“We don’t know if everyone who has had the disease has developed an acceptable protective immunity,” said Dr. Alessandro Venturi, the president of the San Matteo hospital in the Lombardy town of Pavia. The hospital validated the antibody screening test used for mapping the virus in Lombardy, Italy’s hardest hit region.

Infected people develop different quantities of antibodies, and researchers are still studying the level that offers protection, and for how long.

“We don’t know how long they last,” Dr. Venturi said. ‘‘This is the central point.”…

Some virologists said they worried that all the hype around antibodies would lead to bad behavior among those who consider themselves invincible, thus leading to more infections.