Last Tuesday, researchers at the Johns Hopkins Medical Institutions also reported that SARS-CoV-2 antibodies can persist up to 20 months. In that study, 293 out of 295 unvaccinated people generated antibodies after a positive COVID test.
This pattern, with antibodies declining quickly and then plateauing, occurs with any infection, not just SARS-CoV-2, immunologist Deepta Bhattacharya told NPR back in August. "In every single immune response, there is a sharp rise in antibodies, a period of sharp decline," and then it starts to stabilize at a lower level, he said.
About six months after a SARS-CoV-2 infection, right around the time when the antibody level starts to stabilize, the immune system does something extraordinary: It generates special cells, called long-lived plasma cells, that can make potent antibodies against SARS-CoV-2 for decades, possibly even a lifetime. "These cells are remarkable," Bhattacharya said. "They're estimated to spit out something like 10,000 antibody molecules per second." Therefore, you wouldn't need many of these cells to protect against a future infection.
On top of that, the immune system also generates several types of cells, known as B and T cells, which kick into action upon reinfection. These cells quickly ramp up antibody levels and destroy infected cells to ensure a mild course of COVID doesn't turn into a serious one. Several studies, including one published in January, have shown that these cells are durable and likely persist longer than a year.
Does this mean I won't get reinfected if I've had COVID?
Unfortunately, the answer is no.
"Reinfections are not just possible, they're pretty much inevitable," said evolutionary biologist Jeffrey Townsend at Yale University. "At least all the evidence that we have now says that that's true."
To estimate how often reinfections will occur with SARS-CoV-2, Townsend and his team have been studying four other coronaviruses. They are known as "seasonal coronaviruses" and cause about 30% of colds each year.
"They all infect and reinfect on a yearly timescale," he said, "so there's no reason to expect something different from SARS-CoV-2."
The risk of reinfection is likely very low right after you're sick — up to about four months or so afterward, Townsend and his team estimate. Then the risk grows. And their data suggests that, on average, people will be reinfected every year or two with SARS-CoV-2. "It is very rare for reinfection to happen on a very short timescale, but on a longer timescale it seems to happen with some frequency," he said.
But your specific risk for reinfection depends heavily on your personal situation, such as whether you're exposed to lots of people at work or live with children who go to school. "So that time frame for reinfection each year or so assumes that we all just relax everything about our protections and just sort of let it rip, basically," Townsend said.
And, he said, your risk of reinfection also depends on the virus and how much it mutates. For example, during the delta surge, a previous infection offered about 85% protection against reinfection, Abu-Raddad and his team found. But with omicron, that protection dropped to 55%. So the chance of reinfection was much higher.
So why is the immune system so good at preventing severe disease but not really able to stop reinfection?
No one knows for sure. But this strategy may be a deliberate one by the immune system, National Institutes of Health immunologist Jonathan Yewdell explained last week on the podcast This Week in Virology.
In essence, your immune system is allocating resources. And its primary goal is to keep you alive. So the immune system has decided that, with coronaviruses, it's not worth stopping the infection as long as it can stop serious, life-threatening illness.
"That is good enough under most circumstances," Yewdell explained. "You might not think when you're in bed with a 106 [degree] fever, you know, and crying for your spouse to help you because you could barely move that 'this is good enough.' But as long as you've recovered ... you know, mission accomplished."
In other words, the immune system is not built to stop every sickness or asymptomatic infection. And it's definitely not built to "give you a negative PCR test," Yewdell said.