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A health worker shows a negative result for the novel coronavirus (COVID-19) during fast tests perform to people in cars at the parking lot of the Mane Garrincha Stadium on April, 21, 2020 in Brasilia, Brazil.

Andressa Anholete/Getty Images

Imagine, in the not-too-distant future that, before you attend any public event, cross a provincial boundary or board a plane, you have to show your “COVID-19 passport.”

A not-quite-postpandemic world where, before you can return to work, you need an “immunity certificate” that shows you have developed immunity to the coronavirus. And, if do have immunity, then you may be reassigned from your quiet desk job to a role in which you interact with the public.

These are the type of scenarios being envisaged for easing out of the world of self-isolation and physical distancing and getting people back to work and into stores again.

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In other words, the answer to the “how does this end?” questions almost always begins with antibody testing.

Unfortunately, as essential as it will be, antibody testing is not quite ready for prime time.

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Right now we are still in the early stages of the pandemic, trying to figure out who has COVID-19, treating them and doing contact tracing to isolate those who are potentially infected.

We are still focusing on diagnostic testing, tests that are done in a lab with a method known as nucleic acid amplification.

While diagnostic testing has ramped up considerably in recent weeks, we still don’t really know how many people have been infected. We’ve focused on travellers, residents of nursing homes, people who have fallen ill and their close contacts, not the general public.

The confirmed cases – more than 38,000 cases in Canada – are considerable, but just the tip of the iceberg.

We know, based on how viruses spread, that many more people have likely been infected but not sick enough to seek medical attention, or be tested.

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What we don’t know is how much of the iceberg is hidden – and that’s a key data point for determining how quickly and broadly society can reopen.

But some clues are beginning to emerge. According to the World Health Organization, between 2 per cent and 3 per cent of the population seems to have been infected, and as much as 10 per cent to 15 per cent in some hard-hit communities.

Paradoxically, public-health officials were hoping those numbers would be much higher, because higher infection rates would suggest more people are immune, and that the true COVID-19 mortality rate is lower.

We won’t know if that is the case until we’ve done a fair bit of serosurveillance – random testing in the community.

There are two major impediments to that step: the spotty quality of the tests and the lack of availability.

The United States was broadly criticized for being too slow and finicky in its approval of diagnostic tests, and the lack of testing allowed coronavirus to spread like wildfire. Now, it has gone to the other extreme, rubber-stamping dozens of antibody tests with very little evidence they are accurate.

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Health Canada, for its part, is fast-tracking reviews but has yet to approve any commercial tests.

Developing an effective test is not as easy as one would assume. A person generally doesn’t develop antibodies until about 12 days after symptoms begin.

You have to determine which antibodies to measure: transient ones that spike during active infection, or others that only peak weeks after infection.

The sensitivity and specificity of the tests matter. A particular challenge with this virus, SARS-CoV-2, is that it resembles other widely circulating coronaviruses that cause colds.

Once you have a decent test, the more difficult part of the equation begins: figuring out if antibodies confer immunity.

Generally, you develop immunity after a viral infection. But there are some troubling indications that the coronavirus can be reactivated or that people who have been infected can be reinfected. (Then again, those anomalies may be due to inaccurate tests.)

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Just as importantly, we don’t know if immunity will be partial or total, or how long it will last – and that’s impossible to determine with a virus that’s only been circulating broadly for between four and five months.

If we ease public-health restrictions too quickly, we risk seeing a resurgence of COVID-19 cases. This happened recently in Singapore, a country often cited as having among the best pandemic responses.

Everybody wants a “get out of isolation free” card. So, everyone is eager for antibody testing to begin.

But, with so much at stake, it’s more important to do it right than to do it quickly.

Sign up for the Coronavirus Update newsletter to read the day’s essential coronavirus news, features and explainers written by Globe reporters and editors.

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