The past couple of years we have seen fall virus seasons characterized with a tripledemic of COVID-19, RSV, and influenza surging together. The fact that we have had such so much respiratory virus activity at the same time post pandemic has led people to wonder whether there is something about the COVID-19 pandemic or COVID-19 itself that causes us to be more susceptible to other viruses. There are two main schools of thought that have been attempting to explain this phenomenon.
Immunity debt theory
The first one is called the “immunity debt” theory which basically posits that social distancing and mask-wearing associated with the COVID-19 pandemic caused our immune systems to “forget” how to fight certain viruses because we didn't have constant, normal exposure to them. There is no scientific data to support this theory, and it mostly gained traction among groups who were opposed to public health measures and mitigations on social media.
Immunity theft theory
Another school of thought that is emerging is “immunity theft”, or the concept that COVID-19 infections lead to immune system dysregulation and dysfunction. This is an area of growing scientific inquiry, and there are a few recent small, laboratory studies in special populations (severe covid, long covid):
A small 2023 study found that people who recovered from severe COVID-19 infections had long lasting changes to the immune system, up to 1 year. The study found that stem cells from the severe COVID-19 group produced more white blood cells, which then produced more inflammatory signals, than in healthy control subjects. The findings suggest that these cells may play a role in some of the lingering symptoms associated with long COVID, although more studies are needed to confirm this connection.
A 2022 study showed immune dysfunction for up to 8 months in individuals suffering from long COVID. These individuals were described by the authors as having “highly activated innate immune cells and lacked naïve T and B cells”. The authors state that this work describes “an abnormal immune profile in patients with COVID-19 at extended time points after infection and provide clear support for the existence of a syndrome of long covid” but also state this is foundational work, and more research is needed to better understand the pathophysiology of long COVID.
Another 2023 study found that people who were vaccinated and recovered from a COVID-19 infection mounted a weaker CD8 t-cell response against SARS-COV-2 than people who were vaccinated but were never infected with the disease. Levels of CD8 cells among those who had infections and then were vaccinated were still higher than levels among unvaccinated, COVID-19 recovered people. The findings suggest that previous infection can influence the T cell response to vaccination and vaccines should perhaps be designed with this in mind.
It is important to remember that studies done in vitro in a laboratory Petri dish do not always translate to what would happen in a human body. In other words, we don't yet know what these immunological changes mean clinically in a human body. So, while these studies may have found alterations in the immune function in certain groups of patients (long covid, severe COVID-19) in vitro, it's too early to make sweeping generalizations about what they might mean for the entire post-COVID general population.
So, why all the post-COVID infections?
Using children as an example - viruses surging worse in children the past couple of years could be because children were exposed to them later than they normally would have been, due to the pandemic. For example, nearly every child old is exposed to RSV by the time they are two years old. So perhaps the pandemic year shifted when and at what age children were first exposed to viruses such as RSV. This isn't immunity debt theory at work but probably just the logistics of not having been exposed.
A logical explanation as to why people seem more susceptible to illness than ever before is because we just went through a respiratory virus pandemic, and we are now more attuned to looking for illness than we were before the pandemic.
So to answer the question of whether COVID-19 destroys immune systems, we cannot really answer yet on a population level. As more immunology research is done on post-COVID patients, we will understand more about any immune system irregularities the virus causes, and perhaps learn better ways to design vaccines and therapies.