o stop a virus, you need to test for it. So why is it taking so long for the UK to increase its testing capacity? Although Public Health England is now aiming to conduct 10,000 tests per day, the UK’s response still pales in comparison with Germany, which is reportedly managing to test about 70,000 people per day. The government’s inability to increase the number of tests is down to several factors. The kits have to be purchased in a global marketplace where demand has now exceeded supply. This raises the question: why didn’t we prepare sooner? According to some reports, the problem isn’t just one of availability, but timing. The government’s initial controversial policy of mitigation focused on the elderly and vulnerable, and mass testing was a secondary concern. At the point the government shifted its strategy to suppressing the virus and decided to roll out testing, it was already playing catchup. The government had made no preparations to increase the supply of testing kits or expand laboratory capacity – and the UK found itself at the back of the line in a global queue for tests. Historic factors have also shaped the UK’s current testing capacity. Over the past 20 years, the number of laboratories and testing facilities has shrunk. The public health laboratory service, a network of local and regional laboratories that was established as part of the NHS in 1946 to respond to the threat of bacteriological warfare, was subsumed by the health protection agency in 2003. Many of the laboratories in the old network were shut down, taken over by local hospitals or merged into a smaller number of regional laboratories. So even if it did have enough kits, the UK would no longer have the capacity, equipment or biomedical scientists to match the testing capacity of its European neighbours. But what if the tests could be done at home? This brings us to the question of the kinds of tests the UK will be conducting. Part of the confusion about testing has arisen from the fact there are two different types of test, which measure very different things. One detects the presence of the virus, and is sometimes referred to as the “antigen” test; the other detects the presence of antibodies, sometimes referred to as the “serology”, or “antibody” test. The antigen test relies on molecular methods to detect the presence of the viral genetic material, usually in a nose or throat swab. This test is used to tell whether a person is currently infected and possibly infectious, and it needs samples to be sent to a testing laboratory. The antibody test, that will be available to conduct at home, detects whether the person actually had an infection at some time in the past – and indicates whether they are likely to be immune to further infection. This test is usually done on a spot of blood. Antibody tests typically do not become positive for a week or so after the beginning of an illness, so they are not much use for quickly diagnosing an acutely ill person – and won’t detect those who have only recently caught the virus. Criticisms of the UK’s testing numbers mainly concern the antigen tests. Several other countries, which have arguably been more successful at controlling their own epidemics – Germany, China and South Korea – have done many more antigen tests than the UK. South Korea has been running 20,000 tests a day. Even the US has completed more tests than the UK – 2,248 tests per million compared to the UK’s 1,923, according to the Covid Tracking Project. Importantly, the antigen test can show whether someone may be infectious to others, so it can be used to screen essential workers who may have only mild coronavirus symptoms, or who are asymptomatic. For now, the antigen tests are crucial for controlling the spread of the virus. But as we get over the peak of the epidemic, antibody tests will help us recover a semblance of normality. By detecting which individuals are immune from the virus, they could allow them to return to work. Essential workers with immunity would not need to worry about getting infected or passing the virus to their families. Carers, social workers, the police and priests officiating at funerals could carry out their work as normal. Perhaps an even more important use of antibody tests is to fill some of the biggest gaps in our knowledge about the pandemic. How many people get only mild illness, or even do not get ill following an infection? Are younger people getting infected at the same rate as older adults? This information will be vital in helping us decide when and how the current social distancing measures can be relaxed, and when we can finally return to normal life. Prof Paul Hunter is professor in medicine at the University of East Anglia