**2.1 Microfluidic biosensor setup**

As illustrated in **Figure 1**, the 2D configuration of the microfluidic biosensor with flow confinement studied has a length, L, of 250 μm and a height, H, of 40 μm. The reaction surface, of 20 μm in length, is positioned on the bottom border of the microchannel. The carrier fluid, mixed with the analytes (SARS-CoV-2) enters the microchannel (sample flow) from left to right. Ligands having constant concentration, *Bmax*, are initially immobilized on the reaction surface. The sample flow is correlated to the confinement flow in the vertical direction. The confinement flow traps analytes to a thin layer over the detection area.
