Does your immune system need a workout? The bad science behind "immunity debt," explained

Immunity debt — the idea that our immune systems become weak if we're not exposed to pathogens — isn't a real thing

By Troy Farah

Staff Writer

Published December 4, 2022 10:00AM (EST)

Doctor's hand holding syringe of vaccine (Getty Images/Francesco Carta fotografo)
Doctor's hand holding syringe of vaccine (Getty Images/Francesco Carta fotografo)

Viruses may seem all-powerful, given their practical invisibility and ability to take down human hosts, which are 10 million times larger than them. But all viruses have a weakness: they're dependent on living things in order to reproduce and spread, which is why some experts debate if viruses like SARS-CoV-2, the pathogen that causes COVID, might not technically be alive.

When SARS-CoV-2 began spreading in force in early 2020, cities across the globe sheltered in place, trying to flatten the curve of rising cases. Many people stayed home for months and even years, until vaccines and drugs like Paxlovid became widespread. Even though the COVID pandemic is not over, we're in a different stage in late 2022 — hence, many people are choosing to return to a 2019 style of living, including going to restaurants, and attending concerts, holiday parties, and other large gatherings.

The underlying idea behind "immunity debt" is that the immune system is a muscle that needs working out or it will become flabby and weak.

But when everyone locked down, a lot of other viruses died, too. Without human reservoirs to bounce to and from, we almost completely extinguished flu viruses in the United States. In fact, one strain of flu, the Yamagata lineage of influenza B viruses, may even be extinct at this point, thanks to COVID prevention measures like masking, closing schools and staying isolated.

In 2020, health agencies also saw a massive dip in another common respiratory illness caused by respiratory syncytial virus (RSV). It's a common childhood disease that resembles a cold, but can develop into pneumonia and in rare cases, can be fatal, especially in the very young and very old. RSV made a comeback in summer 2021 and again this year, but this time much earlier and with many more cases than the previous two years.

Flu is also making a major comeback in 2022. Probably thanks to relaxed COVID prevention measures, hospitals are experiencing a surge in flu and RSV cases. To make matters worse, COVID hasn't really gone anywhere either and cases are now starting to rise again.

Some on social media are explaining this phenomenon — the surge in flu and RSV cases — as something called "immunity debt," which is not a widely accepted scientific term and does a poor job of clarifying the underlying causes of this "tripledemic." Others even argue that we should have never locked down from COVID in the first place, which is why we are so ill equipped to handle flu and RSV.

"By calling it a debt, there is the implicit argument that catching these viruses is part of our social contract ... And that is somewhat brutish, archaic, and social Darwinist."

The underlying idea behind "immunity debt" is that the immune system is a muscle that needs working out or it will become flabby and weak. If you don't regularly expose yourself or your kids to viruses, bacteria and other pathogens, then the next time you get sick it will be especially bad.

This "immunity debt" rationalization has been used to argue against COVID protection measures like masking or even vaccinations. But some experts see it differently: the immune system is not a muscle — it's far more complex than that — and they warn that framing this situation as "debt" can have serious consequences.

"By calling it a debt, there is the implicit argument that catching these viruses is part of our social contract. It's a debt that must be paid to reengage into society," Dr. Anthony Leonardi, an immunologist specializing in T cells and a master of public health student at Johns Hopkins Bloomberg School of Public Health, told Salon. "And that is somewhat brutish, archaic, and social Darwinist."

"The argument there is let your kids play in the dirt. Let them get exposed to microbes. It's not 'let them run around in a biohazard lab.'"

The term first appeared in scientific literature in August 2021, with authors in the journal Infectious Diseases Now arguing that "The longer these periods of 'viral or bacterial low-exposure' are, the greater the likelihood of future epidemics."

The reality may not be so simple, and it conflates individual immunity with population level immunity. Dr. T. Ryan Gregory, an evolutionary and genome biologist at the University of Guelph in Canada, described the term immunity debt as "confusing" and a "distortion" of the hygiene hypothesis, a widely criticized concept that dates back to 1989, positing that childhood exposure to pathogens is necessary to prevent allergies, asthma or other conditions.

"The argument there is let your kids play in the dirt. Let them get exposed to microbes. It's not 'let them run around in a biohazard lab,'" Gregory told Salon. "Suggesting that unless you're getting infected regularly with pathogenic viruses, your immune system will be weakened, just runs into a logical problem."

Gregory cited a popular xkcd comic that succinctly describes the issue: The character White Hat says to Cueball, "See, it's good to get infected, because it gives you immunity." Cueball responds, "Why would I want immunity?"

"To protect you from getting inf..." White Hat replies. "...Wait."


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In other words, it's smart to protect against certain illnesses, especially those as severe as COVID-19. Widespread COVID infections may also explain why other diseases are surging. While SARS-CoV-2 isn't "airborne AIDS," as some alarmists have described it, there is something unique that the virus does to damage immune memory. SARS- 2 can devastate the immune system, especially harming T cells, a type of white blood cell that helps the body recognize an infection, among many other functions.

Compared to 2018, "children up to the age of six months are seven times more likely to be hospitalized by RSV," Leonardi said, which cannot be entirely explained by an immunity debt. "They're only six months old. What could have happened is that COVID, in mild infections, it'll wipe out, it'll bring to the floor, the plasmacytoid dendritic cells that are responsible for secreting a lot of interferon alpha, which is responsible for dampening the severity of RSV and even preventing infection of RSV."

All of this immunity stuff is very complex, but the response is relatively simple. Masking, paid sick leave, improved ventilation and keeping up to date on vaccines, including the COVID boosters, are all really effective at keeping multiple respiratory illnesses at bay.

"There's no doubt that social distancing and mask wearing prevented a lot of RSV," Leonardi said. "We can't really do what China did. We can't let the cure be worse than the disease. However, we know that if we can start filtering air and doing other things, we can tackle some of these viruses."


By Troy Farah

Troy Farah is a science and public health journalist whose reporting has appeared in Scientific American, STAT News, Undark, VICE, and others. He co-hosts the drug policy and science podcast Narcotica. His website is troyfarah.com and can be found on Twitter at @filth_filler

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