When a virus such as COVID-19 enters your body, it triggers a reaction to fight the infection. This is called an immune response. In an immune response, your body produces antibodies that attack the weak spots of the virus to stop further damage. The race is on to develop a blood test that can measure antibodies to COVID-19.

An antibody test could help prove that a new vaccine works, and show who has had the infection. If we know who has or hasn’t been infected with COVID-19, it is much easier to predict when it will be safe to go back to normal. The best time to start measuring these antibodies is about a week after the infection occurred. However, we need to make sure that the antibody test is reliable – to be sure we are measuring antibodies against COVID-19 and not some other infection such as the flu.

Source: https://www.thelancet.com/journals/lancet/article/PIIS0140-6736(20)30788-1/fulltext