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While the main role of SARS-CoV-2 antibody testing is currently limited to determining exposure to the virus in populations, the scientific community is working hard to define the neutralizing properties of the antibodies detected, and the duration of the immune response. As a result, antibody testing could be here to stay for the long term for monitoring populations, testing vaccines and selecting patients for trials of a variety of new therapies to treat numerous conditions.

Testing for antibodies to the SARS-CoV-2 virus has been the subject of great interest and debate among scientific and laypeople alike over recent months. In the initial scramble to quell the impact of the COVID-19 pandemic, the antibody test market was flooded as unvalidated, potentially inaccurate antibody tests were developed and some manufacturers took advantage of relaxed, emergency regulations. People self-tested using unproven finger-stick blood tests and there was widespread – and unproven – belief that testing positive for antibodies signified future immunity to the disease.

After concerns that some manufacturers were inappropriately marketing antibody tests for diagnostic use, including home use, the US Food and Drug Administration strengthened its guidance on antibody testing in early May. (Also see "COVID-19 Antibody Tests Being Abused To Falsely Diagnose Coronavirus Cases, FDA Says" - Medtech Insight, 4 May, 2020.)

The new guidance required manufacturers to provide sensitivity and specificity data before the FDA would grant emergency use authorization (EUA) for the test. As a result, the agency subsequently pulled 28 antibody tests from the market that made false claims or did not meet its sensitivity and specificity criteria. (Also see "FDA Yanks Potentially Faulty COVID-19 Antibody Tests – And More May Be On The Chopping Block" - Medtech Insight, 22 May, 2020.)

Full Utility Of Antibody Testing Still Unclear

Despite the huge publicity for antibody testing in the mainstream press, its full utility has yet to be determined. Recurrence of COVID-19 in patients previously infected appears to be very uncommon, suggesting that the presence of antibodies could confer at least short-term immunity to infection with SARS-CoV-2. In addition, antibody development correlates with a marked decrease in viral load in the respiratory tract. These factors suggest that the presence of antibodies may decrease a person’s infectiousness and offer some level of protection from reinfection. However, definitive data are lacking, and it remains uncertain whether all infected people produce neutralizing antibodies and if so, what concentration of antibodies is needed to confer protection against reinfection with SARS-CoV-2.

Currently, the main focus for antibody testing in the medical and scientific communities is for seroprevalence and epidemiology studies, rather than for assessing immunity from past infection. Antibody testing can estimate the proportion of people suffering from COVID-19 or have been infected in the past, or even give clues on whether the proportion high enough for herd immunity to protect against a second wave. A number of groups around the world are working on this type of study. For example, antibody testing is used in the UK’s COVID-19 Infection Survey.

As Laura Parnas, senior scientific affairs manager at Roche Diagnostics Corp., which supplies the Elecsys Anti-SARS-CoV-2 antibody test, explained to Medtech Insight: “The role of antibody testing right now is to determine exposure [to SARS-CoV-2] in the population. The scientific community is working hard to have evidence that supports a definitive understanding about the neutralizing properties of the antibodies detected, and the duration of the immune response. Until this is fully understood and supported by clinical and scientific evidence, the use of antibody testing is limited to assessment of exposure to the virus.”

Deepath Nath, president of laboratory diagnostics at Siemens Healthineers AG agrees. He told Medtech Insight: “In terms of the science today, the link between immune response [to SARS-CoV-2] and immunity has not been established. There’s a lot of work underway to establish that link but right now, we are very careful to point out that detection of an immune response, or antibody response, does not equal immunity and it’s important that the public be educated, and that point be reinforced. That said, there’s a lot of work underway to understand that link and whether there is some level of immunity conferred by the presence of antibodies and if so, how long that immunity is likely to last (weeks, months, years or decades).”

Current Centers for Disease Control recommendations for serological testing say that although the presence of SARS-CoV-2 antibodies when detected using a testing algorithm with a high positive predictive value (PPV) – the key measure of diagnostic accuracy of a test – for the context of use likely indicates at some degree of immunity, until the durability and duration of immunity is established, it cannot be assumed that individuals with truly positive antibody test results are protected from future infection. It adds that asymptomatic people who test positive by antibody testing and who are without recent history of a COVID-19 compatible illness have a low likelihood of active infection.


