COVID-19: Immune response

The immune response to SARS-CoV-2 involves both cell-mediated immunity and antibody production.

Cell-mediated immune response

T-cell responses against the SARS-CoV-2 spike protein have been characterised and correlate well with IgG and IgA antibody titres in COVID-19 patients. It is currently unknown whether antibody responses or T-cell responses in infected people confer protective immunity, and if so, how strong a response is needed for this to occur. 

CD8+ T cells are the main inflammatory cells and play a vital role in virus clearance. Total lymphocytes, CD4+ T cells, CD8+ T cells, B cells, and natural killer cells show a significant association with inflammatory status in COVID-19, especially CD8+ T cells and CD4+/CD8+ ratio. Decreased absolute numbers of T lymphocytes, CD4+ T cells, and CD8+ T cells were observed in both mild cases and severe cases, but accentuated in the severe cases. In multivariate analysis, post-treatment decrease in CD8+ T cells and B cells and increase in CD4+/CD8+ ratio were indicated as independent predictors of poor treatment outcome. The expression of IFN-γ by CD4+ T cells also tends to be lower in severe cases than in moderate cases.

Antibody-mediated immune response and protective immunity

The detection of antibodies to SARS-CoV-2 does not indicate directly protective immunity and correlates of protection for COVID-19 have not yet been established.

Most people infected with SARS-CoV-2 display an antibody response between day 10 and day 21 after infection. Detection in mild cases can take longer (four weeks or more) and, in a small number of cases, antibodies (IgM, IgG) are not detected at all (at least during the studies’ time scale). 

Based on the currently available data, the IgM and IgG antibodies to SARS-CoV-2 develop between 6–15 days post disease onset. A study in China showed a median seroconversion time for total antibodies, IgM and then IgG of day-11, day-12 and day-14 post symptom onset, respectively. The presence of antibodies was detected in <40% among patients within 1 week from onset, and rapidly increased to 100% (total antibodies), 94.3% (IgM) and 79.8% (IgG) from day-15 after onset. 

The longevity of the antibody response is still unknown.

Structure of the Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2).
Source: Roche 

References

• European Centre for Disease Prevention and Control. Coronavirus: Immune responses and immunity to SARS-CoV-2. Accessed 6 October 2020.
• Zhao J, et al. Antibody responses to SARS-CoV-2 in patients of novel coronavirus disease 2019. Clin Infect Dis. 2020 Mar 28:ciaa344. doi: 10.1093/cid/ciaa344. Epub ahead of print. PMID: 32221519; PMCID: PMC7184337.
• Wang F, Nie J, Wang H, Zhao Q, Xiong Y, Deng L, et al. Characteristics of Peripheral Lymphocyte Subset Alteration in COVID-19 Pneumonia. The Journal of Infectious Diseases. 2020.
• Chen G, Wu D, Guo W, Cao Y, Huang D, Wang H, et al. Clinical and immunological features of severe and moderate coronavirus disease 2019. The Journal of Clinical Investigation. 2020 04/13/;130(5).