“I cannot give a blueprint” or a “recipe” of statistics for when it will be advisable for people to leave their homes again, said Lothar Wieler, president of the Robert Koch Institute, the German government’s body in charge of disease-monitoring and control. “We evaluate the situation on a daily basis,” Wieler added Wednesday.

The European Union’s national leaders plan to call for an “exit strategy” when they convene over videoconference on Thursday. “We should start to prepare the measures necessary to get back to a normal functioning of our societies,” says an initial draft of the conclusions the European Council is expected to agree on that day.

The statement is being drawn up in Brussels even as experts back home warn that the lockdowns will likely need to be extended.

If, as French President Emmanuel Macron put it, the world is fighting a “war” against the coronavirus, governments didn’t mobilize their armies until the enemy had already crossed the border. Social-isolating measures are believed to be able to slow the pandemic, but it’s almost certainly too late to eliminate — or even contain — the coronavirus.

That leaves the ultimate goal unclear. In the short term, lockdowns serve a simple function. Evidence from Asia, including a new study from Singapore, suggests they are an effective way to slow the rate of infections and prevent hospitals and health care systems from being overwhelmed.

But if the virus is here to stay, when will policymakers consider it safe for people to leave their homes again?

Politicians are not making decisions based on actual data right now, said Christian Drosten, a German virologist who developed the coronavirus test and has advised the German government on containing the disease.

Instead, he said, moves like curfews and school closures are driven by politicians’ gut instincts and observational impressions. Hard data is still weeks away, but epidemiologists are rushing to gather it to help officials make an informed decision by Easter.

“We can’t keep making the measures more and more stringent without knowing whether this will make any difference at all,” Drosten said on a recent podcast.

Last week, U.K. Prime Minister Boris Johnson estimated it would take about 12 weeks to turn the tide of the outbreak. However, when announcing a hard lockdown on Monday, he hedged, saying the government would review the rules in three weeks and “relax them if the evidence shows we are able to.” Johnson did not say what such evidence would need to show.

Asked Tuesday by the BBC how the government would know its efforts were working, Cabinet Office Minister Michael Gove said the aim is to “reduce the rate of the infection in order to reduce the pressure on the [National Health Service],” without addressing when the shelter-in-place orders could be lifted.

In France, a new report from an advisory panel of 11 scientists recommended at least six weeks of social distancing.

The panel said they wouldn’t even be able to consider whether the containment is working until three weeks after it started, and the decision to lift the orders should be based on things like whether hospitals are able to manage the flow of patients — especially for ventilation. But that requires data from hospitals, and “collecting data is difficult when hospitals are overwhelmed,” the panel wrote.

“We’ll only be able to stop confinement when the epidemic curve allows it,” French Health Minister Olivier Véran said Tuesday, warning his country’s 15-day lockdown would likely be extended.

Austrian Chancellor Sebastian Kurz is one of the few leaders who has actually articulated a measurable criterion for lifting the lockdown: getting to the point where cases double only every 14 days.

He added, however, at a Tuesday press conference that authorities have to “find measures to implement in order for [the situation] to stay like that when we reboot public life.”

Part of the difficulty is that politicians and their advisers are still waiting to learn more about the virus. Will it fade away in warm weather, like the flu? How does the virus mutate over time, and how long do people stay immune once they’ve been exposed?

If enough people catch the virus and survive, will a country achieve so-called herd immunity, in which a large proportion of the population has the antibodies that can stop an infection from taking hold? Is there a cure? How fast can vaccines be developed — and will they even work?

The experience of Wuhan in China, the epicenter of the outbreak, could provide some answers. About 11 weeks weeks after it was isolated from the outside world, the city of 11 million will be released from lockdown on April 8, Chinese authorities announced Tuesday.

The world will be watching to see if the virus spreads once again — even if some don’t trust Beijing to accurately report a resurgence, given it concealed the early signs of the outbreak.

Some experts are starting to envision what a transition to the “new normal” could look like once lockdowns slow the rates of infection: A combination of the aggressive testing urged by the World Health Organization since the very beginning, with an expectation that the most vulnerable people will shoulder more of the burden of isolating alone.

A paper published Wednesday by a group of Italian economists proposes gradually sending people under 40 back to work, bringing the most important manufacturing back online and using an app to track how potentially contagious people might have spread the disease.

On Wednesday, the U.K. announced a massive ramp-up of testing capacity, including the purchase of 3.5 million antibody kits that could be used for at-home testing and a new laboratory to analyze swabs outside London.

For countries basing their decisions not on epidemiological studies, but political or economic factors, the exit ramp is easier to spot — even if the road afterward could be rocky.

In announcing new restrictions Tuesday, Polish Prime Minister Mateusz Morawiecki said the aim is to return to a “new normal” after Easter break. The country’s elections are slated for May 10.

And in the U.S., President Donald Trump has expressed concerns about the U.S. economy and said a social resurrection by Easter “would just be a beautiful timeline.”

There are many in Europe who are hoping he’s right, as national leaders watch their tax bases crumble even as demands for bailouts mount.

But experts offer few reasons for short-term optimism.

Asked by U.K. MPs on Wednesday how to end the outbreak, Neil Ferguson, an infectious disease expert from Imperial College London, responded: “The long-term exit of this is clearly the hopes of a vaccine.”

At best, that’s a year away.

Ashleigh Furlong, Jakob Hanke, Elisa Braun, Judith Mischke, Annabelle Dickson, Zia Weise and Zosia Wanat contributed reporting.

This article is part of POLITICO’s premium policy service: Pro Health Care. From drug pricing, EMA, vaccines, pharma and more, our specialized journalists keep you on top of the topics driving the health care policy agenda. Email [email protected] for a complimentary trial.