This is all true, but detecting the viral proteins directly with reasonable sensitivity and specificity is much more difficult. The reason is that there is just a lot less of it to detect. After seroconversion, the amount of antibody against the virus can be very high and relatively easy to check.
+1 the major advantage of RT-qPCR is it's a massively amplified test. Tiny amounts of virus can be detected. When you're detecting a patient's antibodies to the virus, the patient is producing massive amounts of it. That said, after clearing the infection that production will decline, but still should be detectable at least for a while.
Direct detection of viral particles is substantially harder because for N viral particles you basically get O(N) signal (to put it in CS terms). There are some caveats, your readout antibody can generate an exponential signal for example.
You still need to be presenting the right part of the capsid and have the right material. In the direct ELISA scenario, you need 2 antibodies. One to bind the capsid to your surface, the other to bind it again and either have a readout attached directly or be bound by yet another antibody with the readout (likely the latter). Fortunately, viral capsids are repetitive so you might be able to use the same antibody for both. You ALSO need those antibodies to have low cross-reactivity with sputum or whatever other rough sample you have. When you are looking for a patient's antibodies, the patient's immune system has already taken care of that for you. This is all tricky, obviously it's not THAT hard, but it's not trivial.
RT-qPCR is directly a O(2^N) signal. It's extremely sensitive, requires very little assay development, can be made very specific, can work with little sample prep, so it's a much better front-line, first-to-go test than direct ELISA. Still, we'd all love an ELISA, particularly to patient antibodies.
I definitely agree about the superiority of RT-PCR and the drawbacks of direct detection. Nevertheless, seroconversion can take between 5-12 days from symptom onset [1]. Therefore it's important to outline the different ELISA strategies (Thank you for doing so!), when discussing diagnostic tests.
The signal is actually more like N*2^C where C is the number of cycles (usually around 30). So a huge amplification, but still linear in the starting amount, i.e. O(N). That's actually more useful, generally, since ratios of amplified material stay approximately constant during amplification.
Yes, there are of course drawbacks. But the direct detection is applicable at the same stage of infection as a rt-pcr and therefore usefull for immediate disease diagnosis. Indirect tests after seroconversion are the easier alternative, and will probably be used extensively in the aftermath, to get solid epidemiological numbers.
One disadvantage to RT-qPCR I forgot to highlight, and an advantage to direct detection ELISAs (and ELISAs in general) is that ELISAs are relatively robust when fully developed. RT-qPCR requires a lot more gear. See pregnancy tests. Sits on a shelf for ages, all you need to do is pee on it, works reliably, needs no extra equipment.
