In typical years, RSV garners little media attention. It’s incredibly common and usually causes only mild illness. In fact, most children encounter the virus before age 2.

But RSV can be a formidable respiratory infection with serious consequences for children under 5, especially infants. It is the most common cause of lower respiratory infections in young children, and more severe illnesses can lead to pneumonia and other complications, often requiring hospitalization.

Children, especially young children, tend to get sicker from flu and RSV than other age groups. But infants younger than 6 months old stand to suffer the most, with nearly double the risk of RSV-related death compared to other children younger than 5. COVID-19 hospitalization rates are also four to five times higher for infants than older children.

One reason the youngest children are at greater risk is that their immune systems are not yet fully developed and don’t produce the robust immune response seen in most adults. What’s more, infants younger than 6 months – who are most at risk of severe disease – are still too young to be vaccinated against influenza or COVID-19.

These viruses present challenges on their own, but their co-circulation and coinciding surges in infections create a perfect storm for multiple viruses to infect the same person at once. Viruses might even act together to evade immunity and cause damage to the respiratory tract.

Such co-infections are typically uncommon. However, the likelihood of co-infection is substantially higher for children than adults. Co-infections can be difficult to diagnose and treat, and can ultimately lead to greater disease severity, complications, hospitalization and death.

There are a few reasons why the U.S. may be seeing a surge in pediatric respiratory infections. First, COVID-19 protection strategies actually help prevent the transmission of other respiratory pathogens. School and daycare closures likely also minimized exposures children normally have to various respiratory viruses.

These and other efforts to prevent the spread of COVID-19 seem to have suppressed the broad circulation of other viruses, including influenza and RSV. As a result, the U.S. saw an overall drop in non-COVID respiratory infections – and an almost nonexistent flu season in the winter of 2020.

The decreased viral activity means that children missed out on some exposures to viruses and other pathogens that typically help build immunity, particularly during the first few years of life. The resulting so-called “immunity debt” may contribute to an excess of pediatric respiratory infections as we continue into this season.

To further complicate the picture, the changing nature of viruses, including theemergence of new COVID-19 variants and the natural evolution of seasonal influenza viruses, means that we could be seeing a unique combination of particularly transmissible strains or strains that cause more severe illness.

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