COVID Pandemic: Treatments, Vaccinations, and Herd Immunity

COVID-19 and the Path to Immunity

The emergence of adaptive immunity in response to the novel Betacoronavirus, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), occurs within the first 7 to 10 days of infection.1–3 Understanding the key features and evolution of B-cell– and T-cell–mediated adaptive immunity to SARS-CoV-2 is essential in forecasting coronavirus disease 2019 (COVID-19) outcomes and for developing effective strategies to control the pandemic. Ascertaining long-term B-cell and T-cell immunological memory against SARS-CoV-2 is also critical to understanding durable protection.

A robust memory B-cell and plasmablast expansion is detected early in infection,2,4 with secretion of serum IgM and IgA antibodies by day 5 to 7 and IgG by day 7 to 10 from the onset of symptoms. In general, serum IgM and IgA titers decline after approximately 28 days (Figure), and IgG titers peak at approximately 49 days. Simultaneously, SARS-CoV-2 activates T cells in the first week of infection, and virus-specific memory CD4+ cells and CD8+ T cells reportedly peak within 2 weeks but remain detectable at lower levels for 100 or more days of observation. Grifoni et al1 and others5,6 have identified SARS-CoV-2–specific memory CD4+ T cells in up to 100% and CD8+ T cells in approximately 70% of patients recovering from COVID-19. Although severe COVID-19 is characterized by high-viral titers, dysregulated innate inflammatory cytokine and chemokine responses and prolonged lymphopenia, antibody-dependent enhancement or dominant CD4+ TH2-type cytokines (eg, IL-4, IL-5, IL-13) do not appear to contribute to acute COVID-19 severity.