BERLIN (NYTIMES) - Mr Felix Germann was not expecting anyone when his doorbell rang last week. Outside was a doctor who looked like she had just stepped out of an operating theatre, green scrubs, face mask and all - and a policeman.

"I didn't do it!" Mr Germann said, throwing up his hands, and everybody laughed.

The unusual visitors had come with an unusual proposal: Would he allow them to test his blood for Covid-19 antibodies? Every month? For a year? Starting next week?

He would be helping to further the science that would ultimately allow for a controlled lifting of social and economic restrictions and save lives.

"Of course I said yes," said Mr Germann, a 41-year-old project manager at a media company. "I want to help. This is a collective crisis. The government is doing what it can. Everyone needs to do their bit."

With that, Mr Germann and his girlfriend joined 3,000 households chosen at random in Munich for an ambitious study whose central aim is to understand how many people - even those with no symptoms - have already had the virus, a key variable to make decisions about public life in a pandemic.

The study is part of an aggressive approach to combat the virus in a comprehensive way that has made Germany a leader among Western nations figuring out how to control the contagion while returning to something resembling normal life.

Other nations, including the United States, are still struggling to test for infections. But Germany is doing that and more. It is aiming to sample the entire population for antibodies in coming months, hoping to gain valuable insight into how deeply the virus has penetrated the society at large, how deadly it really is, and whether immunity might be developing.

The government hopes to use the findings to unravel a riddle that will allow Germany to move securely into the next phase of the pandemic: Which of the far-reaching social and economic restrictions that have slowed the virus are most effective and which can be safely lifted?

The same questions are being asked around the world. Other countries like Iceland and South Korea have tested broadly for infections, or combined testing with digital tracking to undercut the spread of the virus.

In hard-hit Italy, antibody tests - and the potential of "immunity licences" - have lingered over a national debate over how and when to reopen the country. Regional presidents have turned to serological tests as a way to better chart infections but also to get a sense of which workers might have the desired antibodies to possibly provide protection and return to work.

In the US, President Donald Trump is in a hurry to restart the economy in an election year, but experts warn that much wider testing is needed to open societies safely.

Both Britain and the US, where some of the first tests were flawed, virtually forfeited the notion of widespread testing early in their outbreaks and have since had to ration tests in places as they scramble to catch up. In Italy, one of the worst hit countries in the world, the central government and regional leaders sparred over how widely to test.

Germany, which produces most of its own high-quality test kits, is already testing on a greater scale than most - 120,000 a day and growing in a nation of 83 million.

The Munich antibody study, run by the Division of Infectious Diseases and Tropical Medicine at Munich University Hospital, and co-financed by the government of the state of Bavaria, is the biggest of several regional studies being rolled out in various corners of Germany. Still, scientists caution that there is no proof yet that the detection of antibodies signals effective immunity and even if it does, it is not known how long that immunity might last.

Nationally, the Robert Koch Institute, the government's central scientific institution in the field of biomedicine, is testing 5,000 samples from blood banks across the country every two weeks and 2,000 people in four hot spots who are further along in the cycle of the disease.

Its most ambitious project, aiming to test a nationwide random sample of 15,000 people across the country, is scheduled to begin next month.

"In the free world, Germany is the first country looking into the future," said Professor Michael Hoelscher, who heads up the Munich study, noting that a number of countries had already asked him for the protocol to be able to replicate it. "We are leading the thinking of what to do next."

Prof Hoelscher is co-author of what has become a widely influential research paper about how the virus can be transmitted before someone develops symptoms.

"There's no doubt after reading this paper that asymptomatic transmission is occurring," Dr Anthony Fauci, director of the National Institute for Allergy and Infectious Diseases in the US, told CNN on Feb 1, three days after the paper was published. "This study lays the question to rest."

Asymptomatic transmission is what has made containment so difficult because a large number of infections are not detected.

Measuring the number of hidden infections and getting a sense of the true scale of the disease is key to fine-tuning the gradual loosening of restrictions and minimising income loss and social isolation, scientists say.

"We will have a better idea of the number of undetected infections once we have done these representative studies," said Prof Lothar Wieler, president of the Robert Koch Institute for infectious diseases, which is conducting a number of the antibody tests. "A lot is being done to measure well." Some interim results have already come out.

In Gangelt, a small town of about 12,000 in north-west Germany, tests of a first group of 500 residents found that 14 per cent had antibodies to the virus. Another 2 per cent tested positive for the coronavirus, raising hopes that about 15 per cent of the local population may already have some degree of immunity.

"The process toward reaching herd immunity has begun," Prof Hendrik Streeck, director of the Institute of Virology at the University Hospital Bonn, who is leading the study, said in an interim report.

And even if 15 per cent of Gangelt has some degree of immunity, levels of immunity are almost certain to be lower elsewhere in the country.

"We are at a crossroads," said Prof Hoelscher. "Are we going the route of loosening more and increasing immunity in the summer to slow the spread of this in the winter and gain more freedom to live public life? Or are we going to try to minimise transmissions until we have a vaccine?" he asked.

"This is a question for politicians, not for scientists," he added. "But politicians need the data to make an informed risk assessment."