Antibody Testing. Why It’s Not a Guarantee of Immunity

About a 7 minute read

By: Jenny Blair, MD

Learn what antibody testing is and how it could help inform public health during the pandemic.

As COVID-19 cases around the world continue to climb and the economic fallout from weeks of lockdown restrictions intensifies, a growing number of people are understandably anxious to resume a more normal daily existence and get back to business as usual. And with that, the focus for many has shifted to antibody testing.

After all, a positive antibody test means you’re now immune to COVID-19, right?

Sorry, we just don’t know that yet.

A recent statement from the World Health Organization (WHO) attempted to clear up the misconception that a COVID-19 antibody test is the equivalent of a “get out of jail free” card, dealing a major blow to many people who were banking on this very idea.

“Some governments have suggested that the detection of antibodies to SARS-CoV-2, the virus that causes COVID-19, could serve as the basis for an ‘immunity passport’ or ‘risk-free certificate’ that would enable individuals to travel or to return to work assuming that they are protected against re-infection,” the WHO stated in an April 24 scientific brief.

Now, here’s the part that might sting: “As of 24 April 2020, no study has evaluated whether the presence of antibodies to SARS-CoV-2 confers immunity to subsequent infection by this virus in humans.”

In short, there is no evidence—at least not yet—that people who have recovered from COVID-19 and have antibodies to SARS-CoV-2 are immune to the coronavirus or won’t ever be re-infected.

But why?

To answer that question, it’s important to understand how antibody testing works and how it applies to the current pandemic.

What antibody testing is and how it’s helpful
When experts talk about testing for SARS-CoV-2, they’re really referring to two different types: diagnostic tests and antibody tests.

A diagnostic test checks for the coronavirus itself, using saliva or a respiratory sample collected by nasal swab. The results are intended to reveal whether or not you’re currently infected.

Antibody tests, on the other hand, help determine if you’ve been infected in the past—not if you’re sick right now.

Rather than testing respiratory specimens collected by swabs, antibody tests (also called serology tests) check your blood for antibodies to SARS-CoV-2, which would indicate a past encounter with the virus.

Antibodies are proteins that attach to a foreign invader, such as a virus—similar to the way a key fits into a lock. Once the antibody recognises the virus and snaps into place, it destroys the virus or marks it for destruction by other parts of the immune system. Neutralising antibodies, in particular, attach to viruses and essentially block the infection.

In general, large-scale antibody testing helps epidemiologists estimate the size and spread of a pandemic, as well as zero in on groups who are at higher risk.

For the current pandemic, we need to know who’s already been exposed to the novel coronavirus and recovered, including silent spreaders—those who may not have been tested or may not have even shown symptoms while they were infected. This information will likely play an important role in getting people, especially health care workers, back to work.

People with antibodies in their blood may also be able to donate their plasma for the treatment of people who are sick with COVID-19—a means of lending another person some antibodies.

Why antibody testing isn’t a guarantee
At this stage of the pandemic, one promise of antibody tests—that people can find out whether they’re immune—may be greater than what they can actually deliver.

Although this testing is an important step in controlling the COVID-19 pandemic, it’s not a guarantee of immunity and there are some other challenges scientists and health officials must overcome, including:

Not all antibody tests are created equal.
From 22nd April 2020 only approved point of care test kits for COVID-19 will be able to be imported and sold in New Zealand. Medsafe has banned the importation and sale of all point of care COVID-19 test kits, unless they gain approval. No point of care test for COVID-19 has so far been approved.

Medsafe acknowledges that internationally there has been a lot of work done in developing point of care testing, but believes that more development is required to produce a reliable point of care test. In particular, Medsafe has concerns about both the quality of testing from many of these kits and the likely impact of misinterpreted results.

Antibody detection tests could deliver false-positive results, or incorrectly show that a person has antibodies against the coronavirus that causes COVID-19. An antibody test could also falsely report COVID-19 antibodies when it’s actually detecting those made against other coronaviruses. (There are many other coronaviruses, in fact, most cause mild illnesses and you’ve probably already had one).

