Antibody tests are ones which look for signs of past infection in someone’s blood.
Antibodies are substances produced by the immune system which store memories of how to fight off a specific virus. They can only be created if the body is exposed to the virus by getting infected for real, or through a vaccine or other type of specialist immune therapy.
Generally speaking, antibodies produce immunity to a virus because they are redeployed if it enters the body for a second time, defeating the bug faster than it can take hold and cause an illness.
An antibody test, which involves analysis of someone’s blood sample, has two purposes: to reveal whether an individual has been infected in the past and may therefore be protected against the virus, and to count those people.
Knowing you are immune to a virus – although whether people actually develop immunity to Covid-19 is still unknown – can affect how you act in the future. Someone may need to protect themselves less if they know they have been infected, for example, or medical staff may be able to return to work in the knowledge they are not at risk.
Counting the numbers of people who have antibodies is the most accurate way of calculating how many people in a population have had the virus already.
This can be done on a small sample of the population and the figures scaled up to give a picture of the country as a whole.
In turn, this can inform scientists and politicians how devastating a second outbreak might be, and how close the country is to herd immunity – a situation in which so many people have had the virus already that it would not be able to spread quickly a second time.
Experts believe that around 60 per cent exposure would be required for herd immunity from Covid-19, but the UK does not appear to be anywhere close to that.
Early estimates suggest 17 per cent of Londoners have had the virus, along with five per cent of the rest of the country – about 4.83million people.
This means the virus might spread slightly slower in future but the risk of second outbreak and hundreds or thousands more deaths remains very real.