Because indirect ELISA tests for antibodies that bind to the antigens.

It may be that there are other challenges to making those specific ELISA assays and with PCR tests being developed there may be less of a need. However, having a testing kit that allows for Ab detection may be a good test to have especially for front line workers to show that they have built a immune response to Covid and will be "safer" than others without an immunity and also less likely to be a carrier so can act in a more important capacity. Or can test workers to get people back doing more regular work where required

I imagine the biggest hurdle in testing for antigen would be sensitivity, when you test for viral genome you amplify the signal - which is relatively straight forward to do - much less so for an antigen. Which would make reliably detecting what may be small amounts of the proteins in question tricky. Also we would need to have antibodies that bound two different epitopes of the antigen - and that we knew were not going to cross react with one of the many other coronaviruses.

The antibody test as the previous reply suggests would tell you that the individual was probably safe from catching the virus again and therefore out of the infection cycle

why do the antibodies used in the test need to bind two different epitopes of the atigen?

I think what Hannah was trying to say is that you have the capture Ab bound to the plate that captures the covid epitope of interest following this another detection Ab can bind a second epitope and the detection method of choice can be used. Ideally these should be two differing targets and be specific for the virus in question.

To do that is tricky but not impossible by any means at all but you also want it to be relatively specific for the Covid-19 virus. Sensitivity is more important though at this stage and we have other tests that are very sensitive and specific for the virus with quick turn around times too so there isnt much need, there is more need to detect the Ab's produced by previous patients though and thats where this technology is going

ELISA used in COVID-19 (click on picture):

SARS-Cov-2 recombinant protein on a cassette (antigen)

Nico Crudele

During the first encounter with a virus, a primary antibody response occurs. IgM antibody appears first, followed by IgA on mucosal surfaces or IgG in the serum (1). During testing, if you have only one band (IgM), then it tells you that this patient is infected by coronavirus recently. If you have both IgM and IgG, then it means that this patient has been infected for a period of time already.

(1) http://www.virology.ws/2009/07/22/adaptive-immune-defenses-antibodies/

I think the use of Ag virus coated plate to detect the Ab against the virus is more easy than the detection of the Ag virus, because the amount of Ab is more than the virus Ag itself as the immune system forms more and more Ab.. So it's more easy and more accurate..

You will find the article interesting.

https://www.nature.com/articles/s41467-020-16256-y