BERLIN — 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 theater, 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.”