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Population-wide immunity to the novel coronavirus could be "unachievable" with antibodies to the virus disappearing after just a few weeks in some patients, according to a major new Spanish study.

The Spanish government teamed up with some of the country's leading epidemiologists to discover what percentage of the population had developed antibodies that could provide immunity from the coronavirus.

The study found that just 5% of those tested across the country maintained antibodies to the virus, in findings published by the medical journal The Lancet.

The study also found that 14% of people who had tested positive for coronavirus antibodies in the first round of testing no longer tested positive in subsequent tests carried out weeks later.

"Immunity can be incomplete, it can be transitory, it can last for just a short time and then disappear," Raquel Yotti, the director of Spain's Carlos III Health Institute, which helped conduct the study, said.

Other researchers said the study corroborated findings elsewhere that immunity to the virus might not be long-lasting in people who develop only mild or no symptoms.

"No symptoms suggests a mild infection, which never really gets the immune system going well enough to generate immunological 'memory,'" Ian Jones, a professor of virology at the University of Reading, said.

Jones added: "Anyone who tests positive by antibody test should not assume they are protected. They may be, but it is not clear."