Improving Early Diagnosis, Identifying Infection Stage And Vaccine Development

While the main focus for antibody testing currently is in determining exposure to the virus, Patrice Hugo, chief scientific officer at laboratory services company Q2 Solutions, said in a recent webinar that antibody testing can also be used for improving early diagnosis of COVID-19, because of the high false-negative rate of PCR, and identifying early versus late phase of infection.

For example, said Hugo, one strategy is to use antibody testing after PCR testing. PCR testing is used to detect viral genomic RNA in a sample swab of mucous. A positive result may be from active live infection, but there could also be remnants of viral RNA in patients who may not be still infectious, and swab samples are prone to handling errors and contamination. As a result, PCR can have a relatively high false-positive and false-negative rate for live infection. Were a PCR test to be followed with a test for IgM antibodies (even if the sensitivity of the PCR test was just 50%), 99% of people suffering from the disease would be identified. Another example is to use a rapid, first line antibody test (eg, a lateral flow test) and if the result is positive, perform a laboratory antibody test.

Testing can also be used to monitor antibody response post infection in patients under treatment with antiviral or anti-inflammatory drugs, Hugo added. There are currently over 1,600 clinical trials ongoing in this area, he said, and in all cases, the presence of IgM or IgG must be qualified to include the patient for recruitment in the trial.

Antibody tests will also prove particularly valuable for vaccine development and once an effective vaccine has been developed as they can be used to measure immunogenicity (antibody immune response) in vaccine development and post vaccination. In vaccine trials, antibody tests can be used to exclude patients from the trial who have been exposed to the virus.

Q2 Solutions, a joint venture between IQVIA and
Quest Diagnostics Inc., is developing an assay that can detect neutralizing antibodies, said Hugo. It uses a virus reporter gene system for use in a lower risk BSL2 lab environment. The company hopes to have this capability by the third quarter of 2020 for use in vaccine clinical trials.

When a vaccine is available, "you need to reach about 60% of population having neutralizing antibodies to reach herd immunity," said Hugo. This will require a sound testing algorithm for PPV, using antibody testing in combination with PCR, for example. "You need a test that can determine if antibodies are neutralizing or not," said Hugo. "Then you need to know whether the half-life of these neutralizing antibodies is long enough to survive a second wave of COVID-19."

Also, in trials for non-COVID-19 treatments, such as treatments for cardiovascular diseases or diabetes, researchers may want to exclude patients from getting into trials or document the presence of antibodies, as past COVID infection may have caused damage from the cytokine storm/inflammation, risking a serious adverse event in the trial.

Antibody Testing Market Developing Rapidly

As a result of the emerging opportunities for antibody testing, many laboratory diagnostics companies have been encouraged to develop assays for their installed base of analyzers in anticipation of a large uptick in the volume of testing. Coupled with this is the development of rapid, point of care tests (POC) based on easy-to-obtain samples, such as finger-stick blood and even saliva.

The main advantages of antibody tests on automated laboratory immunoanalyzers are high throughput, reliability of results and low sample cost. However, this approach relies on expensive instrumentation, highly qualified laboratory staff and turnaround times for results of around 24 hours. POC tests offer rapid turnaround of results and can be performed by hospital labs but have a higher cost-per-test and lower throughput than laboratory instruments. Lateral flow devices, or cassettes, offer an even faster turnaround time (around 15 minutes), require no instrumentation and can be used by healthcare professionals. However, results are less accurate, less reproducible and are subject to reading errors.

All the major diagnostics firms have launched laboratory tests for detecting antibodies to SARS-CoV-2, with Siemens Healthineers the latest to throw its hat in the ring with its COV2T test. (Also see "Siemens Healthineers Adds Muscle To Global Antibody Testing Effort, In Talks With Vaccine Developers" - Medtech Insight, 1 Jun, 2020.)

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