Antibody tests for COVID-19: What are they, and what do they do?

Both the European Union and the Food and Drug Administration (FDA), in the United States, have now approved certain antibody tests for COVID-19. But what are these tests, and how can they be useful? Read our Special Feature to find out.

If you have been following the news about research into COVID-19 treatments, you may have seen mentions of “antibody testing.”

But what exactly are antibody tests, how do they differ from similar-seeming tests — such as antigen tests — and are they of any use?

In this Special Feature, we pursue these questions and describe the possible role of antibody testing during the ongoing pandemic.

We also explain why antibody testing, though useful, is only one part of a global effort needed to reduce the negative impact of SARS-CoV-2, the virus that causes COVID-19.

Antibody tests vs. antigen tests

First, what is antibody testing? It involves screening a sample of blood for tiny molecules called antibodies that have “learned” to respond to a particular pathogen, such as a virus.

Antibodies learn to recognize and fight off specific viruses through exposure to them.

This means that if a doctor finds antibodies that respond to a certain virus in a person’s blood, they can confirm that the person has overcome an infection of that virus.

By contrast, antigen tests, which typically rely on swabs from the nose or throat, show whether the person currently has the viral infection.

Antigens are molecular structures on the surface of viruses. Their presence in a sample indicates that the virus is also present and active — and therefore infectious.

What can and can’t they do?

In the context of the COVID-19 pandemic, some researchers argue that deploying antibody tests more widely in communities could ultimately help ease the restrictions put in place to slow the spread of the virus.

For instance, Dr. Jenny Harries, the deputy chief medical officer for England, stated in a Euronews interview that “If we know how many people have already had [the new coronavirus], we will understand the proportion of the population that could still get it, and that gives us the clue to a number of things.”

Specifically, a better understanding of how many people have recovered from SARS-CoV-2 infection would enable specialists to more accurately estimate the dynamic and rate of the virus’ spread. This would help them design better strategies to safeguard the community and possibly also allow for more freedom of movement.

Meanwhile, antibody tests could play an additional role — confirming who qualifies to donate convalescent plasma, a component of blood that contains antibodies.

Currently, researchers across the U.S. and in various European countries are testing the use of convalescent plasma therapy in the treatment of COVID-19.

This therapy involves transfusing people who have COVID-19 with plasma from people who have already recovered from the disease and who have formed antibodies to the virus. The transfusion could help the body fight off the infection.

Also, some officials have conjectured that antibody tests could indicate who may have developed a degree of immunity to SARS-CoV-2.

This could allow a person to acquire an “immunity passport” — documented proof that they can likely no longer spread the virus. The documentation would potentially guarantee the person freedom of movement in spite of lockdown measures.

However, researchers warn that these passports would be untenable, as there is no clear evidence that people who have recovered from SARS-CoV-2 infections have acquired long-term immunity to the virus.

In fact, the World Health Organization (WHO) released an official statement on April 24 that rejected the option of these passports.

The health authority cited the lack of evidence regarding acquired immunity to the new coronavirus and pointed out that people who believe that they have immunity could end up endangering their own and others’ health. The WHO warn:

“People who assume that they are immune to a second infection because they have received a positive test result may ignore public health advice. The use of such certificates may, therefore, increase the risks of continued transmission.”

Are antibody tests available?

In light of the potential benefits of antibody testing, researchers affiliated with various healthcare and pharmaceutical companies have rushed to develop accurate tests that could be distributed to points of care and, eventually, to people’s homes.

National and global authorities have been assessing these tests, some of which have now received official certification.

The EU and the FDA have each recently validated an antibody test developed by the healthcare and medical device company Abbott Laboratories.

Abbott claim that their test has “99.6% specificity and 100% sensitivity for patients tested 14 days after symptoms began,” which would indicate a high level of accuracy.

Similar claims have come from the pharmaceutical company Roche about their own antibody test, which has also received FDA emergency use authorization. Roche report that their test has “specificity greater than 99.8% and sensitivity of 100%.”

However, some researchers remain unconvinced by these claims, explaining that they could be “misleading,” as the samples trialed by the company may not represent those of the population exposed to SARS-CoV-2.

Also, there are other reasons to treat such tests with caution for the time being.

“Whilst both tests make no or very few false positive errors — they only very rarely wrongly state a non-COVID-19 sample as showing antibodies to COVID-19 — both tests sometimes miss detecting COVID-19 in samples which are from infected patients,” notes Prof. Jon Deeks, of the University of Birmingham’s Institute of Applied Health Research, in the United Kingdom.

For people in the U.S. who do wish to take advantage of antibody tests, the Centers for Disease Control and Prevention (CDC) encourage them to contact their healthcare providers for more information.

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