These are potentially dangerous scenarios because they could encourage a vulnerable person who believes they are immune to do things that put them at risk for infection.

A May 3 announcement by Roche Diagnostics offers a glimmer of hope. The pharmaceutical company said that its coronavirus antibody test was granted emergency authorisation by the FDA. Roche plans to ramp up its production capacity to high double-digit millions each month. The company also notes that its antibody test is highly accurate, with specificity greater than 99.8 percent and sensitivity of 100 percent. This means it correctly generates negative results for people who don’t have COVID-19 and generates positive results for those that are infected.

Antibodies to SARS-CoV-2 don’t guarantee immunity.
We still don’t know whether antibodies equal immunity. The WHO issued a tweet clarifying its April 24 statement, explaining that most people who are infected will develop antibodies that will provide some amount of protection.

What remains unclear is how much protection these antibodies will provide and for how long.

“If you get measles, you’re immune for your whole life. It’s very robust immunity. But if you get a vaccine, or you get whooping cough, your immunity wanes after 6 or 7 years,” says Dr Christopher Ohl, professor of infectious diseases. “If you do get immune [to the novel coronavirus], does your immunity last one week? Does it last one month? One year? One decade? We don’t know. So, a positive antibody test is not a certificate of ‘Hey, you’re free and clear’—at least, it isn’t right now.”

An April report from the Johns Hopkins Center for Health Security states that antibody tests first need to be made widely available to public health authorities to conduct surveillance and to estimate how many people have already been infected. The report concluded that these tests shouldn’t go into widespread use among the general population until there is a highly accurate, reliable test and there is more clarity on what level of immune protection, if any, SARS-CoV-2 antibodies provide.

Dr. Ohl believes that it could be up to a year before scientists know how much immunity antibodies confer and how long that protection lasts.

The body develops antibodies over time.
“It takes a couple of weeks for those antibodies to form enough so that you can reliably measure them,” Ohl explains. “If you were just exposed yesterday, it’s not going to tell you anything. It hasn’t developed yet.”

Why you should still get tested (in a study)
“For any one individual person, right now, antibody testing is not going to help you much. It’s not going to tell you for sure if you’re immune,” Ohl says. But your test results could help epidemiologists—the scientists working to better understand the COVID-19 pandemic, he points out.

Researchers working with large numbers of test results can account for known testing weaknesses by applying statistical corrections. Ohl adds that taking part in research involving antibody testing could help inform public health.

Medically reviewed in May 2020.

Sources:
World Health Organization. “‘Immunity passports’ in the context of COVID-19.”
The Edmond J. Safra Center for Ethics at Harvard University. “Roadmap To Pandemic Resilience.”
Mayo Clinic. “COVID-19 saliva tests: What is the benefit?”
U.S. Food and Drug Administration. “FAQs on Testing for SARS-CoV-2.”
SusanPayne. “Viruses From Understanding to Investigation.” Chapter 6 - Immunity and Resistance to Viruses. Academic Press. 2017, Pages 61-71.
U.S. Food and Drug Administration. “Coronavirus (COVID-19) Update: Serological Test Validation and Education Efforts.”
U.S. Food and Drug Administration. “Policy for Coronavirus Disease-2019 Tests During the Public Health Emergency (Revised).”
U.S. Food and Drug Administration. “Insight into FDA’s Revised Policy on Antibody Tests: Prioritizing Access and Accuracy.”
World Health Organization. “Advice on the use of point-of-care immunodiagnostic tests for COVID-19.”
Roche. “Roche’s COVID-19 antibody test receives FDA Emergency Use Authorization and is available in markets accepting the CE mark.”
Ministry of Health New Zealand. (2020). Covid 19 point care test kit. Retrieved from https://www.health.govt.nz/news-media/media-releases/covid-19-point-care-test-kits



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