Some people are worried that another Covid surge is coming, powered by omicron's highly contagious BA.2 subvariant.

But experts say a significant jump in cases is unlikely, at least for now — possibly due to a recent estimate that nearly all Americans currently have some level of Covid antibodies in their systems.

According to a Centers for Disease Control and Prevention survey of blood donor samples, conducted in December and updated last month, an estimated 95% of Americans ages 16 and older have developed identifiable Covid antibodies. Those come from both vaccinations — roughly 77% of the U.S. population has received at least one Covid vaccine dose, according to the CDC — and prior Covid infections.

That's one potential reason some experts, including White House chief medical advisor Dr. Anthony Fauci, don't expect another sharp rise in Covid cases just yet — even though BA.2 is surging across Europe, and cases of the subvariant have doubled over the last three weeks in the U.S.

It's certainly good news, but it doesn't mean a surge will never happen. Antibodies are temporary, with some disappearing faster than others. Some of them don't actually help your body fight the virus at all. And the virus' unpredictability means that few projections are ever completely trustworthy.

The CDC's survey acknowledges this, estimating that in June 2021 — ahead of the delta variant's July peak in the U.S. — more than 87% of Americans had Covid antibodies in their systems. With that in mind, here's what experts want you to know about the country's current antibody levels:

Different types of immunity provide protection for different amounts of time

The 95% estimate is "probably close to accurate," says Dr. Timothy Brewer, a professor of medicine in the division of infectious diseases at the David Geffen School of Medicine at UCLA. But there's a potentially big difference between people who got their antibodies from Covid vaccines and people who got them solely from prior infections.

Antibodies from vaccinated individuals tend to decline after about four to six months post-shot, Brewer says. So if you got a booster dose in December, your antibody boost will probably wear off between April and June.

The data around so-called "natural immunity" is much more of a mixed bag.

According to an October 2021 study by the Yale School of Public Health, published in The Lancet Microbe, unvaccinated people could have immunity against reinfection for anywhere from three to 61 months after getting Covid. The study's authors wrote that more data is still needed to confirm those results, noting that their research didn't include variants like omicron or subvariants like BA.2.

A different study in the journal Science, published in January 2022, found that natural immunity could last as long as eight months. And a CDC study published in September 2021 showed that roughly one-third of participants with Covid developed no apparent natural immunity.

"I am not aware of good data for the duration of antibodies from natural infection," Brewer says.

Unvaccinated individuals are also more than twice as likely to get re-infected than those who got vaccinated after having Covid, according to a study published in the National Library of Medicine last August. Brewer notes that people who have recovered from an omicron infection probably have better protection from BA.2 than people who recovered from other Covid variants, or those who were last vaccinated more than four to six months ago.

"However, [a] previous infection or vaccination still offers protection against serious disease and death," he says.

Not all antibodies are 'Covid-fighting' antibodies

Dr. Salman Khan, an infectious disease specialist at Lenox Hill Hospital in New York City, says it's true that most people in the U.S. have antibodies against Covid, either through past infection or vaccination. But "not all of these are 'Covid-fighting' or neutralizing antibodies," he says.

Sometimes, Khan says, "non-neutralizing antibodies" are produced in response to a pathogen. "These antibodies do not bind sufficiently to the specific site on the pathogen to stop it from continuing to cause infection," he says. "In the case of SARS-CoV-2, this may have to do with mutations to the spike protein."

Those non-neutralizing antibodies function "almost like a GPS," says Hannah Newman, Lenox Hill Hospital's director of infection prevention. They still attach themselves to the virus, but instead of working to stop the pathogen from spreading — like neutralizing antibodies do — they "serve as a locator, indicating to other parts of the immune system that there is a problem," Newman says.

In other words, both types of antibodies are helpful to a degree — but testing positive for Covid antibodies is no guarantee that your body will be able to fight off a future Covid infection.

Nobody knows what the future holds

If there's anything to learn from the Covid pandemic, it's that nobody can predict the future, says John P. Moore, a professor of microbiology and immunology at Cornell University's Weill Cornell Medicine.

"Because people want to believe the pandemic is over, they let down their guard and behave less carefully," says Moore. But in reality, nobody knows for sure what'll happen tomorrow, next week or next month.

As of Monday, the seven-day average of daily new U.S. cases is 30,662, according to Johns Hopkins University data. That's fairly low by pandemic standards, but it's too early to tell if this trend is "real or just a blip," says Moore.

Moore also says it's important to remember that the CDC's antibodies statistic is just an estimate. "It's 95% of blood donors," he says, noting that the study also doesn't include anyone under age 16. "The question is: How representative of the general population are blood donors?"

Moore says he's "not suggesting that everyone needs to go down into their basement and not come back out." Rather, he advises, each person should monitor their own perception of risk, and make Covid-related decisions based on their own lives and circumstances.

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