Coronavirus patients suffering mild symptoms may carry protective antibodies for just a matter of weeks, new research suggests.
A study of almost 70,000 people in Spain found that 14 per cent who initially positive for antibodies then showed a negative result two months later.
The apparent antibody disappearance was predominantly seen among those reporting very mild symptoms or who had been asymptomatic, according to the analysis published in the Lancet journal.
“Immunity can be incomplete, it can be transitory, it can last for just a short time and then disappear,” one of the report’s co-authors Raquel Yotti told The Times.
“We must keep protecting ourselves and protecting others.”
Other researchers told the paper that the findings were in line with a growing body of evidence that largely symptom-free coronavirus sufferers may not amass lasting antibodies, although other elements of the immune system may still protect them.
“No symptoms suggests a mild infection, which never really gets the immune system going well enough to generate immunological ‘memory’,” Ian Jones, professor of virology at the University of Reading, explained.
Prof Jones said this means anyone who tests positive for antibodies should not assume they are protected.
“They may be, but it is not clear,” he added.
The research also suggested that just 5.2 per cent of the Spanish population have developed antibodies against the disease, despite the country being one of the worst affected in Europe.
This pours water on hopes that herd immunity could offer wide-scale protection against the disease and therefore quash the need for constant lockdowns.
Around 70 to 90 per cent of a population needs to be immune to protect the uninfected.
"Despite the high impact of Covid-19 in Spain, prevalence estimates remain low and are clearly insufficient to provide herd immunity," the study's authors wrote in the report.
"In this situation, social distance measures and efforts to identify and isolate new cases and their contacts are imperative for future epidemic control."
The study is thought to be the largest of its kind on the disease in Europe, however, China and the US have conducted similar analyses.
“The key finding from these representative cohorts is that most of the population appears to have remained unexposed" to the coronavirus, "even in areas with widespread virus circulation," a Lancet commentary alongside Spain’s findings concluded.
"In light of these findings, any proposed approach to achieve herd immunity through natural infection is not only highly unethical, but also unachievable," the study’s peer-reviewers warned.