A study has found that 17% of Londoners and 5% of people in the whole of the UK have already had coronavirus. The participants in the government-commissioned study had to give blood samples that were tested for antibodies.
The test looks for antibodies in the blood to see whether someone has already had coronavirus. Antibodies are made by the immune system when it fights an infection.
According to the World Health Organisation (WHO), there is still no evidence that people who have developed are immune from being infected again.
The test has been approved for use by Public Health England, and will be rolled out for wider use next week. Over the coming months 10 million antibody tests will roll out in a phased approach.