Does Exposure to Omicron Help Our Immunity, Even If We Don’t Get Sick?

Your immune system probably benefits only if you get infected, many scientists say, because a near miss probably won’t have put enough virus in your body to meaningfully rev up your defenses. You can benefit from an asymptomatic infection that you didn’t realize you had, or a case that was too low-level to show up on a rapid test.

The only safe way to build immunity is vaccination, as any exposure to Covid-19 comes with a risk of serious illness, hospitalization or death. Avoiding infection is still important, but if you are exposed, there are circumstances where you might benefit if you already have antibodies, some scientists say.

The impact of a Covid-19 exposure on your immune system is influenced by how much virus you’re exposed to, says Marion Pepper, an immunologist and associate professor at the University of Washington School of Medicine in Seattle.

“The immune response doesn’t really get activated until it sees a certain level of infectious proteins or infectious parts of the virus,” Dr. Pepper says.

Yet multiple exposures that don’t result in a detectable infection may still result in the body sensing enough viral particles to reactivate the immune system, boosting your defenses against the virus, she says. “It’s possible, it’s not definite,” she says.

B cells, a type of immune cell, can sense virus particles that are floating around, and can bind to their proteins and get activated, Dr. Pepper says.

Some scientists say that if you already have antibodies—from vaccination or prior infection—it’s plausible that exposure to viral particles may give you some immune benefit even if you don’t get infected.

Say, for instance, that the virus comes into contact with the tissue that lines the inside of your nose. Antibodies there could “tag” the virus before it is eaten by immune cells and has a chance to replicate and cause an infection. Now, those antibodies can train the rest of the immune system to better recognize the virus, says Nicole Baumgarth, a professor of immunology at the Center for Immunology and Infectious Diseases at the University of California, Davis.

“You keep sort of reminding the immune system that you should make antibodies to the virus,” she says.

Other scientists don’t think such a process has much of an impact on the immune system.

If some virus particles get into your body but neutralizing antibodies from the vaccine prevent them from getting into the cells, that means the vaccine worked, John Wherry, director of the Institute for Immunology at the University of Pennsylvania. However, he says he doesn’t think that process significantly improves your future immune responses.

It’s also possible for a person—especially someone who is vaccinated or has had a prior infection—to be infected and completely unaware because the person is asymptomatic. In some cases such an infection can be very short, even a day or two, before it’s cleared.

“That’s enough for the immune system to find it and get boosted,” says Frances E. Lund, a professor and director of the Immunology Institute at the University of Alabama at Birmingham. “You never knew you were sick.” Still, a low-level infection is likely to be less of a boost to your immune system than a symptomatic infection with more viral replication.

So if you’ve been dodging Covid-19 left and right, you might think your immune system is so good, superhuman good, and maybe you’re immune to getting the coronavirus and can let your guard down? You are wrong.

If you’ve been wearing masks or social distancing during your exposures, that’s providing protection, Dr. Wherry says. So even if you live with someone who is infected, your precautions might mean you aren’t exposed to a large enough dose of virus to cause an infection.

“You might breathe in a handful of virus particles but they don’t get into your cells,” he says. “That doesn’t mean you have great immunity. It’s a little bit of luck with physical barriers, which are very important.”

Write to Sumathi Reddy at sumathi.reddy@wsj.com