【】

The coronavirus is a vexing parasite.

Around one in four infected people may have no symptoms, as far as experts currently know (the disease is new, so these numbers aren't yet fully certain). So it leaves many of us with a burning question: Is it possible you were infected, but didn't get sick?

It's a question of paramount importance. A bedrock of immunology (the study of how the body defends against infections) is if a virus infects you — and you fight off the infection — you develop an immunity to it for some time, even if you had mild symptoms. This means, after you eventually get a blood test to show immunity, you could then safely reenter society without immediately catching and spreading the respiratory disease COVID-19.

"The golden rule of immunology is if you are infected with a virus, get sick, and recover, you probably won't get reinfected with the same virus," said Mark Cameron, an immunologist at Case Western Reserve University who previously helped contain the outbreak of another deadly coronavirus, SARS, in 2003.

When exposed to a new virus, the body will soon create defensive proteins in your bloodstream, called antibodies, that block the virus from successfully hijacking the body's cells in the future. (That's why vaccines — which encourage your body to make antibodies — work, and the U.S. eradicated polio over 30 years ago).

This is happening with the new coronavirus, too, but disease experts must observe the response of recovered coronavirus patients to understand how effective this immunity is, how long it will last, and if some people can be reinfected. The CDC, for example, just started recruiting Americans to see who has been infected and made antibodies. People infected with SARS developed immunity for an average of two to three years.

"It's very likely if you got exposed that you mounted a response and you would have antibodies," explained Dr. Vince Silenzio, an M.D. and professor at the Rutgers School of Public Health. "We are fairly certain people are becoming immune [to the new coronavirus]."

And critically, just because someone had a mild response — or showed no symptoms at all — there's no conclusive evidence they build up a weaker immune defense against the coronavirus, officially named SARS-CoV-2.

"The bottom line is it's not necessarily true yet that people with mild infections have less of an immune response," said Silenzio.

"Asymptomatic infections are just as likely to grant immunity to the bearer as frank infections [meaning infections that cause disease], despite the challenge in identifying these infections and the risk of spread they entail," agreed Cameron.

Tweet may have been deleted

Tweet may have been deleted

Why, though, might a significant number of people have such mild (or no) symptoms to this new coronavirus? There could be genetic or health differences that make it more difficult for the coronaviruses to infect a cell. It's also possible that people with milder infections were only exposed to a tiny amount of the virus (like someone picking up just a few particles off a piece of mail versus an ER doctor getting sprayed with millions of viral particles). "It's possible that asymptomatic people were exposed to a much lower dose," said Brian Baker, a biochemist at the University of Notre Dame.

The reason there are severe, mild, and asymptomatic infections will be intensively researched throughout this year, and beyond. Although researchers have learned a great amount about this microbe, the virus is still new to science. After all, this coronavirus only spilled over to humans from other animal species in the last five months. "We don’t have all the answers yet," Silenzio said. "But on the other hand," he added, "it's amazing we have as many answers as we do."

As far as peoples' future immunity is concerned, there are still some weighty questions out there. For example, there are recent reports out of South Korea and China of people who supposedly already had coronavirus, but then tested positive for the infection again. This would challenge the idea they developed immunity. "There is some controversy currently regarding whether people are truly immune to COVID-19 once they have it, because there have been stories to the contrary," said Cameron.

There's an explanation, however, for these reinfections. There's evidence the virus may have temporarily eased off in these patients, and later picked up steam again. So it could have been the same infection all along — not someone getting sick from a new infection. "Infection causes different courses of disease in people — that's not unusual," explained Cameron.

Another looming question is how long immunity to the new coronavirus may last, either after recovering from an infection or getting a vaccine (when one becomes available in 2021 at the earliest). For example, if the coronavirus mutates too much, the body's immunity is lost. "Some viruses change rapidly and some don't change rapidly," said Notre Dame's Baker.

The polio virus, for example, doesn't mutate quickly. So a polio vaccination (which sparks the body to produce new antibodies) generally lasts one's entire life. But other viruses, like influenza, mutate constantly and require those annual flu shots you're so familiar with.

The good news is the new coronavirus hasn't been mutating quickly so far, explained Baker. This means it's relatively stable. We don't know how long immunity might last, but this virus is unlikely to change so fast it will reinfect people or outpace forthcoming vaccines, said Cameron.

At the end of the day, this coronavirus may unwittingly help humanity tame the pandemic. While it's true that asymptomatic people can spread the virus when they're infected (that's why everyone must social distance right now), if one in four infected people are truly asymptomatic, that means that likely millions of people will ultimately develop immunity — whether they know it or not — and won't be able to spread the virus around until we get a vaccine. "That would be a good thing," said Baker.

"We can count on the fact that the vast majority of COVID infections will cause immunity," said Cameron, noting that a vaccine will then only add to the number of immune people. Ultimately, this is how we end this grim pandemic, now that we've failed to contain it.

Though immunologists have an ever-strengthening grip on this coronavirus, they're aware the microbe is still revealing itself. Each day scientists around the globe learn more about how it behaves and infects people. So stay tuned for an improved understanding of how immunity will play out with SARS-CoV-2, a virus that's on pace to kill some 68,000 Americans by August (but likely many more if we don't sustain extreme social distancing measures).

"This virus is teaching us new stories," said Cameron. "The story is not yet written."

(责任编辑：綜合)