When Lucia Abascal calls a client, she first asks them whether they have a home and enough to eat for 14 days. "There are many homeless people in San Francisco. Not everyone can afford a quarantine." Abascal, who was born in Mexico and is now a clinician in the US, often speaks to clients in her mother tongue. "The majority of at-risk people in the Bay Area are Latinos. Some have to go outside to work to provide for their large families. This makes them vulnerable to catching the virus."

Abascal works as a contact tracer. She is part of a 40-person team in San Francisco consisting of public health officials, clinicians, medical students, but also librarians currently out of work. They call the contacts of people infected with COVID-19 and arrange tests for them. If needed, they send food packages or medicine and arrange hotel rooms.

At the beginning of each day Abascal gets a list of people to call. She works from home in four-hour shifts. Until recently clients without symptoms were sent an automated text message every day to check their state of health. Now, all of an infected person's contacts get tested twice, at the beginning of their two-week "shelter in place" and again at the end, even if the first test result was negative.

Every call is supposed to last 15 minutes, but often there are more questions than anticipated. "Some are worried we are checking on their immigration status, others ask whether they can fight the virus with herbs," says Abascal.

Abascal does not work with any COVID apps, even though tech companies have offered their support to the initiative run by the University of California, San Francisco (UCSF), and the local public health department numerous times. She saves processed cases with an open-source software that was used in the fight against Ebola.

"Apps can tell you whether a person went to Starbucks, but they cannot tell you whether they tried to get testing but couldn't, whether they struggle to provide for their families," says Abascal. "There's a complexity to this job that tech is not able to solve right now."

Receipts, spreadsheets, language barriers

Singapore also employs contact tracers rather than relying solely on apps. Edwin Philip has been working as a tracer at Singapore General Hospital since the beginning of February. In contrast to Lucia Abascal's work, which has her calling people who have been exposed to the infected but not tested positive, he first has to identify contacts by asking COVID-19 patients detective-like questions.

To do so, Philip gets only two hours to reconstruct a person's complete history of movement and contacts over the previous 14 days. He starts by asking detailed questions about their meals. "I ask them what they had for breakfast, lunch and dinner. I ask who they passed the salt or ketchup bottle to, how long they talked to them. This gives me a clue about who they could have infected."

Philip then goes on to ask whether they visited churches, mosques, temples or malls. "Some of them send me photos of all their taxi and restaurants receipts, which are sometimes faded. I have to scan them and make them readable," says Philip. When people do not remember everything, Philip talks to their spouses. "They look in their work diaries, some even provide Excel spreadsheets with all activities."

After the lockdown was imposed in early April, Philip was confronted with new challenges. Most infections now occur in the crowded dormitories that house 300,000 of the city's 1.4 million migrant workers. At least 23,000 of them are infected. As most of them come from South Asia, Philip faces language barriers. "In the beginning of the outbreak most infected were Chinese, which many Singaporeans including myself can speak. I do not know Bengali and translators are in short supply," he says.

Like Lucia Abascal in the US, Philip does not use apps in his work. The government app, TraceTogether, is seen as secondary in the fight against the coronavirus. Human contact tracing proved to be an effective method when it was employed during the 2003 SARS epidemic. Many other Asian countries, such as China and South Korea, also rely on it.

Suspicion and moments of relief

In Bavaria, Germany, around 2,500 contact tracers have begun their new jobs since the end of April. One of them is Franziska Weiß. The civil-servant-to-be was asked to put her training on hold to work as a tracer for the Nürnberger Land region. She works in an office with another team member, their desks spaced 1.5 meters (five feet) apart.

When she calls people, they sometimes ask suspiciously whether she really works for the public health department. She gives them the option to call the citizen phone line and get reconnected to her to prove her identity. Her job also entails telling people their test results. "Most people keep calm when I tell them they are infected, but they have many questions and I talk to them for half an hour on average," she says.

Afterwards, Weiß calls all of the people they have had contact with in the previous 48 hours. They have to stay in quarantine for 14 days, and Weiß phones them every day to ask how they are. If they develop symptoms, a test is arranged at home or in a laboratory. Even though Weiß only has to ask about people's movement over two days rather than two weeks, in contrast to tracers in Singapore, many people find it hard to recall their actions, she says. "I ask them if they have been to the hairdresser or doctor. If older people or people with dementia cannot remember, I ask their family members."

Despite being on the telephone all day long, Weiß enjoys her job. "I get to tell people when their quarantine ends. That is always a happy moment."