The mixed promise of antibody tests

Image credit: Fernando Zhiminaicela via Pixabay

As many countries in Europe tentatively begin to ease restrictions, experts are debating the confounding question of how to exit lockdown and restart the economy under the enduring threat of coronavirus.  Antibody tests have been touted as one possible means to assess who may have acquired immunity to the virus and could therefore return to work. However, development and application of the tests have been marred by controversy, and the hope they can offer remains unclear. 

An antibody test, or serological test, aims to identify whether an individual has previously mounted an immune system response to Covid-19. With emerging studies suggesting that up to 50% of carriers could be asymptomatic, and even more experiencing just mild symptoms, many will be keen to know if they have already had the virus without realising. Increasingly, governments across the world are discussing “immunity passports” as a means of increasing freedom for those who have already recovered. This is all very well apart from a major caveat: we still don’t know how immunity to coronavirus works. 

When our body detects a viral infection, it generates a tailor-made defense strategy. Immune cells generate antibodies which are specifically shaped to complement the unique “spikes” decorating the outer coat of the invading virus. By coating these spikes, antibodies can stop the virus in its tracks by blocking it from latching onto and infecting new cells. Often, some of these bespoke antibodies remain after the virus is fought off, providing an immunological memory of the infection which should, in theory, help the body to protect itself in the future. 

Antibody tests work similarly to pregnancy tests, but using a pin-prick of blood. On the test stick there is a surface covered in antigens – the viral “spike” molecules that the antibodies bind to. These molecules “catch” the antibodies in the patient’s blood as it washes over the surface, and a chemical reaction induces a signal, a change in colour for example, to show that they’ve been detected. 

The promise of the test depends on an assumption that Covid-19 antibodies in a patient’s blood will protect them from reinfection. However, this is information that we currently do not have. Studies in two closely related coronaviruses, SARS and MERS, have indicated that antibodies persist for 1-3 years after recovery, but we don’t know exactly how effective these antibodies were in the case of reinfection. Some coronaviruses, such as the common cold, can infect an individual up to three times in one year. WHO epidemiologist Maria Van Kerkhov has expressed concern over the discussion of immunity passports: “these antibody tests will be able to measure that level of seroprevalence – that level of antibodies – but that does not mean that somebody with antibodies is immune.” Researchers are currently running ongoing experiments to assess which antibodies are protective and how long for, but these kinds of studies will inevitably take time. 

A further complication has been the challenge in accessing reliable tests. In March, the government ordered 3.5 million antibody tests from China, which upon further testing were found to be too unreliable to use. The UK testing coordinator has warned that inaccurate tests could put people at risk of infection, with false positives luring people into a misguided sense of security. Now, pharmaceutical company Roche has announced the development of a highly accurate antibody test which it aims to roll-out to the UK by mid-May. 

How the government will utilise these tests remains to be seen, though it is likely they will first be provided to healthcare professionals and other key workers. Accurate and widespread antibody testing would undoubtedly be a good thing, especially as a tool to understand the spread of the disease and how different population groups develop an immune response. Until we have more answers, though, we should not rely too heavily on the promise of immunity. The gamble is simply too costly.

Written by Emma Clarke and edited by Ailie McWhinnie.

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