What we know and don't know about immunity to COVID-19

Medical researchers are gaining cluesaboutSARS-CoV-2, the virus that causes COVID-19,but there are big gaps in understanding our immune response to it how well, and for how long, we canbecome protected from being infected in the future.

That understanding is crucial in the push to loosen lockdown restrictions that will allow people to go back to their workplacesand gather in public.

Some clues come from what we know about other viral infections, and from ongoing studies tracking those who've had COVID-19 and recovered.

What is an infection?

Infection refers to the growth of microbes in the human host.

It's not the same as disease because an infection doesn't always lead to harm.

What's known about infection in COVID-19?

The incubation periodbetween infection and appearance of symptomsseems to be about five or six days. It can vary from one to 14 days.

Symptoms are mild about 80 per cent of time, the World Health Organization says.

Those who are hospitalized are admitted about a week after symptoms start.

And immunity?

Immunity is the ability to resist infection.

It can occur naturally when someone isinfected and their body mountsa successful immune response against it. To do so, the immune system creates both general antibodies and specific protection against the particular infection and retains a sort of memory to resist it coming back.

Or we can gain immunity by vaccination with inactive or weakened forms of the virus to stimulate an immune response.

What don't we know aboutimmunity toCOVID-19?

Once the antibodies have fully cleared the virus from someone's system, the personrecovers.

"In most cases you would expect those antibodies to provide you with protection for a period of time," the World Health Organization's head of emergencies, Dr. Michael Ryan,said last week. But we don't yet know the extent of protection for someone who has successfully fought off this newcoronavirus.

Immunologists say that in people who have symptoms, antibodies start to appear after about a week and then peak a week or two later.

Even less is known about people who test positive forSARS-CoV-2 but never show symptoms.

Diagnostic nose or throatswabs used at COVID-19 assessment centresand hospitals look for RNA, or genetic material, from the pandemic virus because it is faster than trying to grow the virus in a lab the old-fashioned way.

But it's only by growing the virus that scientists can tell if it's viable or capable of causing disease.

Dr. Mark Loeb chairs the infectious diseases division at McMaster University, where he studies viral infections such as influenza (flu) and West Nile. Loeb said scientists and physicians need to find more specific ways of measuringimmunity toCOVID-19.

"The technical term for this is 'correlates of protection,'" Loeb said. "What sort of level of antibody do you need to be protected? How long does it last? Those are all important research questions."

Why do scientists want to track immunity in populations?

Mona Nemer, Canada's chief science adviser and a member of the federal COVID-19 immunity task force, said knowing more about the immune response is also important for understanding what percentage of Canada's population has been exposed to the virus.

"Right now, we're testing only people who are showing symptoms," Nemer she said.

Wedon't really know how many people have been infected but lack symptoms,she said.

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Antibody testing could help map COVID-19 outbreaks

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Antibody testing can determine whether someone has had COVID-19 even if they didnt have symptoms, and could shed more light on where outbreaks have occurred.

To conduct these antibody tests, a person gives a pinprick sample of blood that's put on a slide or cartridge. The cartridge is loaded into a devicethat indicates the presence or absenceof two types of antibodies, called IgM and IgG, that indicate an immune response to COVID-19.

Daniel Altmann, an immunologist at Imperial College's Hammersmith Hospital in London, says the simple yes-or-no antibody test is not enough.

"Would it give me the confidence that I scored positive on that test that I was now ready to queue up for a pint of beer in a crowded pub, shoulder to shoulder with all the other people there? Not really, no," said Altmann, who co-wrote a commentary in the medical journal The Lancettitled "What policy-makers need to know about COVID-19 protective immunity."

In labs, researchers use a more sophisticated version of antibody tests than the rapid tests being assessed for clinical use in Canada for so-called seroprevalence studies that will follow people in the general population over time to test their antibody levels and to see if they develop symptoms.

"By following people forward into a second wave, we'll have a much better idea about all these questions," Loeb said.

What is herd immunity and why does it matter?

Herd immunity occurs when enough people get protection from infection, either naturally or through vaccination, that most of the population isresistant to a virus's invasion and spread. When most of the population is resistant, that helps protect those who are more vulnerable.

But the proportion of population that must be immune to achieve herd immunitydiffers depending on the illness.

"We may be banking on herd immunity, and herd immunity may be very weak and fragile thing," Altmann said of the pandemic. "We may all be sitting ducks waiting for re-infection."

WATCH | Immunity after recovery:

Researchers warn of risks with COVID-19 antibody testing

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Researchers say the science still isnt clear on how people that test positive for antibodies are protected from being reinfected.