Good news. There is a new Elisa kit to detect COVID-19 nucleoprotein as below.

EU3124 COVID-19 nucleoprotein ELISA Kit
Sandwich Elisa
Range: 62.5-4000pg/ml
Sensitivity: 37.5pg/ml

It can be used for quantitative detection of nucleoprotein in serum, plasma, tissue homogenates, cell culture supernatant, cell culture lysate and other biological fluids(throat swab, nose swab).