How to think about your immunity if you're one of the nearly 39 million Americans who've had COVID-19

• A coronavirus infection doesn't guarantee long-term immunity.
• Experts advise people who've had COVID-19 to get vaccinated.
• Still, natural immunity seems to provide stronger, more lasting protection than vaccines alone.

It's a staggering number: Nearly 39 million Americans have had confirmed coronavirus infections - almost 12% of the population.

A spate of new research suggests that natural infection can offer powerful long-term protection against the coronavirus, but that immunity isn't guaranteed. So researchers still recommend a full vaccine course to lower the risk of reinfection for people who've had COVID-19 before.

Here's how to understand your immunity if you've had COVID-19.

How does vaccine-induced immunity stack up against immunity developed after an infection?

Immunity derived from an infection has shown to be a powerful force against the Delta variant, which is more transmissible than any other variant so far and can partially evade protection from vaccines.

In a new study that's still awaiting peer review, Israeli researchers found that a prior coronavirus infection offered stronger, more lasting protection against the Delta variant than two doses of Pfizer's vaccine. The results were based on data from tens of thousands of Israelis, making it the largest real-world study of its kind.

Vaccinated people who'd never had COVID-19 were between 6 and 13 times more likely to get infected with Delta than unvaccinated people who'd had COVID-19 before, the researchers found.

But that isn't a reason to forego a vaccine if you've gotten COVID-19 before, since several studies also suggest that people who get an mRNA vaccine after getting infected have perhaps the strongest defense against variants, including Delta.

As of June, unvaccinated Americans who'd had COVID-19 were more than twice as likely to be reinfected than vaccinated Americans who'd had COVID-19, according to a study from the Centers for Disease Control and Prevention.

What effect does getting a vaccine have on people who previously got COVID?

If you've had COVID-19 before, your response to a vaccine may vary based on how you sick you got.

A recent study from Northwestern University looked at 27 people who got either Pfizer's or Moderna's vaccine. Thirteen of them had coronavirus antibodies - an indicator that they'd had a mild or asymptomatic infection at some point - and four had previously confirmed COVID-19 cases.

Three months after their vaccination, the people who'd never had COVID-19 and those who'd had prior asymptomatic infections showed similar antibody levels. But the people who had developed multiple COVID-19 symptoms during their illness had higher antibody levels after their first dose than the others.

"For people who had mild or asymptomatic infections, their antibody response to vaccination is essentially the same as it is for people who have not been previously exposed," Thomas McDade, the study's lead author, said in a press release.

However, even the people with confirmed COVID-19 cases still saw their antibody levels decline about 20% two months after their second dose. Researchers say this supports the idea that everyone should get both doses, regardless of their infection history.

"Some people with prior exposure don't think they need to get vaccinated. Or if they do get vaccinated, they think that they only need the first dose of the two-dose Pfizer/Moderna vaccines," McDade said in the press release. "Our study shows that prior exposure to SARS-CoV-2 does not guarantee a high level of antibodies, nor does it guarantee a robust antibody response to the first vaccine dose."

Will a prior infection make a second one less severe?

An April study, conducted before the Delta variant became dominant, suggested that people who got a second coronavirus infection had lower rates of pneumonia, heart failure, and acute kidney injury than during their first infection.

A May study, too, found that most people who got mild COVID-19 cases didn't experience worse symptoms when they got sick a second time. There were a couple exceptions, though: In one instance, a 25-year-old man was put on oxygen support following his second infection, even though his first case was mild. And an 89-year-old woman was admitted to the intensive care unit after getting reinfected.

Do previously infected people need booster shots?

Vaccines are highly effective against severe disease for up to six months, but emerging data suggests the shots become less effective against infection or mild illness over time. A recent UK study that hasn't been peer reviewed indicates that vaccine protection against the Delta variant starts to wane within six months.

For that reason, the US recommends booster shots for fully vaccinated Americans eight months after their second dose. The recommendation applies to people who've had COVID-19, though studies suggest they may be protected for longer. A June study found that people who've had COVID-19 continue to develop coronavirus-fighting antibodies for at least a year.

The Israeli researchers said it's still unclear how much long-term protection booster shots will give to people who've had COVID-19 before.