Some are blaming 'immunity debt' for the 'tripledemic' — but experts disagree

This fall and winter, many hospitals across the country have been overwhelmed with an unusually high number of patients stricken with respiratory illnesses — predominantly respiratory syncytial virus (RSV), flu and COVID-19. Some are blaming this surge on "immunity debt."

Throughout the U.S., hospitals have reached capacity and have had to make room for patient overflow in hallways, offices and even tents in parking lots. The situation, described as a “tripledemic,” has also led to a shortage of certain antibiotics and fever-reducing medications for kids, such as acetaminophen and ibuprofen, in parts of the country.

Some of what’s happening is not completely surprising, because most viral respiratory infections tend to follow seasonal patterns and are more prevalent during winter months. However, doctors have said that what’s been unusual this year is how early some of these viruses peaked.

RSV, which typically starts circulating at higher levels in the winter, began in the late summer and continued through the fall. Flu season also started about a month early, and flu-related hospitalizations have been at the highest in a decade nationwide, according to the Centers for Disease Control and Prevention.

In addition to the overwhelming number of patients, some doctors have said also cases of RSV have been more severe.

What is immunity debt?

Although the underlying reasons for the convergence of these viruses and the increase in infections this year are unclear, one popular idea that has emerged to explain what’s causing the tripledemic is a concept called “immunity debt.”

According to this theory, people were exposed to fewer viruses over the past two years because of COVID-19 mitigation measures, such as masking, isolation and social distancing. Because these infection prevention techniques are also effective at preventing other respiratory viruses (like the flu and RSV) from spreading, the number of infections they have caused sharply decreased while such measures were in place. But now that most COVID-19 restrictions are gone, and society has reopened, these respiratory viruses are making a comeback and getting more people sick.

“I think there's some general agreement that that is part of what's going on, that like these mitigation measures helped prevent the spread in the past, and now we don't have them anymore. So it's not surprising that we're seeing this rapid surge in these other viruses too,” Jane Thomason, an industrial hygienist for National Nurses United (NNU), the largest organization of registered nurses in the U.S., told Yahoo News.

However, the concept of “immunity debt” has become controversial, because it has also been used to suggest that the lack of exposure to viruses due to pandemic restrictions may have weakened people’s immune systems, particularly children’s.

The idea of “immunity debt” first emerged in 2021 in an opinion paper published in Infectious Diseases Now by a French pediatric group. The authors warned that while COVID-19 mitigation measures were important to prevent hospitals from becoming overwhelmed, these precautions would also cause people’s immune systems to become less effective at fighting pathogens because of a “lack of immune stimulation.”

Since the paper was published, the term has been widely discussed on social media, in news outlets and in medical journals. Like many other aspects of the COVID-19 pandemic, “immunity debt,” a scientific concept, has also become heavily politicized, with opponents of masking using the theory to prove their belief that masks have caused more harm than good.

Our immune systems never stop working

Catherine Troisi, an infectious disease epidemiologist at the University of Texas School of Public Health in Houston, told Yahoo News that it is wrong to suggest that mitigation measures that prevented the spread of COVID-19 and saved “hundreds of thousands of lives” have harmed children’s immune systems.

“It shows a lack of understanding of how our immune systems work,” Troisi said. “When I was a child and exposed to, you know, measles, my body made these memory T-cells [an important part of the immune system that helps us fight some viruses]. Those cells stay around for the rest of your life, and should I come in contact with the measles virus, they will spring into action, make more cells that can fight the virus. … So the idea that for two years we weren't exposed to flu, and now there is something wrong with our immune system — that's just not how immunity works.”

Additionally, Troisi explained, our immune systems are constantly working, even in the absence of exposure to pathogens. “There are many, many things we're not exposed to all the time. And yet, should we get exposed, our body, our immune systems ramp up, and we're protected,” she said.

Deborah Burger, president of NNU, said that in the more than 45 years that she has been a registered nurse, she had never heard of “immunity debt.” She told Yahoo News it’s concerning that the term is gaining any kind of traction, because it is “highly misleading” and dangerous, because it suggests people need to be constantly exposing themselves to pathogens in order for their immune systems to remain robust.

Last month, the organization criticized those blaming the surge in pediatric RSV infections and related hospitalizations on the “immunity debt” theory.

In a statement, NNU called out a CDC expert who during a November media advisory, said children needed “to become infected to move forward” because RSV is “very common” in kids.

This statement, Burger said, was problematic because it could be easily misinterpreted by some parents, who might be prompted to think that they needed to deliberately expose their children to RSV.

“It's like the old times, when I was growing up, and they would have these ‘chickenpox parties,’ right? Oh, and they say, ‘Oh, come get your kid exposed to chickenpox.’ But it can be deadly. They did it for measles, mumps, you know, all those things, but people still died from it. It makes no sense,” she added.

Troisi explained that most kids encounter RSV sometime in their first year and a half of life and develop antibodies that later protect them against future infections. Even though the immunity provided by an RSV doesn’t last very long, enough of it remains, typically, to mitigate subsequent infections, she said.

There are currently no vaccines available for RSV, but some vaccine makers are working on developing one.

What could be the reason behind the children’s ‘tripledemic’?

One possible reason there have been so many sick children with RSV in 2022 is that last year, many children were still not attending school in person and weren’t exposed to the virus. “They didn’t have a chance to build up antibodies,” Troisi said. She explained that when babies are born, their immune systems are not very strong, and nature protects them with antibodies passed onto them by their mother, which are transmitted to the baby through the placenta and breast milk.

“For about six months after birth, babies are protected with their mother's antibodies. So, again, because the moms weren't being exposed [during the pandemic], their antibody levels might have been a little bit lower,” she said, adding that “it had nothing to do with their immune systems.”

Other experts, however, have argued that there was also a surge of RSV infections in the summer of 2021 after some pandemic restrictions were loosened, and many children should have acquired some immunity from those infections, which would have protected them this year. Therefore, the “immunity debt” idea still doesn’t fully explain the sharp increase and severity in RSV infections this year.

Troisi said health experts are still trying to understand what caused these viruses to converge and why so many people fell sick at the same time.

Thomason said one possible reason why pediatric hospitals have been overwhelmed is because of a lack of beds. She told Yahoo News that from 2008 to 2018, the hospital industry cut about 19% of pediatric hospital units across the country.

Another idea that has floated around is that this is another result of COVID.

“COVID has long-lasting impacts on people's immune systems,” Thomason said. “By April 2022, at least 75% of children had had at least one COVID infection, and more than 60% of the general population had had at least one COVID infection. That's a lot of people with potentially long-lasting immune system dysfunction, which puts them at higher risk for getting infected with RSV and flu when they're exposed to those other viruses, and at higher risk of severe disease.”

But Troisi said that so far, there’s just not enough evidence to prove this theory. She noted that there is a lot of evidence that a measles infection can hamper a person’s immune system, but that has not yet been proven to be the case with COVID.

“I think the jury's out right now. There's no good evidence that it's happening. But, you know, we've changed our minds as we’ve learned more about this virus,” Troisi said. “I'm certainly not saying for sure that SARS-CoV-2 doesn't affect your immune system post-infection, but right now, we don't have the evidence that it does.”