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 police officer.

“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.”

Alone in isolation: Scenes from a global pandemic Show all 20 1 /20 Alone in isolation: Scenes from a global pandemic Alone in isolation: Scenes from a global pandemic A person crosses 42nd Street in a mostly deserted Times Square as New York City is in lockdown Reuters Alone in isolation: Scenes from a global pandemic A man walks across an empty Charles Bridge in Prague Reuters Alone in isolation: Scenes from a global pandemic A person wearing a mask to prevent contracting the coronavirus walks along an empty street in Seoul Reuters Alone in isolation: Scenes from a global pandemic A man walks along the National Mall in Washington Reuters Alone in isolation: Scenes from a global pandemic A quiet train on the London underground Reuters Alone in isolation: Scenes from a global pandemic An empty street is seen in Manhattan as New York City is in lockdown Reuters Alone in isolation: Scenes from a global pandemic A person sits alone in the empty Notre Dame des Victories Cathedral in Dakar, Senegal Reuters Alone in isolation: Scenes from a global pandemic A guest talks on his mobile phone at a hotel in Hong Kong Reuters Alone in isolation: Scenes from a global pandemic A man walks alone through the nearly empty Oculus transportation hub in Manhattan Reuters Alone in isolation: Scenes from a global pandemic A passenger waits for a train at an empty platform in Richmond station, London Reuters Alone in isolation: Scenes from a global pandemic A woman walks through empty streets outside the Public Market in Seattle Reuters Alone in isolation: Scenes from a global pandemic A woman walks alone on Mariacka street in the usually crowded Main Town in Gdansk, Poland Reuters Alone in isolation: Scenes from a global pandemic A lone commuter stands at the platform of a deserted train station during morning rush hour in Sydney Reuters Alone in isolation: Scenes from a global pandemic A man rides his electric scooter on an empty street near Kaiser Wilhelm Memorial Church in Berlin Reuters Alone in isolation: Scenes from a global pandemic A woman walks on Recreio dos Bandeirantes beach, amid the coronavirus outbreak in Rio de Janeiro Reuters Alone in isolation: Scenes from a global pandemic A man crosses a nearly empty 5th Avenue in midtown Manhattan as new York City is in lockdown Reuters Alone in isolation: Scenes from a global pandemic A volunteer walks as military beds set asid efor coronavirus patients in Novi Sad, Serbia Reuters Alone in isolation: Scenes from a global pandemic A woman wearing a protective face mask rides a bus in Bangkok Reuters Alone in isolation: Scenes from a global pandemic A man plays basketball alone in Manhattan Reuters Alone in isolation: Scenes from a global pandemic Pope Francis delivers an extraordinary "Urbi et Orbi" (to the city and the world) blessing - normally given only at Christmas and Easter - from an empty St Peter's Square Reuters

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 licenses” — have lingered over a national debate over how and when to reopen the country. Regional presidents have turned to serological tests, which look for the presence of antibodies, 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.

But even the best laid plans can go awry; Singapore attempted to reopen only to have the virus reemerge.

In the United States, 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 United States, 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.”

Mr Hoelscher was 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 1 February, 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 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.

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In Gangelt, a small town of about 12,000 in northwest 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 towards reaching herd immunity has begun,” professor 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 Mr Hoelscher, the professor. “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.”