Why Rates of RSV and Flu Are Surging in the US This Year — And Why It's Probably Not a Population-Wide Immunity Debt
Most children contract respiratory syncytial virus (RSV) by the time they turn two, and for many kids it resembles little more than a cold. But recent data from the CDC shows that pediatric RSV hospitalizations have more than tripled in the last few months. Flu season is also kicking off earlier than expected this year, with documented cases skyrocketing since October.
Upticks in RSV & Influenza—What Gives?
The U.S. is taking on a very unusual season of respiratory virus transmission: RSV cases rapidly increased in rate and severity early this season, flu cases emerged earlier and hit harder than expected, and new COVID-19 variants continue to circulate through the population. Public health experts expected abnormal viral seasons due to the pandemic—but healthcare entities were caught unprepared.
The surge in RSV and influenza —especially in children under the age of two— poses a significant public health crisis as hospitals struggle to accommodate new patients. Over 77% of US hospital inpatient beds have been full since October, according to data from the Department of Health and Human Services, with only 6% of inpatient beds in use for COVID-19.
This crisis is leaving both the population and public health officials with more questions than answers. Why is this viral season behaving so strangely?
What’s Different This Year?
Cases of RSV infections in the U.S. typically occur from mid-September to mid-November, peaking from late December to mid-February. This year, however, cases skyrocketed in late October—infection rates were more than 500% higher than rates in early September, according to data from the CDC. From early September to mid-November, positive RSV cases increased by nearly 15%. Since the beginning of December, however, the CDC’s PCR Detection (more sensitive than antigen detection tests) results suggest that RSV cases are on a slight downturn.
Flu cases acted similarly, with the CDC reporting that the upward surge was significantly earlier and more severe than it has been for the past decade. Positive cases increased by over 23% in about nine weeks (early Oct. to early Dec.), and hospitalizations were higher than they’ve been in ten years. The CDC estimates that this season has resulted in over 22 million illnesses and up to 230,000 hospitalizations. Nearly 14,000 U.S. citizens, including 74 children, have died from the flu since October.
Both RSV and flu cases have been falling over the past few weeks, but COVID-19 is unfortunately picking up the reins. Subvariants of Omicron are rapidly emerging and surging through the states. As of January 6th, the most prominent variant is BQ.1.1—infecting more than 34% of the nation. This is an incredibly tough season.
Several respiratory viruses that seemed to almost disappear over the past two years have reappeared in full force over these past several months—and many experts believe that we have an “immunity debt” to blame.
What is Immunity Debt?
According to these experts, an “immunity debt”—or immunity gap—can occur when the immune system is no longer regularly exposed to common pathogens (viruses, bacteria, and other potentially harmful microorganisms), resulting in reduced immunity. Some experts posit that this reduced immunity following the peak of the pandemic directly caused the “tripledemic” of RSV, flu, and COVID-19.
One aspect of the theory suggests that since most of the U.S. population took to COVID-19 mitigation measures in 2020—such as masking, increased sanitization habits, and isolation—people were exposed to far fewer viral pathogens than usual.
Mitigation measures allegedly deprived people’s immune systems of pathogenic exposure, therefore weakening their immunity to viruses like RSV and flu. Because of their reduced immunity, the theory claims, much of the U.S. population has decreased “herd immunity” and is a lot more likely to catch a nasty viral infection.
Is It a Credible Theory?
The short answer: not really, no. The concept only gained traction less than a year ago, and it has no scientific evidence to prove that “declined herd immunity” caused this season’s viral surge.
According to experts, social distancing and similar behaviors don’t inherently weaken the immune system. The idea of an “immunity debt” came to life when a team of French researchers published a position paper in the journal Infectious Diseases Now in mid-2021, and the concept has circulated through media outlets as people demand answers.
The paper explores the idea that a population’s herd immunity can be reduced as a result of decreased “immune stimulation.” And they posit that precautionary behaviors like social distancing and masking are to blame.
