The incubation period for COVID-19 ranges from 2 - 14 days following exposure, with most cases showing symptoms approximately 4 - 5 days after exposure.2,6 The spectrum of symptomatic infection ranges from mild (fever, cough, fatigue, loss of smell and taste, shortness of breath) to critical.7,8 While most symptomatic cases are not severe, severe illness occurs predominantly in adults with advanced age or underlying medical comorbidities and requires intensive care. Acute respiratory distress syndrome (ARDS) is a major complication in patients with severe disease. Critical cases are characterized by e.g., respiratory failure, shock and/or multiple organ dysfunction, or failure.7,9
Upon infection with SARS-CoV-2, the host mounts an immune response against the virus, including production of specific antibodies against viral antigens. Both IgM and IgG have been detected as early as day 5 after symptom onset.10,11 Median seroconversion has been observed at day 10 - 13 for IgM and day 12 - 14 for IgG,12-14 while maximum levels have been reported at week 2 - 3 for IgM, week 3 - 6 for IgG and week 2 for total antibody.10-16 Whereas IgM seems to vanish around week 6 - 7,17,18 high IgG seropositivity is seen at that time.10,17,18 While IgM is typically the major antibody class secreted to blood in the early stages of a primary antibody response, levels and chronological order of IgM and IgG antibody appearance seem to be highly variable for SARS-CoV-2. Anti-SARS-CoV-2 IgM and IgG often appear simultaneously, and some cases have been reported where IgG appears before IgM, limiting its diagnostic utility.11,12,14,19,20
After infection or vaccination, the binding strength of antibodies to antigens increases over time - a process called affinity maturation.21 High-affinity antibodies can elicit neutralization by recognizing and binding specific viral epitopes.22,23 In SARS-CoV-2 infection, antibodies targeting both the spike and nucleocapsid proteins, which correlate with a strong neutralizing response, are formed as early as day 9 onwards, suggesting seroconversion may lead to protection for at least a limited time.19,24