Our Pediatric ‘Immunity Debt’ Is Really an ‘RSV Debt’

In the headlines each day, there are reports of beleaguered pediatric hospitals all over the country stressed with an onslaught of respiratory syncytial virus (RSV) cases. The way the press is currently covering RSV, the general public couldn’t be blamed if they thought this virus was something new, rather than a pathogen that has been a perennial scourge of infants. It has historically been the leading cause of hospitalization among children under 1 year old in the U.S.; a killer of older adults at rates fairly similar to influenza; and a virus for which vaccine development failed decades ago. But regardless of the under-appreciated impact of RSV in the pre-COVID era, many people are questioning what is behind this current surge of RSV cases and whether it has anything to do with COVID and the virtual cessation of the spread of respiratory viruses for the past two seasons.

The term “immunity debt” has been offered as an explanation. Like everything that has to do with infectious disease in the COVID era, it has generated acrimonious debate. Personally, I don’t think there is anything nefarious about the term “immunity debt,” but I do believe it is being misused and misapplied in a manner that doesn’t really get to the core of what is behind the RSV surge.

A few simple facts about RSV are critical to setting the context for what is happening today. RSV is a ubiquitous, efficiently spreading respiratory virus. Infection with this virus is inescapable for humans. We do not escape toddlerhood without at least one bout. This was true pre-COVID and it remains true today. What this has meant historically is that every year a cohort of children would become infected with RSV, leading to a wave of infection that, in climates such as that in the U.S., began in October and stretched through May with activity concentrated during a period of about 6 months spanning winter.

During the height of COVID, with the resultant social distancing, school closures, and mask wearing — and possibly viral interference — almost all respiratory viruses other than COVID ceased circulating. However, the viruses did not completely disappear. Instead, they were circulating at some low level somewhere in the world (though certain strains of influenza may have become extinct such as B/Yamagata and H3N2 3c3.A), and when social interaction increased they, predictably, began to circulate more widely.

Another interesting facet of the COVID pandemic is that the disruption it caused in respiratory virus circulation seems to have, for unknown reasons, altered some of the traditional seasonality of the viruses. For example, the spring and summer of 2021 exhibited intra-seasonal circulation of RSV, temporarily stressing pediatric hospitals — which do not have much excess capacity at baseline — but without a subsequently normal winter season. So, it is clear that the pandemic has impacted other respiratory viruses, and I believe that studying how and why this happened will be useful to more fully understanding these viruses and their trajectories.

So, if RSV is a “right of passage” for every human and most individuals are initially infected during their first years of life (when they are probably most likely to be require medical attention) and the pandemic postponed the inevitable, then the number of susceptible children who never had RSV increased over the past several years. Couple this with RSV finally being able to circulate in earnest again as people socially interact and schools operate more in the usual fashion, and the result is that higher numbers of susceptible people are now getting hit with RSV.

This is not really an “immunity debt” but an “RSV debt” we all must pay at some point. And now, the virus is collecting what it is owed. There is no basis to claim that immune systems are somehow worse off because of the social distancing that was put in place. These RSV cases would all have occurred regardless of the pandemic — and the mitigation measures employed — but more evenly spread over multiple seasons. This would have been easier for pediatric hospitals to cope with.

The fact that we all have to pay an RSV debt and that the media is intently focused on this virus should provide an impetus to accelerate the final development and regulatory approval of RSV medical countermeasures. In the next year, it is likely that multiple products will be available to combat RSV, including vaccines directed at pregnant women to protect neonates, vaccines to protect older adults, and an improved monoclonal antibody (already available in the EU). Hopefully future seasons will see a much lower human RSV debt.

Amesh Adalja, MD, is a senior scholar at the Johns Hopkins Center for Health Security, and a practicing infectious disease, critical care, and emergency physician in Pittsburgh.