COVID-19 Testing

How Long Does It Take for COVID-19 Patients to Develop Antibodies?

BACKGROUND AND PURPOSE:

Data regarding time course to develop antibody response to SARS-CoV-2 is limited
Zhao et al. (Clinical Infectious Diseases, 2020) investigated the dynamics of total antibody (Ab), IgM and IgG antibodies against SARS-CoV-2 in a series of blood samples taken from patients with confirmed COVID-19

Cohort study
Setting
- Shenzhen Third People’s Hospital, China
Participants
- Acute respiratory infection syndromes and/or abnormalities in chest CT images accompanied by detectable SARS-CoV-2 RNA in respiratory sample since illness onset (COVID-19 Diagnosis Based on National Health Commission of China Protocol)
- All in critical condition requiring ventilation (invasive or non-invasive)
Study design
- Enzyme linked immunosorbent assays (ELISA) were performed on successive plasma samples to determine
  - Total antibodies (Ab) against SARS-CoV-2 (specificity 99.1%)
  - IgM antibody (specificity: 98.6%)
  - IgG antibody (specificity: 99.0%)
- Antibody dynamics analyzed during disease progression

173 patients | 535 plasma samples
Seroconversion rate
- Ab: 93.1%
- IgM: 82.7%
- IgG: 64.7%
Negative antibody findings: 12 patients
- These patients had samples taken earlier in their disease course (10 <day 10; 2 on day 11 and 13 after onset)
Median seroconversion time
- Ab: day 11
- IgM: day 12
- IgG: day 14
<40% of patients had antibodies within 1 week of disease onset
>15 days after onset, antibody testing was more sensitive, detecting a greater percentage of patients with COVID-19
- Ab: 100%
- IgM: 94.3%
- IgG: 79.8%
Combining RNA and antibody detection significantly improved sensitivity (p<0.001)
- This was true even in the early stages of disease (<1 week since onset; p=0.007)
A higher titer of Ab was independently associated with a worse clinical classification (p=0.006)

Antibodies against SARS-CoV-2 were present in 100% of COVID-19 patients after 15 days since disease onset
- Antibodies were only detected in 40% of patients within 1 week of disease onset
The authors highlight important points related to antibodies, including
- This study could not evaluate the persistence of antibodies because samples were collected during the acute phase
- Cross-reactivity of the assay to other coronaviruses needs to be further assessed
The authors suggest that combining serological antibody tests with the PCR-based tests for viral RNA could improve the accuracy of disease diagnosis

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