An off-season surge in hospital admissions due to RSV, influenza and other viral infections in the pediatric population commonly has been attributed to immunity debt. Introduced in 2021, the term suggests that nonpharmaceutical interventions (NPIs) instituted to reduce exposure to SARS-CoV-2 led to a reduction in other respiratory viruses and resulted in decreased immunity, rendering a large proportion of the pediatric population susceptible to future infections.
Let us be clear: There is no statistical or published evidence to support this concept.
The term immunity debt purports that early exposure to respiratory viruses protects against future infections. On the contrary, there is no benefit or evidence of harm from early respiratory infections such as RSV. In fact, antibodies gained from early RSV infections dissipate over time and ongoing immunity requires recurrent seasonal exposures.
RSV in infants under 6 months of age can be particularly severe owing to an immature immune system and smaller airways. RSV is a major cause of lower respiratory tract infections and hospitalizations in the pediatric population.
While we acknowledge that a susceptibility gap may exist due to the COVID-19 pandemic, it is important to note that infants 0 to 6 months of age who are at highest risk of severe sequelae from RSV and other respiratory infections were not alive when NPIs were instituted; children 6 to 12 months of age were not affected by the strictest NPIs such as lockdowns. In addition, areas that employed short-lived NPIs are also seeing the same surge in RSV infections and hospitalizations. Finally, in many areas, RSV infection rates are lower than the same period last year. In fact, most Canadian provinces saw a surge in RSV infections in the fall of 2021.
Numerous studies have shown that SARS-CoV-2 causes immune dysregulation both during the acute phase and after recovery. Post-COVID-19 immune dysregulation can cause reactivation of latent infections. One study showed persistent T-cell abnormalities in convalescent COVID-19 patients three months after initial infection. Another study revealed evidence of immunodeficiency caused by downregulation of CD19 expression on B cells in recovered COVID-19 patients.
Most children have been infected by SARS-CoV-2, particularly the Omicron variant. The Centres for Disease Control and Prevention estimated that by February 2022, 75 per cent of children had been infected by COVID-19. Furthermore, according to the Public Health Agency of Canada, only 1 per cent of the pediatric population under the age 5 have received a full primary series of the COVID-19 vaccine. As such, it is plausible that children are experiencing immune system dysfunction as a long-term effect of COVID-19 infection, making them more susceptible to severe respiratory infections.
Many viruses are known to also cause persistent immune dysregulation, secondary infections, autoimmunity and malignancies.
While data demonstrating the correlation between COVID-19 infection and subsequent respiratory infections from other viruses such as RSV is not available, the concept of post-infectious immune dysfunction is not new. Many viruses, including Epstein Barr Virus, hepatitis C and measles are known to also cause persistent immune dysregulation, secondary infections, autoimmunity and malignancies. Although measles is a vaccine preventable illness, rates of measles vaccination have been declining, in part due to misinformation on social media. This has long-term consequences. The measles virus can non-specifically infect up to 70 per cent of immune memory cells, including B cells, T cells, and plasma cells in the lymphoid tissue and peripheral blood, erasing existing immunity to previously acquired infections. As a result, measles related mortality is predominantly due to increase in susceptibility to bacterial or viral infections.
Public health measures including NPIs and vaccines have played an important role in reducing hospitalizations, morbidity and mortality from COVID-19 infection. This is due in part to following the precautionary principle of using a proactive approach to risk mitigation in the face of serious and irreversible threats to the environment amid scientific uncertainty. This is critical when confronted with novel pathogens such as SARS-CoV-2. Even at the height of the pandemic, however, these protective measures were criticized, gaining traction on social media. Similarly, the term immunity debt is now being exploited to provide negative connotation to public health measures that were used to minimize health-care pressures, morbidity and death during the pandemic.
Skepticism around public health measures has dire implications now amid a health-care crisis. By creating a false narrative in which the blame is placed on NPIs, the focus is shifted away from building long-term solutions such as improving ventilation in public spaces, addressing health-care resource and staffing crises, ensuring access to antipyretics, creating variant-proof vaccine strategies and promoting vaccine uptake in the general population.
Superficially, immunity debt is an attractive concept. However, there is little evidence for immunity debt as an explanation for the resurgence in pediatric respiratory infections. The real cause is likely multifactorial. For now, it may be better explained by post-COVID-19 immune dysregulation.
We are only beginning to understand the true impact of COVID-19, and data in the coming months and years will elucidate some of the current unknowns.