Antibody tests fail to identify many with mild form of virus

ANTIBODY tests may miss large numbers of people who contracted Covid-19 because they do not work for people with a mild infection, research from Oxford University suggests.

A study of more than 9,000 healthcare workers suggested that significant numbers of people tested negative despite probably having had the virus.

The work has major implications for health policy, and could mean reviewing where the threshold between negative and positive results lies.

Antibody tests are used to map an outbreak, but are also central to potential plans for “immunity passports”, which could allow people to return to normal life. However, it is not clear whether antibodies, the protective proteins produced to help fight off the virus, provide any long-lasting immunity to the disease.

The Oxford University study compared the results of antibody tests among healthcare workers who had also reported losing their sense of taste or smell, a key coronavirus symptom.

Of the 903 people who tested positive for antibodies on one test, 47 per cent reported a loss of their sense of taste or smell. But among those whose test results fell just below the threshold for a positive antibody result, meaning they would currently be classed as not having had the virus, 30 per cent also reported a similar loss.

“This suggests the test threshold is missing people with mild disease,” said Dr Tim Walker, one of the study’s authors. “Of course, there will be plenty of people, too, who will have had no symptoms and still have antibodies.”

The background rate of people who would report these symptoms for another reason – from seasonal colds to other conditions – is around three per cent, researchers added.

A further 387 people also tested just below the threshold in terms of antibodies for a positive test, but did not have symptoms. They may have been asymptomatic coronavirus patients, but the researchers could not say with confidence that they had the virus.

The study used several antibody tests, including the Abbott diagnostic, which is one of the four main commercial tests used in the UK. The results suggest these were around 11 per cent less sensitive than is currently believed – around 98 per cent – in part because the tests were developed using samples from symptomatic patients.

The team suggested that samples from mild and asymptomatic patients with confirmed infections should be included in the evaluation process to investigate further whether mild illness is associated with a mild immune response – responses that are currently being missed by the antibody tests.

Dr Walker said that antibody tests have been designed to make sure that people who did not have antibodies were not mistakenly told that they did, giving them a false sense of security. Because of this, some people with antibodies would be missed, he said.

Professor Will Irving, from the University of Nottingham, who was not involved in the study, said: “It is impossible to know to what extent we are under-reporting positives. But we are missing some patients, and one particular reason for this is that the cut-off is set too high.”

A Department of Health and Social Care spokesman said: “We do not yet know whether antibodies indicate immunity or if they prevent transmission. However, antibody testing is an important part of our testing strategy.”

‘We are missing some patients, and one particular reason for this is that the cut-off is set too high’