NEW YORK (NYTIMES) - A new study offers a glimmer of hope in the grim fight against the coronavirus: Nearly everyone who has had the disease - regardless of age, sex or severity of illness - makes antibodies to the virus.

The study, posted online on Tuesday (May 5) but not yet reviewed by experts, also hints that anyone who has recovered from infection may safely return to work - although it is unclear how long their protection might last.

"This is very good news," said Angela Rasmussen, a virologist at Columbia University in New York who was not involved with the work.

Antibodies are immune molecules produced by the body to fight pathogens. The presence of antibodies in the blood typically confers at least some protection against the invader.

Health officials in several countries, including the United States, have hung their hopes on tests that identify coronavirus antibodies to decide who is immune and can go back to work.

People who are immune could replace vulnerable individuals, especially in high-transmission settings like hospitals, building what researchers call "shield immunity" in the population.

But most antibody tests are fraught with false positives. The new study relied on a test developed by Florian Krammer, a virologist at the Icahn School of Medicine at Mount Sinai, that has a less than 1 per cent chance of producing false-positive results.

Several small studies have given reason to hope that people who have had Covid-19, the illness caused by the coronavirus, would gain some immunity. The new study is the largest by far, with results from 1,343 people in and around New York City.

The study also eased a niggling worry that only some people - only those who were severely ill, for example - might make antibodies. In fact, the level of antibodies did not differ by age or sex, and even people who had only mild symptoms produced a healthy amount.

Having antibodies is not the same as having immunity to the virus. But in previous research, Krammer's team has shown that antibody levels are closely linked with the ability to disarm the virus, the key to immunity.