Who is immune to coronavirus? 2 questions must be answered to actually know

Because of coronavirus, billions of people are living in lockdown. Immunity to the Covid-19 is theorized as one path toward "reopening" countries and ending quarantine.

Scientists aren't exactly sure what qualifies as being immune to Covid-19. To get there, and for life to resume, two key questions have to be answered first.

Both have to do with antibodies, which are blood proteins created by the immune system that fight off infection.

Before scientists can determine who is immune to coronavirus, they have to answer two questions:

1. Which antibodies are produced after an encounter with the disease?
2. How much protection do those antibodies actually provide?

Immunity to the coronavirus, either through a vaccine or through exposure to the virus, has become a focal point as countries decide how to reopen. In countries like England and Germany, leaders have proposed "immunity passports," which is the idea that if someone has the coronavirus and has an adequate amount of virus-fighting antibodies, then those people might be allowed back into a socially-distanced society — if they have a certificate to prove it.

In practice, science can't quite support the idea that once you've had Covid-19, you're permanently immune.

Stephen Goldstein, a postdoctoral researcher at the University of Utah who studies virus evolution, says that having had the coronavirus likely confers some amount of immunity to reinfection, but testing positive for antibodies doesn't guarantee the person can't get sick again or transmit the virus.

"My expectation is that the vast majority of people are protected against reinfection," Goldstein tells Inverse. He stops short of supporting the idea of an immunity passport.

While scientists can test for Covid-19 anti-bodies — a blood protein that is present in a person's body after they've recovered from Covid-19 — they don't know enough about how those antibodies affect immunity.

Antibody tests, also known as serological tests, determine whether a person has developed these proteins — but the tests don't confirm a person's immunity.

However, if we get answers to a few key questions, they could bring us a step closer to freedom.

Immunity to coronavirus – As with any viral infection, scientists don't know exactly which function of the immune response might slow the virus, explains Goldstein.

The immune system can destroy a virus either by stopping it from entering a cell, by disrupting the virus' ability to replicate, or by simply killing infected cells.

When scientists try to determine how someone becomes immune to a virus, they're looking for signs called "correlates of protection." These are clues that scientists use to determine immunity.

The "gold standard" of those correlates, says Goldstein, are called neutralizing antibodies – these antibodies stop viruses from infecting other cells.

Neutralizing antibodies likely correlate with protection from coronavirus, says Goldstein, but they're not the only thing that can make you immune to an infection: Different types of antibodies and other components of the immune response might confer some type of immunity to the coronavirus in their own specific ways.

When it comes to the novel coronavirus, scientists are unsure of a number of factors:

• Which antibodies are neutralizing, meaning they attach to a virus to block infection
• Which offer some additional protection
• Which can't offer much protection at all
• How all of these factors combine to make someone truly protected

"It's this complex milieu of the immune response that contributes to whether someone is protected," Goldstein says.

Scientists only gain a piece of the puzzle when they look at neutralizing antibodies, but it's an important piece of the puzzle -- it's the one we know how to study. It makes sense that doctors base their recommendations on neutralizing antibodies, Goldstein says, "but it's still just a piece."

Crucially, there are still players in the game: The amount of antibodies the body produces differs from person-to-person. This may also affect how we gauge someone's immunity.

The "total measurable antibody is not precisely the same as a protective, virus-neutralizing antibody."

In one preliminary, non-peer reviewed study released in April, scientists examined 175 recovered Covid-19 patients in Shanghai and found that 30 percent of patients produced very low levels of neutralizing antibodies. Of those patients, the elderly and middle-aged patients produced more antibodies than younger patients. Whether or not this means those patients with neutralizing antibodies are more likely to be immune than others isn't quite clear.

Because we don't know very much about how levels of antibodies (and the antibodies themselves) affect immunity, the World Health Organization has cautioned against issuing immunity passports. But that could change as we learn more. In a statement, the WHO declared it would continue to review evidence on how the presence of antibodies affects immunity to coronavirus.

What a coronavirus antibody test actually reveals — To really know if someone is immune to coronavirus, we need to know which antibodies are produced and whether those antibodies themselves are neutralizing.

The widely available antibody tests for coronavirus can't tell us those things, says Goldstein. That's because these tests are just looking for the presence of antibodies.

"It’s not saying that the antibodies that it’s detecting will prevent the virus from infecting a cell," Goldstein explains. "It says that the antibodies you have will react to a piece of the virus the test is designed to find."

The authors of a recent report in The Lancet echo Goldstein, adding that "total measurable antibody is not precisely the same as a protective, virus-neutralizing antibody."

Elected officials should understand that the presence of antibodies doesn't always tell the full immunity story, contend the authors in that paper published in The Lancet. We know antibodies are there, but it is unclear how much those antibodies can slow the virus down, if they can, and for how long.

Does a positive antibody mean people can leave quarantine? — Testing positive for coronavirus antibodies is not a free pass to go back out into the world, even though, that definitely sounds great. But hold up: It also doesn't mean those rapid antibody tests aren't useful, Goldstein adds.

Rapid antibody tests can tell us what percentage of the population has been exposed to the virus. Knowing levels of exposure can help determine whether we have reached the threshold of herd immunity, Goldstein says.

Herd immunity occurs when enough people have immunity to the virus, that it can no longer easily spread. Somewhere between 70 to 90 percent of the population has to be immune for that to happen, and that immunity must be long-lasting or permanent.

Early results from New York City, which has been hit especially hard, estimate that about 25 percent of New Yorkers have been exposed to coronavirus. Given the severity of New York's epidemic, those numbers seem about right, says Goldstein, but there are some places in the country that likely have far lower levels of exposure. Herd immunity can't happen until those numbers go up — or a vaccine is produced, which is likely months or years away.

Taken together, there are still millions of people in the United States who are still at risk of becoming infected with Covid-19 for the first time. The herd immunity threshold is still very far off. Even then, we simply don't know enough about immunity itself to give anyone a free pass to go back out into the world yet.

Knowing more about how coronavirus antibodies work is the key to understanding and establishing immunity. Only then can we confidently begin the process of, ever so slowly, going back to the way we were.