The UK has ordered 3.5 million antibody tests designed to reveal if people have been infected with the coronavirus. The UK’s prime minister Boris Johnson, who today announced he himself has tested positive for the virus, has said these tests will be a “game changer”, but the reality is they might not have that much of an impact in the short term.
Almost all testing around the world is based on looking for the presence of the virus, by looking for its genetic sequence. But such tests require or nose or throat swabs to be taken by trained personnel and sent to a specialised lab, and there’s a global shortage of equipment. Genetic tests also detect only active infections.
Antibody tests, by contrast, detect the antibodies our bodies produce to kill the virus, which we keep producing even after the virus is eliminated. These tests can reveal who has been infected even after they have recovered. Handheld tests that require only a drop of blood can give results in ten minutes, and can be mass produced quickly and cheaply.
If we know someone has had the virus, they can potentially leave their home without risk of being re-infected, which would help countries getting moving again. However, the accuracy of the tests has yet to be established. “The one thing that’s worse than no test is an inaccurate test,” Chris Whitty, the UK’s chief medical officer, said on 25 March. Someone wrongly told they have already had covid-19 could go out and get infected.
How accurate do they need to be? “It’s very difficult to say,” says Emily Adams at the Liverpool School of Tropical Medicine, who is helping assess the tests developed by Mologic, one of the companies supplying the UK. Part of that process will be working out what accuracy is required for different uses, Adams says.
Ideally, we want to find out whether the thousands of health workers who are currently self-isolating because they or their housemates have symptoms that might be covid-19 can get back to work. Unfortunately, the antibody test may not help with this.
The antibody response to the coronaviruses may be delayed compared with other infections. The tests can be used only 14 days or more after people develop symptoms, Adams says.
This also means antibody testing will also be only of limited use for tracing the contacts of infected people – which many think is crucial for controlling the outbreak – because health authorities will be weeks behind.
Widespread antibody testing will also reveal whether large numbers of mild infections have gone unnoticed. It would be great news if this is the case, allowing many to return to work and meaning that the infection fatality rate is lower than thought. Unfortunately, places like South Korea that have been doing lots of genetic testing have not found vast numbers of mild cases.
On the plus side, many groups are working on faster genetic tests and on antigen tests that can detect the virus in, say, saliva. Testing widely for both active infections and past infections should be a highly effective combination.
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