Easy-to-Use Paper Test Predicts COVID-19 Immunity

Researchers at Massachusetts Institute of Technology (MIT, Cambridge, MA, USA) have developed the test which uses the same type of “lateral flow” technology as most rapid antigen tests for COVID-19 to measure the level of neutralizing antibodies that target the SARS-CoV-2 virus in a blood sample. The test is designed so that different viral spike proteins can be swapped in, allowing it to be modified to detect immunity against any existing or future variant of SARS-CoV-2. Easy access to this kind of test could help people determine what kind of precautions they should take against COVID infection, such as getting an additional booster shot, according to the researchers.

Currently, the gold standard approach to measuring immunity involves mixing a blood sample with live virus and measuring how many cells in the sample are killed by the virus. That procedure is too hazardous to perform in most labs, so the more commonly used approaches involve noninfectious modified “pseudoviral” particles, or they are based on a test called ELISA (enzyme-linked immunosorbent assay), which can detect antibodies that neutralize a fragment of a viral protein. However, these approaches still require trained personnel working in a lab with specialized equipment, so they aren’t practical for use in a doctor’s office to get immediate results.

The MIT researchers wanted to come up with something that could be easily used by a health care provider or even by people at home. The team drew inspiration from at-home pregnancy tests, which are based on a type of test called a lateral flow assay. Lateral flow assays generally consist of paper strips embedded with test lines that bind to a particular target molecule if it is present in a sample. This technology is also the basis of most at-home rapid tests for COVID-19. The researchers then went on to develop a device that can detect the presence of antibodies which block the SARS-CoV-2 receptor binding domain (RBD) from binding to ACE2, the human receptor that the virus uses to infect cells.

The first step in the test is to mix human blood samples with viral RBD protein that has been labeled with tiny gold particles that can be visualized when bound to a paper strip. After allowing time for antibodies in the sample to interact with the viral protein, a few drops of the sample are placed on a test strip embedded with two test lines. One of these lines attracts free viral RBD proteins, while the other attracts any RBD that has been captured by neutralizing antibodies. A strong signal from the second line indicates a high level of neutralizing antibodies in the sample. There is also a control line that detects free gold particles, confirming that the solution flowed across the entire strip.

Along with a testing cartridge, which contains the paper test strip, the testing kit also includes a finger prick lancet that can be used to obtain a small blood sample, less than 10 microliters. This sample is then mixed with the reagents needed for the test. After about 10 minutes, the sample is exposed to the test cartridge, and the results are revealed in 10 minutes. The output can be read two different ways: One, by simply looking at the lines, which indicate whether neutralizing antibodies are present or not. Or, the device can be used to obtain a more precise measurement of antibody levels, using a smartphone app that can measure the intensity of each line and calculate the ratio of neutralized RBD protein to infectious RBD protein. When this ratio is low, it might suggest that another booster shot is needed, or that the individual should take extra precautions to prevent infection.

The researchers tested their device with blood samples collected in December 2020 from about 60 people who had been infected with SARS-CoV-2 and 30 people who had not. They were able to detect neutralizing antibodies in the samples from people previously infected to the virus, with accuracy similar to that of existing laboratory tests. They also tested 30 serial samples from two people before they received an mRNA Covid-19 vaccine and at several time points after vaccination. The level of neutralizing antibodies in the vaccinated individuals peaked around seven weeks after the first dose, then began to slowly decline. The test could be easily adapted to different variants of SARS-CoV-2 by swapping in a reagent that is specific to the RBD from the variant of interest. The researchers have filed for a patent on the technology and are now hoping to partner with a diagnostic company that could manufacture large quantities of the tests and obtain FDA approval for their use.

“Among the general population, many people probably want to know how well protected they are,” said Hojun Li, the Charles W. and Jennifer C. Johnson Clinical Investigator at MIT’s Koch Institute for Integrative Cancer Research. “But I think where this test might make the biggest difference is for anybody who is receiving chemotherapy, anybody who’s on immunosuppressive drugs for rheumatologic disorders or autoimmune diseases, and for anybody who’s elderly or doesn't mount good immune responses in general. These are all people who might need to be boosted sooner or receive more doses to achieve adequate protection.”

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