Fig. 1: Changes in modes of testing across a generalized COVID-19 infectious course.

a | Shedding of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) RNA genome (yellow shading) typically increases rapidly following infection and peaks at the presentation of symptoms (although many patients can be asymptomatic) before gradually declining. Levels of viral proteins (red shading) also rapidly increase, albeit within a narrower window. In response to infection, the body produces IgM (green shading) and IgG (blue shading) antibodies, which persist for weeks to months. b | The SARS-CoV-2 RNA genome can be detected by molecular assays, such as reverse transcription–quantitative polymerase chain reaction (RT–qPCR), before the development of symptoms. Serological tests can detect reactive IgG and IgM, which indicate past infection with SARS-CoV-2. c | Assay sensitivity is dependent on both technical performance aspects and viral load. A test might not detect viral RNA even when an individual is infectious (false negative, red shaded area) or, alternatively, might detect persistent viral RNA after an individual is no longer infectious (false positive), which demonstrates that test positivity correlates poorly with infectivity. Owing to the rapid increase in viral shedding, only a narrow window exists wherein a more sensitive assay (test B) will outperform a less sensitive assay (test A). Note that this figure illustrates a generalized COVID-19 infectious course, and in practice the relative duration of detectability and analyte abundance differ considerably between individuals. NAAT, nucleic acid antigen testing; NGS, next-generation sequencing.