Odds are, you’ve had COVID-19—whether you know it or not. Almost 60% of people in the U.S. have antibodies in their blood that suggest they’ve been infected by SARS-CoV-2, the virus that causes COVID-19, according to new estimates from the U.S. Centers for Disease Control and Prevention.
But that doesn’t mean 60% of people in the U.S. are immune to COVID-19. “We know that is not what this means. Reinfection happens,” said Dr. Kristie Clarke, co-lead for the CDC’s COVID-19 Epidemiology and Surveillance Taskforce Seroprevalence Team, during a press briefing on April 26. “Protection from vaccination and protection from previous infections does wane over time.”
Even two years and millions of infections into the pandemic, researchers are still learning about immunity to the virus, and how long you can expect to remain disease-free after a shot or illness. Here’s what the latest science says about immunity to COVID-19.
If I already had COVID-19, will I get it again?
Your immune system mounts a few different defenses against SARS-CoV-2. White blood cells called B cells produce antibody proteins, which help fight off the invader. Meanwhile, T cells, another type of white blood cell, can kill off cells that have been taken over by the virus.
A study from U.K. researchers published in the New England Journal of Medicine in March found that infection-related immunity stays strong for up to a year, then began to wane—whereas the protection offered by two doses of a Pfizer-BioNTech vaccine begins to wane at around six months. Meanwhile, a research review published in the Annals of Internal Medicine in April concluded that people who’d recovered from COVID-19 had significant protection against reinfection for at least seven months.
But all of that research was done before the emergence of the Omicron variant, which is more contagious than previous strains. “We can’t really predict how the protection will work for the next variant or over a period of time,” says co-author Dr. Mark Helfand, a professor of medicine at the Oregon Health & Science University.
Children may hold onto their antibodies even longer than adults, according to a new study from researchers at the Indiana University School of Medicine. Six months post-infection, more kids than adults still had antibodies capable of neutralizing the virus, they found.
While those findings should bring some comfort to people who have had COVID-19, there are no guarantees when it comes to immunity. (A woman in Spain recently made headlines for testing positive for COVID-19 twice, only 20 days apart.) Some people develop more antibodies after an infection than others. It’s also not clear exactly how antibody levels correlate with protection against infection, so a positive antibody test doesn’t necessarily mean you’re immune to the virus, Clarke said during the CDC briefing.
Dr. Bruce Farber, chief of infectious diseases at Northwell Health in New York, says there’s no way to say for sure how long someone who has recently recovered from COVID-19 can stop worrying about being reinfected. But, he allows, it would be “very unusual” to get COVID-19 within 90 days of a previous case.
If I’m fully vaccinated, am I immune to COVID-19?
COVID-19 vaccines also prompt the body to produce antibodies and trigger a T-cell response, though they’re different from those the body produces naturally.
Vaccines provide strong and durable protection against severe disease and death, but their effectiveness against infection wanes over time, so your risk of getting sick is lowest shortly after your most recent dose. A research review published in the Lancet in March analyzed data from prior studies that assessed the effectiveness of different COVID-19 vaccines, including those made by Moderna, Pfizer-BioNTech, AstraZeneca, and Johnson & Johnson. Across brands, vaccines’ ability to prevent symptomatic disease dropped by 20% to 30% after six months, even though they remained good at preventing severe disease and death, the researchers found. This review was also published before the Omicron variant—against which vaccines are less effective—began spreading widely.
Even with Omicron circulating, however, someone who’d been vaccinated was 2.4 times less likely to test positive for the virus in March than an unvaccinated person, according to the CDC.
Anyone who is unvaccinated, whether they’ve had COVID-19 or not, should still get their shots, says Dr. Jessica Ridgway, an associate professor of medicine at the University of Chicago who co-authored a recent study on COVID-19 reinfection. “They’re incredibly safe and having that additional level of immunity would definitely be helpful for preventing COVID,” she says. Further, research shows that getting a booster dose can help bring protection to even higher levels.
What if I’m vaccinated and had COVID-19?
If you’ve recovered from COVID-19 and gotten a COVID-19 vaccine, you have what’s known as “hybrid immunity.” Studies suggest this type of protection is better than either vaccination or exposure alone, because you benefit from both natural and vaccine-derived defenses.
“The best immunity, no question in my mind, is hybrid immunity,” Farber says.
Of course, you should never try to catch COVID-19 for the sake of gaining hybrid immunity. While it’s very unusual for someone who is vaccinated and generally healthy to have a severe case of COVID-19, complications can happen. People have gotten Long COVID even when they’re fully vaccinated and experience few symptoms, for example.
Will I need to keep getting COVID-19 boosters forever?
The U.S. Food and Drug Administration (FDA) recently authorized a second round of boosters for people ages 50 and older, as well as some immunocompromised people across age groups. Additional boosters haven’t yet been authorized for the entire population—but there’s been lots of speculation about whether they will be, since vaccine-related immunity wanes with time.
The FDA’s vaccine advisory committee met in April to discuss ways to streamline and improve booster strategy moving forward. An annual dose, as with flu shots, is a possibility—but the SARS-CoV-2 virus mutates in less predictable ways than the influenza virus, so it’s challenging to make in advance a booster that would target whatever strain is circulating later on.
A better model could be developing boosters that provide immunity to multiple variants. That science is underway now, but it’s too soon to say exactly what future boosting strategies will look like.
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Contributor: Jamie Ducharme