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A serological assay to detect SARS-CoV-2 seroconversion in humans

Authors: Fatima Amanat et al. Link to paper: https://www.medrxiv.org/content/10.1101/2020.03.17.20037713v1

Journal/ Pre-Print: Pre-print

Key Words: Elisa, Serology, SARS-Cov-2, Antibody testing, Isotyping, spike, RBD.

Research Highlights 

1. Development of an ELISA to detect anti-Sars-CoV-2 IgM, IgG and IgA.

2. Validation against 62 patient serum samples – 59 negative and 3 positive samples.

3. Two indirect ELISA methods, coating either with a recombinant conformational Spike protein or the RBD domain both expressed in mammalian cells (and also in insect cells).

Summary 

· The development of serological assays is an essential step in building knowledge of the epidemiology of Sars-CoV-2 infection. ELISA tests are needed as they are high-throughput, relatively fast and reliable.

· The paper reports the development of two formats of an ELISA test, either using the trimeric Spike protein or the RBD. They test IgM, IgA and IgG1, 3 and 4. The method is relatively standard, with appropriate use of controls.

· The evaluation with human serum samples suggest high sensitivity and specificity in 62 human serum samples.

· This study does not add anything new to our understanding of the SARS-Cov-2, but reinforces few findings proposed by other papers.

Impact for SARS-CoV2/COVID19 research efforts

 Develop diagnostic tools for SARS-CoV2/COVID19:

Study Type

· In vitro study

Strengths and limitations of the paper

Novelty: IgG3, IgG1, IgM and IgA bind the SARS-Cov-2 Spike protein. Samples can be heat inactivated to eliminate risk of infectious virus without losing ELISA sensitivity. Serum neutralization titre correlated with ELISA titre.

Standing in the field: Standard methods used, with good reporting of controls, consistent with results of other studies such as Okba et al showing the development of anti-spike antibodies in patients.

Appropriate statistics: No power calculation attempted. The sample size is relatively small. Unpaired and paired t tests used assuming normal distribution of data.

Viral model used: SARS-CoV2 Spike and RBD. Sera samples from patients infected with other viruses (dengue etc).

Translatability: Very translational, as this is a basic ELISA test. So long as the SPIKE preparations are standardised, this test may be ready for use.

Main limitations:

· Sample size: ELISA needs to be validated in a larger sample. The SARS-Cov-2 patient sample number is too low to conclude anything substantial, despite achieving statistical significance.

· The authors do not state why they chose to express the spike proteins in both mammalian and insect cell expression systems so that seems a bit redundant and lacking in purpose for this study.

· There is a control, that has some cross reaction with the mammalian and insect RBD but because the authors used a single colour to represent the controls, it is unclear what control serum it is and from which patient it is from.

· No power calculation attempted.

· Further evaluation is essential, involving a larger sample of positive and negative samples. The positive samples should include patients with EBV infection, and autoimmune diseases such as RA with RF positive.