enough to reduce rates of transmission so cases don’t soar exponentially and overwhelm health systems. A lack of urgency in previous weeks may have wasted valuable time, they say.

On Friday, the director-general of the World Health Organisation, Tedros Adhanom Ghebreyesus, said, “Europe has now become the epicenter of the pandemic, with more reported cases and deaths than the rest of the world combined, apart from China.”

Irish doctors on Friday warned in a letter to the British Medical Journal that the country is “exactly 14 days behind Italy.” They said only stringent measures could head off a medical crisis like Italy’s and produce a more manageable outbreak like South Korea’s.

At the beginning of an outbreak, experts say viral spread can be contained by isolating individuals and tracing those in contact. But when cases begin to emerge where the infection source is unknown, as they have in parts of Europe and the United States, mass behavioural changes and social distancing appear to be the better strategy.

Italy is one country that has had to accept quickly the need for drastic action. Just three weeks ago, it did not have much of a coronavirus problem. Now it has had more than 15,000 confirmed cases. Prime minister Giuseppe Conte extended a lockdown on a handful of small villages to wide swaths across northern Italy on Sunday morning and then to the entire country Monday night. On Wednesday, he went even further, with the announcement of a shutdown of most commercial activity.

Italy may have been particularly hard hit in part because the virus spread in the community for several weeks unchecked, and the government has had to play catch-up.

Dirk Brockmann, a professor at the Humboldt University in Berlin who does infectious-disease modeling for Germany’s Robert Koch Institute, the federal agency tasked with disease control, said looking at the number of confirmed cases at a particular moment can be misleading – if people think it is just a few thousand cases here and there.

“It’s almost like taking a still picture of cars on a highway, it doesn’t tell you anything,” he said. “It’s about how the numbers change.”

The current snapshot may be as much as two weeks old, given the time it may take for the virus to cause symptoms and for patients to be tested and diagnosed.

Britain’s chief science adviser, Patrick Vallance, said on Thursday that the United Kingdom had 600 confirmed cases, which he said means that 5,000 to 10,000 people there probably are infected.

If the virus continues to infect people at its current rate, which is not a certainty, a European country such as Spain, with 3,000 cases, could see 250,000 infected individuals in about a month, based on doubling times of five days.

“Those are the numbers that epidemiologists are expecting,” said Roy Anderson, professor of infectious disease at Imperial College London.

“I think it’s highly likely we all go the way of Italy” in the coming weeks, he said.

Whether the caseloads explode will depend in part on public health controls, decisions by governments and businesses, and individual actions, the disease modelers say.

Many people were taken aback when Chancellor Angela Merkel on Wednesday warned that 60 to 70 per cent of Germany’s population of 83 million could contract the disease.

But infectious-disease specialists say that’s quite possible.

“If governments do nothing, or do things that are not effective, and the population does nothing, then you certainly could see 60 percent of a population infected,” professor Anderson said.

Some European countries appear to be “renouncing efforts to contain the epidemic,” said Giovanni Rezza, director of the Infectious Disease Department at the Italian National Institutes of Health.

“We are applying very tough measures” in Italy, he said. “But in other countries where the incidents of the disease are increasing, it’s like they are not looking at Lombardy,” he added, referring to Italy’s worst-hit region.

“For the other countries around us, we should be a lesson. They know the virus is there, so they should act more promptly,” Dr Rezza said.

For the United States, Dr Rezza said, there was still a “window of opportunity” to try to contain the virus, given the large geographical area and isolated outbreaks.

Infectious-disease specialists say it is possible the case numbers in the Northern Hemisphere will soar in March, and then begin to go down if the coronavirus proves to be seasonal, like common flu, which produces less infection in the warmer summer months.

The British government, especially, appears to be hoping the infection rate drops in April and May, but so far there is no evidence to support the optimism.

In northern Italy, where the wards are running out of beds for all the coronavirus patients, doctors report they are battling out-of-control transmission.

Earlier, a group of Italian doctors sent out an appeal to their colleagues working in intensive care units in Europe.

“Get ready,” they wrote.

Angelo Pan, head of infectious diseases at a hard-hit hospital in Corona, Italy, warned that without effective containment measures, a similar scenario could be ahead for France, Germany and possibly the United States.

“It’s just a matter of time,” he said. “I think we are just maybe 10 days ahead.”

Benjamin Maier, who is Brockmann’s research partner at Robert Koch Institute, has plotted the infection curves of countries in Europe and Asia that have seen outbreaks.

Spain is showing galloping exponential growth, he said, while Britain, Germany and France are still on a steep trajectory.

“You can see that many countries will follow Italy’s example if they don’t follow strict and stricter measures,” Dr Maier said.

Those measures can have a significant impact on how many cases develop. Dr Brockmann and Dr Maier examined the epidemiological curves of the outbreak in China. Although Hubei province initially had an exponential growth rate, the government’s massive containment effort, involving mandatory behaviour changes, managed to slow the rate of infection.

South Korea, which has carried out widespread testing, has identified nearly 8,000 cases, but the numbers of new infections are tapering off. Singapore, Malaysia and Japan also have seen flatter growth.

The Spanish Flu of 1918 was the last pandemic at this scale, so is used by epidemiologists for comparison to the Covid-19 (Johns Hopkins CSSE/The Washington Post)

For those countries facing steep increases in coming weeks, all is not lost, said Alexander Kekulé, a German virologist who heads the institute of medical microbiology at the University of Halle.

Acting in advance, rather than in reaction to numbers that lag behind the picture on the ground by weeks, he said, can make a huge difference.

“This really decides if you win the battle, or you lose it,” he added. For weeks, Dr Kekulé has publicly urged Germany to act more swiftly with measures to ban mass events, close schools and test everyone who has flu-like symptoms.

But China’s swift and drastic action is more of a challenge in Western democracies.

Politicians in Europe have been slow to make proactive decisions that could be lifesaving until the danger has sunk in for the public, said Dr Kekulé, meaning they are reacting, rather than getting ahead of the curve.

The fact that German Health Minister Jens Spahn recommended events with more than 1,000 people be stopped on the same day that the first German national died was no coincidence, he said.

“It’s always the problem of a Democratic system; you have to explain yourself to your voters on why this measure has to be taken,” he said. “You have to wait until it’s hurting.”