as 8 understand it all the detection kits key on a spike that is specific to covid 19. if this had wide crossreactivity to other corona viruses I would expect the positive rate to be near 100%. We have all had colds
As far as I can tell from the paper, the immunoassay detects both IgG and IgM antibodies. The first immune response after infection is the IgM, which spikes quickly and soon falls off. So any test that picks up IgM is only detecting recent infection. The IgG response is slower, with a longer delay until it spikes and a longer drop off. I do not know specifically how long the antibody titer remains elevated after a cold infection, but seriously doubt that the titer a year or more after is going to be even close to the convalescent titer. So, depending on the reaction conditions, the kit they used might not pick up antibodies to a coronavirus infection from previous cold seasons. So, the “pre-COVID” sera might very well fall below the limit of detection (or whatever cut-off the study authors selected). The bottom line is that the kit is suited to detecting recent infections.
Yes, we have all had colds. But the ones caused by coronavirus are a small subset of those colds, and not everyone gets a cold every year. (I get one about every 5-10 years.)
Another piece of evidence that the test may be detecting recent infection with coronavirus, not specifically Covid-19, is the infection rate. To believe that the real number of Covid-19 infections is 85 fold greater than the actual case count is to believe that Covid-19 is orders of magnitude more infectious than any other respiratory virus. I am highly skeptical of such findings.
I think the Stanford paper has a high chance of being published because of the subject matter, but if I were one of its peer-reviewers, I’d send it back with recommendations for major revision and further study. Study, as in including controls who have recently had common colds caused by ordinary coronaviruses. I want to see the data on cross-reactivity of the test.