During the early days of the pandemic, scientists and doctors were concerned that being infected with SARS-CoV-2 might not trigger a strong immune response in many people – thus an infection might not provide long-term protection.
“Immunity to Covid-19 could be lost in months, UK study suggests,” a headline from The Guardian alerted back in July 2020. “King’s College London team found steep drops in patients’ antibody levels three months after infection,” the story warned.
But that idea was based on preliminary data from the laboratory — and on a faulty understanding of how the immune system works. Now about a year and a half later, better data is painting a more optimistic picture about immunity after a bout of COVID-19. In fact, a symptomatic infection triggers a remarkable immune response in the general population, likely offering protection against severe disease and death for a few years.
And if you’re vaccinated on top of it, your protection is likely even better, studies are consistently showing.
If you’re under age 50 and healthy, then a bout of COVID-19 offers good protection against severe disease if you were to be infected again in a future surge, says epidemiologist Laith Abu-Raddad, at Weill-Cornell Medical-Qatar. “That’s really important because eventually, every one of us will get infected,” he says. “But if reinfections prove to be more mild, in general, it will allow us to live with this pandemic in a much easier way.”
Abu-Raddad and his team have been tracking reinfections in Qatar for more than a year. In one study, the team analyzed about 1,300 reinfections among more than 350,000 people in Qatar. They found that a prior COVID-19 infection reduced the risk of hospitalization upon reinfection by about 90% compared with in people having their first infection.
And here’s the “really good news,” Abu-Raddad says: This protection against severe disease persists, perhaps for years. “We’ve been following this same group of people for over a year and a half now, we don’t see much waning. If it’s there, it’s too small to discern.”
“Reinfections are not just possible, they’re pretty much inevitable,” says 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 says, “so there’s no reason to expect something different from SARS-CoV-2.”
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, 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.
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 says.
So the future of COVID-19 is starting to become clearer: We’re going to have a lot more infections but hopefully a lot fewer hospitalizations and deaths.
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