Pre-existing T-cell reactivity from other viruses may make antibody tests (IgM and IgG) unreliable for determining population-level SARS-CoV-2 exposure.

“these people might not develop antibodies to SARS Kobe - which means that we don't have a great test for who has been exposed to SARS Kobe - we don't know how widely this fires have spread in the population so all of this discussion all of this argument around herd immunity and how many people have been exposed it's now based on antibody testing IgM and IgG and if those tests are not going to detect people who have pre-existing reactivity from other viruses we're just shooting in the dark you guys we're never gonna know this”