I am re-linking to the review article of studies on the cycle time and culture positivity, as several people have asked about the articles and charts on this topic. I believe this review has most of the relevant material cited in the references. (Medrxiv Paper)
From the estimable researchers at the Center for Evidence Based Medicine comes a blog post on the testing topic. (CEBM Post) It is worth a close read to examine how various public health agencies define a CV-19 “case”. The obvious tension is between someone who is “infected”, meaning they had detectable virus, and someone who has the “disease”, meaning they have some signs or symptoms of bodily dysfunction due to the virus. A PCR test can tell you the former (although not necessarily that a person is infectious) but not the latter. The latter takes some clinical judgment. It seems to me that for the individual, having clinical manifestations of the disease is what is important, because that is what would guide any treatment decisions. From a public health perspective, what matters is: is a person infectious? So any testing, or designation of a case as a case, should be based on these considerations.
This article discusses the development of adaptive immunity from a CV-19 infection. (JAMA Article) Adaptive immunity begins to develop about 7 to 10 days after infection. As we have mentioned before, there are two primary arms to adaptive immunity, B cells and associated antibodies, and T cells. The article notes the variable antibody and T cell response across individuals, but that most develop some adaptive immunity. The authors dispel concerns about reductions in antibody levels in the weeks or months after infection, noting that this is common and not suggestive of a failure to develop long-lived memory B cells that can detect and react to reinfection. T cells are discussed as a potentially more significant component of response to coronaviruses and the role of cross-reactive T cells is covered. There is a great chart displaying the likely time line of adaptive immune response development.
A similar study in Cell, with the great title “COVID-19 makes B cells forget, but T cells remember”. (Cell Article) The researchers also review what we know about the immune response to CV-19, citing two recent studies in the journal. In one, patients with severe CV-19 disease were found to have a minimal antibody response, but an adequate T cell one. In the second study, patients with a range of disease, from asymptomatic to severe, were found to have a robust T cell response, even if they had limited antibody development. Among other things, this has implications for only using antibodies for surveys of population prevalence.