MARANELLO, Italy—Ferrari NV’s normally bustling factory and headquarters here stood mostly deserted earlier this week, except for the line of employees waiting to have their blood drawn and tested for coronavirus antibodies.

The workers, most of them dressed in Ferrari-red factory outfits, stood at least three feet apart as they filed into the company’s medical offices. There, health care workers drew blood for the tests. Eight hours later, they were to be notified of the result of the so-called antibody, or serology, tests—designed to identify people who have been infected with the coronavirus and developed antibodies to it.

With virus infections leveling off or falling in many Western countries, manufacturers are looking for ways to restart or ramp up production, while avoiding new outbreaks. Italy—among the countries hardest hit by the coronavirus pandemic—gave companies the green light to start up their factories May 4 if they follow guidelines that include social distancing and frequent disinfecting of workstations and common areas.

Ferrari employees who are going back to work pass through a series of steps designed to keep the coronavirus out, including blood tests for antibodies. WSJ’s Eric Sylvers reports from the car maker’s factory near the center of Italy’s outbreak. Photo: Francesca Volpi for The Wall Street Journal

Ferrari, the luxury car maker, suspended production on March 14. It and a group of other manufacturers here are going a step further than the government requirements by offering employees the antibody tests, too.

The epicenter of Italy’s outbreak was in its industrial heartland in the north, including in the Emilia-Romagna region where Ferrari is based. The virus continues to circulate across large parts of the north, according to official statistics.

Antibody testing has been controversial and, so far, unproven as an effective tool against the new coronavirus. Politicians around the world have held such tests up as “immunity passports”—a way to identify people who can return to work safely.

A person infected by a virus typically creates antibodies to fight it off. Those stay in the body and can defend against reinfection. Most health experts expect antibodies will give some level of immunity to the new coronavirus. But because the disease is so new, no one knows for sure that they will provide protection, or for how long.

The antibody tests are difficult to design, and their effectiveness can be stymied by what is sometimes a high level of incorrect results. A false positive, in which somebody appears to have the antibodies, but actually doesn’t, could be particularly dangerous if it gives somebody a false sense of security.

“We know there are limits to these tests, but we’re doing the best we can with what’s available,” says Michele Antoniazzi, Ferrari’s head of human resources, in his office on the grounds of Ferrari’s headquarters.

Francesca Volpi for The Wall Street Journal

Ferrari opted for a type of antibody test that is done using blood drawn from a vein. Those tests have proven to be more reliable than an alternative type done using a tiny amount of blood extracted from a pinprick.

This past week, the eerie silence on the Ferrari factory grounds was pierced by a loop of announcements in English and Italian broadcast over a public address system: “Always wear a mask carefully covering nose and mouth…avoid touching eyes, nose and mouth.” The revving motor of Ferrari test cars occasionally animated the otherwise empty roads on the compound.

Half of Ferrari’s 4,000 employees had been tested for the antibodies by the end of the day on Thursday and most of the remainder are expected to be tested before they come back to work. Less than 1% of those tested so far have come back positive for the coronavirus antibodies. While the relatively low rate could indicate that the vast majority of Ferrari’s workforce never had the virus, it could also point to the limits of the test’s efficacy. Some public health experts have forecast that the percent of the population in northern Italy that has been infected is many times higher. Ferrari said the two figures aren’t comparable because there are many variables that can influence infection rates.

Those shown to have antibodies were referred to regional authorities that then carried out a test to see if the Ferrari employees were currently positive for the virus. Several turned out to be asymptomatic carriers.

“We potentially stopped an outbreak here because those still positive could infect other people,” says Mr. Antoniazzi. “The tests give peace of mind to the workers and their families, but they also provide important information for the company.”

Francesca Volpi for The Wall Street Journal

The company will bring back most of its workers—not just the ones that have the antibodies—as it looks to get its factory up and running. On Monday, Ferrari will restart a line for the Monza, a limited-series, modern version of 1950s race cars. The company, which produced about 10,000 cars last year, is aiming to restart all lines by the end of next week.

The antibody tests are just one part of Ferrari’s bid to keep the coronavirus in check. Employees’ body temperatures are checked with a thermoscanner as they enter the company grounds in single file. Plexiglass has been installed to create personal eating spaces in the cafeteria, workers are issued new masks every day and the production lines have been tweaked to keep employees further apart.

About 5% of Ferrari employees refused to be tested. They too will be back at work, as Italian law prevents the company from requiring its workers take the test.

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More than 95% of Ferrari employees globally work at the Maranello plant and offices here, so the company has the advantage of being able to focus its virus-containment efforts in a concentrated area. The factory’s location in a relatively remote town with poor public transportation options could help. Almost all workers arrive in their own cars. Public health experts are particularly worried about people catching the virus on trains, buses and metro lines.

A smattering of other Italian companies are also doing antibody tests, including power plant maker Ansaldo Energia and Europea Microfusioni Aerospaziali, a unit of U.K. engine maker

Rolls Royce Holdings

PLC. Iveco Defence Vehicles, a unit of

CNH Industrial

NV, has tested about a third of its 750 employees in the city of Bolzano near the Austrian border.

Colosio, a family-owned company that makes die casting machines and accessories in Bresica, one of the northern Italian cities most devastated by the virus, will offer tests to its 90 employees. Production is set to start gradually beginning on May 4, said Chief Financial Officer Emanuela Colosio.

“The test doesn’t give you a 100% guarantee, but it’s another element we can use to try and assess who is healthy and can come back to work,” says Ms. Colosio. “The serology tests will hopefully help us as we try to make up the time lost during the forced shutdown.”

This week, a

Ford Motor Co.

executive said testing will be critical to limiting the spread of the coronavirus in the company’s factories. But she said it could be months before widespread testing is possible and didn’t specify what type of testing Ford is considering.

Ferrari is offering employees who test positive for the virus apartments where they can stay for their quarantine, so they don’t risk infecting family members. Intrafamily transmission has been a key factor slowing Italy’s emergence from the pandemic, according to health officials. Ferrari is also extending free testing to the immediate family of its employees, which will bring the total number of people tested to about 16,000.

“We don’t consider it a cost,” says Mr. Antoniazzi, the human resources executive. “It’s an investment.”

Francesca Volpi for The Wall Street Journal

—Mike Colias contributed to this article.

Write to Eric Sylvers at [email protected]

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