It's unclear whether people who recover from COVID-19 will be immune to reinfection from the coronavirus and, if so, how long that immunity will last.
"We don't know very much," says Matt Frieman, a coronavirus researcher at the University of Maryland in Baltimore. "I think there's a very likely scenario where the virus comes through this year, and everyone gets some level of immunity to it, and if it comes back again we will be protected from it — either completely or, if you do get reinfected later, a year from now, then you have much less disease."
"That is the hope," he adds. "But there is no way to know that."
Researchers do know that reinfection is an issue with the four seasonal coronaviruses that cause about 10 to 30 percent of common colds. These coronaviruses seem to be able to sicken people again and again, even though people have been exposed to them since childhood.
"Almost everybody walking around, if you were to test their blood right now, they would have some levels of antibody to the four different coronaviruses that are known," says Ann Falsey of the University of Rochester Medical Center.
After infection with one of these viruses, she says, antibodies are produced but then the levels slowly decline and people become susceptible again.
"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," says Falsey.
In studies, human volunteers who agreed to be experimentally inoculated with a seasonal coronavirus showed that even people with pre-existing antibodies could still get infected and have symptoms.
That happens even though these viruses aren't as changeable as influenza, which mutates so quickly that a new vaccine has to be developed every year.
"We work with some common cold coronaviruses. We have samples from 30 years ago, strains that were saved from 30 years ago, and they're not appreciably different than the ones that are circulating now," says virologist Vineet Menachery of the University of Texas Medical Branch in Galveston.
Still, seasonal coronaviruses probably do mutate a bit over time to evade the body's defenses, says Frieman. But there's little known about what those changes might look like, since researchers don't do annual surveillance of coronaviruses as they do for influenza.
It's also possible that, for some reason, the body's immune response to seasonal coronaviruses is just not that robust or that something about the infection itself may inhibit the body's ability to develop long-term immunity.
"Maybe the antibodies are not protective and that is why, even though they are present, they don't work very well," says Frieman.
The other known human coronaviruses, SARS and MERS, can cause more severe disease, and basically nothing is known about the possibility of reinfection with those viruses.
Some people sickened by SARS, the dangerous coronavirus that emerged in China in 2002, did develop a measurable immune response that lasted a long time.
"We've gone back and gotten samples from patients who had SARS in 2003 and 2004, and as of this year we can detect antibodies," says Stanley Perlman of the University of Iowa. "We think antibodies may be longer lasting than we first thought, but not in everybody."
Still, it's hard to predict how those survivors' bodies would react if they were exposed to the SARS virus again. "There were 8,000 cases, the epidemic was basically brought to an end within six months or eight months of the first case, so we don't have anyone who was reinfected that we know of," says Perlman.
The other severe coronavirus, MERS, emerged in the Middle East in 2012. "We have almost no information about reinfection because there has only been a total of 2,500 cases over 8 years," says Perlman, who notes that the odds of anyone getting reinfected with that virus are not great, especially considering that 35 percent of people who had it died. Survivors of MERS did generate an immune response to the virus that can be detected up to two years later, he says. And the more ill the patient was, the more robust and long-lasting the immune response.
Until the recent emergence of SARS-Cov2 and this pandemic, scientists say, there just hasn't been much of a research push to fully understand how and why reinfection with coronaviruses can occur.
"You get colds over and over again, and I don't think we think that we're really so well protected against any of them, second time around," says Perlman. "You don't care, either, because it's just a cold virus. I mean, you'd like to not get a cold again, but it's not really a big deal."
This pandemic, he notes, "is a big deal."
He would bet that the virus that causes COVID-19 won't reinfect people. But he wouldn't guess how long their immunity might last.
What's more, some people might have stronger protection from reinfection than others.
"Based on other infections where you get a deep lung infection, you are usually protected against the second infection. If you just have a mild COVID-19 infection that involves your upper airway, maybe it will behave like a common cold coronavirus and maybe you can be reinfected again," says Perlman. "We just really don't know. It's even hard to speculate."
Understanding the natural immune response to this virus is important for vaccine development, he notes.
"If the natural infection doesn't do very well in giving you immunity, what is going to happen with the vaccine?" says Perlman. "How are we going to make sure that that vaccine not only induces a response that works for the next six months, but two to three years?"