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What is a serological test?

A serological test is a blood test. This test detects the presence of virus-specific antibodies in blood samples so that previous exposure to the COVID-19 virus can be detected.

Why are serologic tests important?

You can think of it as useful for policy decisions and individual decisions.

At a policy level, we need to understand how many people have been exposed to the virus and might be expected to have some level of immunity to it. We know that number is a lot higher than the number of people with obvious symptoms who were tested and confirmed to have COVID-19 disease. If that number were high, it would have given us more latitude to start easing off some of the physical distancing measures that have heavily constrained our social lives and our economy.

At an individual level, we all have an interest in knowing whether we are sufficiently immune to the novel coronavirus SARS-CoV-2 that is unlikely to make us seriously ill if we are exposed to it. And that is particularly important for those in key public-facing jobs, such as healthcare workers, police officers, fire-fighters, and other emergency workers. But it also goes for so many others — such as teachers, because children and youth can be infected with the virus but generally show very few symptoms.

How do you test for immunity to novel coronavirus SARS-CoV-2?

We can conduct tests using blood samples (and now saliva tests are being rolled out) to assess whether or not people have measurable levels of antibodies to the novel coronavirus SARS-CoV-2. These test results may give us a rough overall estimate of who has been exposed to the virus and may have built some innate defenses against re-infection leading to onward transmission of COVID-19.

How does that differ from most of the ongoing testing to diagnose COVID-19?

Tests for active COVID-19 focus on identifying the presence of the SARS-CoV-2, the novel coronavirus. The virus can be identified in swabs of the nose and/or throat, or in stool samples, but does not often appear in the bloodstream. SARS-CoV-2 is a single-stranded RNA virus. The definitive test involves looking for a match between RNA in the test sample and the known RNA structure of the virus. This is time-consuming, but faster methods are now coming into use.

In contrast, the immunity tests to date focus on measuring the presence in the blood of antibodies to the novel coronavirus SARS-CoV-2 produced by someone who is infected. A person who is infected may or may not have full-blown COVID-19 symptoms. A huge advantage of measuring antibodies is that this test can identify people who showed very mild or even no symptoms but were never tested. Antibody testing can also give us a look backwards – extending many weeks if not months – at people who had enough exposure to mount an immune response to the virus, but would not be positive if tested for active infection now.

Both tests can produce negative results early in the infection, before the virus is being actively shed and appearing in the nose and throat of a patient, and before the body’s immune system mounts an immune response. But relying on antibody tests for diagnosis of active disease is dangerous, because it usually takes several days for a strong antibody response to occur. Most patients have a measurable antibody response within a week or so, but that is not always the case.

Beyond simple screening, what other information do you need for policy?

First, we need to know if different regions or groups vary. Is there more background immunity in big cities that have dense social networks than in, say, rural and remote communities? We have had many terrible outbreaks in nursing homes and long-term care facilities. How many of the residents and staff show signs of immunity now?

More generally, older persons and persons of all ages with chronic illnesses appear to be more vulnerable to develop severe COVID-19 illness after exposure to novel coronavirus SARS-CoV-2. What proportions of those groups show immunity? Similar questions can be asked about Indigenous communities, as well as children and youth when determining when and how to restart educational programming. Getting reliable first estimates of background immunity in different groups will help us navigate the next phase of this epidemic.

You have drawn a distinction between policy decisions and individual decisions. Are there different considerations for individual decisions?

Yes. The basic problem is that viral immunity is complicated. To a given person, a positive quick test using one of the kits on the market conveys this message: “Yes, it looks like you have been exposed to the novel coronavirus SARS-CoV-2 because you have antibodies in your blood that seem specific to that virus.” However, we do not know the level of those antibodies, how long they will remain elevated, or how closely the level of antibodies correlates with protection against a serious infection in the future.

In fact, the body’s protection depends not just on antibodies but on specialized cells that are essential to viral immunity. Testing this cellular immunity is complicated, but it is important if we really want to make sense of immunity at an individual level. This is why the Task Force supports three types of studies. It does fast cross-sectional studies of different populations, regions and communities; these studies will help with policy decisions. It follows groups of people with and without past novel coronavirus infection over time, repeating more detailed blood tests to understand how their immunity is evolving. And it also supports some laboratory research to figure out the best and most efficient tests that will help us make decisions about policy and as individuals.

This sounds as if we cannot just issue a ‘perpetual and perfect immunity passport’ based on a quick blood test for antibodies against the novel coronavirus SARS-CoV-2. Is that correct?

Correct, based on our limited knowledge to date. We can confirm when someone has been exposed and has some immunity. That is valuable information. But we cannot currently provide an iron-clad guarantee that they will not ever get COVID-19 disease. That is a key reason why the Task Force will be supporting more research into how immunity develops, how complete it is, and how long it lasts. We will also be looking to see if other tests can give us a clearer picture and more certainty about individual results.

For now, however, it is important for information to be generated that will help Canadians understand what their individual immunity tests mean. The Task Force will be analyzing these issues carefully. For example, we know that the risk of severe COVID-19 varies according to several well-proven factors, including age, obesity, diabetes, and other medical conditions. Someone with, say, a positive screening test for antibodies who is 40 years old and entirely healthy, may justifiably have a very different view of their risk than someone else with the same test result who is 70 and has multiple health problems. Our work-plan includes providing timely and relevant information to public health officials and clinicians who will be advising the general public and individuals on the interpretation of results of immunity tests.