But the hypothesis falls apart towards the final page of the paper, where the authors scramble to conclude that low pathogenic exposure caused by COVID-19 precautions could have strong pediatric consequences once the NPIs are lifted. Pediatric, not a population-wide decline in herd immunity as they initially claimed.
However—the basis for a pediatric immunity debt follows recent research on “trained immunity,” which means that the innate immune system can be “trained” to be as effective as the adaptive immune system. Here, the authors suggest that children are more prone to “immune stimulation” through pathogens and early childhood vaccines, and are therefore less likely to be severely affected by COVID-19. Because of mitigation measures, they claim that children had less immune training and are more susceptible to infections following the pandemic.
Now, not only is the paper’s entire argument shaky and unsure of itself but the assertion that our immune systems need to be frequently stimulated throughout our lives in order to be effective isn’t based on scientific fact.
How Do Our Immune Systems Actually Work?
Despite what the immunity debt theory implies, your immune system is NOT a muscle that will atrophy without use. It’s far more complex than that. Your immune system has two lines of defense: innate immunity and adaptive immunity.
Innate immunity is what you’re born with—it’s your rapid response system against what your body might deem “an invader.”
It’s comprised of physical barriers like skin, the cornea, mucous membranes that line respiratory and gastrointestinal tracts, etc. Think of it like a deep, alligator-infested moat that protects a medieval castle.
Adaptive immunity, or acquired immunity, recognizes a familiar microorganism or antigen and targets it. It functions from a state of immunological memory, which allows the adaptive immune system to respond to every subsequent encounter with an antigen faster and more efficiently.
Your immune system’s memory lasts, even after an infection or vaccination. Think of it like the soldiers and cannons that defend a castle.
The immunity debt theory doesn’t take into account how immunological memory works. You don’t get a bad case of the flu every other year because your immune system just forgets how to respond to it. In the same vein, much of the U.S. population isn’t overwhelmed with viral infections because our immune systems have gotten lazy over the past two years.
Why is There a Viral Surge, Then?
As of right now, there’s no definite answer as to why. And it’s frustrating. While we’re pretty sure we can wipe the idea of a population-wide immunity debt from our list of credible explanations, we can only theorize and build off of these ideas until things start making sense.
This year’s season isn’t the first unusual one around the time of a pandemic, though. Unusual viral seasons aren’t uncommon by any means—viruses rarely evolve in a predictable pattern.
For example, the 2009 H1N1 flu pandemic was preceded by a virtually absent winter viral season and historical peaks around May, while seasonal peaks in the U.S. usually occur around February. H1N1 spiked again in November of 2009 but very quickly tapered off into low positivity around November and December of that year. Even without a global application of social distancing, the timing of flu season was skewed.
Viruses are dubious beasts. And we’re still trying to understand what circumstances cause them to mutate and adapt. What we do know, however, is that this season’s viral surge is probably due to several things. There’s not just one answer.
The most promising piece in this puzzle, though, is the fact that most respiratory viruses have incredibly high mutation rates. This allows them to circulate and evade our immune systems more efficiently. Influenza, in particular, evolves so frequently that it’s nearly impossible for you to build complete resistance. The nature of respiratory viruses begs the question: Could RSV and flu have evolved into aggressive, more contagious variants in the past two years?
And of course, this question sparks others that may contribute to this complicated puzzle:
Did nationwide & global applications of precautionary behaviors affect the evolution of RNA viruses? And what effects do these inevitable viral mutations have on young children or seniors?
Did significantly limited travel affect the circulation of viruses, and could the massive uptick in travel have contributed to more viral circulation this season?
Could immune systems dysregulated by COVID-19 infections be more susceptible to these aggressive respiratory viruses?
Were children under four years old sufficiently exposed to RSV in 2020? Is there data on this?
Are you more likely to get infected with more than one virus following the removal of most mitigation measures?
What enables the pandemic potential of RNA viruses? And how do we counteract these factors within reason? etc.
It’s important to do whatever we can to learn, adapt, and push forward—kind of like evolving viruses—when we’re faced with constant crises. Especially those pertaining to our collective health.
But change first starts by asking questions.
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Thanks for reading,
Kendall Proffitt