We know some people can successfully clear the infection, while others have a harder time.
But if you beat the virus, what happens when you encounter it a second time?
Are people immune if they catch — and recover from — COVID-19?
According to Mark Slifka, a microbiologist at Oregon Health and Science University, the answer is yes, at least partially.
“In the short term, you have to be protected or the virus would kill you,” Slifka tells Inverse.
“The good news is, based on the basic biology of the host fighting the pathogen, we know that we're mounting an immune response because people recover and the symptoms go away, and the virus becomes undetectable.”
Successfully fighting COVID-19 creates immunity, at least for a while, Slifka says.
But how long that immunity lingers remains a mystery.
When humans encounter a coronavirus like SARS-CoV-2, perhaps by breathing in a droplet from an infected person or touching a surface carrying the virus, then wiping your mouth, the immune system springs into action.
Virus particles move quickly through nasal passages and the back of the throat, where they attach to cells and hijack their function. Viruses can't reproduce on their own, so they invade human cells and create mini virus factories instead.
“If you recover from this infection... you should be protected from severe disease.”
When faced with viral invaders, the body's white blood cells start producing Y-shaped molecules called antibodies, which stick to viruses and alert immune cells to kill or neutralize them. The human body is capable of producing one quintillion, or one million trillion, unique antibodies. Quite the arsenal.
These antibodies help the immune system clear the COVID-19 infection, and equip the body to fight future COVID-19 infections, too. If a person encounters the same virus twice, their immune system uses these antibodies to “remember” the pathogen, and take it down more efficiently.
“What we've learned from other respiratory infections is that you can get long term immunity against severe disease,” Slifka says. “If you recover from this infection [COVID-19], and I'm basing this on our knowledge of other diseases, you should be protected from severe disease.”
That means if you encountered the novel coronavirus a second time, at least in the short-term, you would be unlikely to suffer a lethal infection, Slifka says. It isn't a guarantee that you won't feel any symptoms, however: You may still get a cough or a mild fever, but you would be unlikely to die.
This theory appears borne out in some preliminary research. A small study, done in macaque monkeys, shows that, when exposed to the virus a second time after recovering from it, monkeys weren’t reinfected.
But monkeys are not humans, and we won’t know for sure how the immune system behaves until we test antibodies in the blood samples of people months or even years after they have recovered from COVID-19.
How long does immunity last?
At this stage, there simply isn't enough data to say how long any immunity might last post-infection. But with other seasonal coronaviruses, like the common cold, immunity seems to last about a year, and people can be reinfected over and over.
"Most respiratory viruses only give you a period of relative protection. I'm talking about a year or two. That's what we know about the seasonal coronaviruses," Ann Falsey, a respiratory viral infection researcher at the University of Rochester Medical Center, told NPR.
"Not only are we mounting an immune response, the immune response can be transferable."
Other infectious diseases similar to COVID-19, like SARS, can generate longer-lasting immunity for two to three years.
Viruses including SARS-CoV-2 can mutate, causing small changes in their genome that may make it difficult for antibodies to recognize them down the line. Based on what we know now, it's impossible to say if other immunity timelines apply to COVID-19.
“The immune response to COVID-19 is not yet understood,” the CDC says.
“Patients with MERS-CoV infection are unlikely to be re-infected shortly after they recover, but it is not yet known whether similar immune protection will be observed for patients with COVID-19.”
Importantly, Slifka and Falsey's predictions may seem confusing based on recent reports that some people in China and Japan tested positive for the virus after recovering from COVID-19.
But Slifka emphasizes that we don’t know whether these people were truly reinfected or if they never truly kicked the virus before being retested.
“This virus for most people is cleared very quickly,” Slifka explains.
“But for some people it might actually hang on for a while.”
Some people might test negative for the virus, but have a “relapse” days or weeks down the line, he says. That doesn't mean they were "reinfected" — rather, the infection was missed first time around.
As more people bounce back from COVID-19, scientists will have new opportunities to better understand how long immunity may last, and even discover effective treatments.
Currently, scientists are racing to harness potentially helpful antibodies from those who have recovered fully and transfer them into people with new infections or compromised immune systems.
The treatment, known as "passive antibody therapy," has been used successfully in past infectious disease outbreaks like the H1N1 influenza pandemic, and the Ebola crisis. People with the virus in China are also reportedly being treated with this approach, leading to reduced symptoms.
This early research suggests, "not only are we mounting an immune response, the immune response can be transferable," Slifka says.
Until more clinical trials confirm these promising results, people can't bank on gaining indefinite immunity to COVID-19 if they beat the virus, or that they could benefit from other peoples' immune system victories.
Some researchers predict the novel coronavirus will resurge annually, popping up every winter. If that prediction becomes reality, that means we could face a disease outbreak on a regular basis, and understanding the immunity question becomes even more important.
In the meantime, we know what the best defense is against COVID-19. Never getting in the first place.