Ultrasensitive high-resolution profiling of early seroconversion in patients with COVID-19

In our recent Nature Biomedical Engineering paper we describe the development of an ultrasensitive serological assay for SARS-CoV-2.

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The COVID-19 pandemic has devasted communities around the globe and placed tremendous burdens on governments and medical systems. However, it has also been an international call to arms for the scientific and medical communities. The collaborative nature of the response has generated impactful results with unprecedented speed. One of the earliest challenges of the pandemic response was establishing accurate molecular and serological testing for infection.

Serological assays, specifically, have a crucial role to play in this crisis for several reasons. By identifying asymptomatic infections, past and present, these assays provide critical information about community prevalence.  Serological assays are essential for stopping the spread and in providing guidance about when it is safe to reopen communities.  Furthermore, serological assays can help answer the all-important question of how long immunity persists in response to infection or vaccination and how this timeframe is affected by age, comorbidities, and immune status.  Finally, they are a sin qua non for running accurate, highly powered vaccination trials.

In the early phase of the pandemic, serological assays with poor sensitivity and specificity flooded the international markets leading to false positives, overinflated estimates of population exposure, and uncertainty about proximity to herd immunity. In addition, these Enzyme Linked Immunosorbent Assays (ELISAs) were unable to quantitate small differences in levels and types of antibodies, which are critical for monitoring the immune response of hospitalized patients.

Our interdisciplinary team of biologists, engineers and physicians set out to aid testing efforts by using Single Molecule Array technology (Simoa), invented in our laboratory. Taking advantage of our unique ability to rapidly immuno-isolate, label, and quantify single protein molecules in solution, we developed a SARS-CoV-2 serology assay that simultaneously characterizes the interaction of three immunoglobulin isotypes to four viral epitopes. By testing COVID-19 patients as well as both healthy and sick pre-pandemic controls, we selected informative markers and developed a model to predict seroconversion with 99% sensitivity and specificity as soon as five days after infection.  A key challenge was ensuring that we were not unwittingly detecting other Coronaviridea such as those that cause the common cold. We overcame this challenge by developing additional assays for the four common strains of Coronavirus and demonstrating that antibodies to these other strains were not contributing to the signal in our SARS-CoV-2 assay.

The assay described in our recent publication in Nature Biomedical Engineering has the best analytical sensitivity and is among the most accurate tests developed to date for SARS-CoV-2 seroconversion. Because of the analytical sensitivity, the assay also has the unique ability to identify both very early and very late stages of the immune response, when antibody concentrations are low. This sensitivity is crucial for vaccine development, as it is important to rule out individuals who were previously infected and monitor small fluctuations in their antibody levels during the course of the trial. Using the HD-X platform we are making our assay widely available for studies of convalescent plasma transfer, vaccines and prophylactic medications.  We hope our test will be one more tool in the global armamentarium that may help save lives in this time of crisis.

Maia Norman

MD/PhD student, Brigham and Women's Hospital

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