Immunity debt: Why so many kids are sick with RSV

Recently, children’s hospitals across the nation have been full of sick kids, and it seems like every child has a cold these days. One of the viruses responsible, respiratory syncytial virus (RSV), is not new to pediatricians, but due to a recent rapid increase in cases, this may be the first time many are hearing of it. RSV is a common childhood virus that typically circulates in the winter months causing respiratory illnesses. This year, children’s hospitals have seen a surge of cases earlier in the year, in the late summer and early fall.

Although RSV can infect people of all ages, it typically does not cause severe symptoms in older children or adults with intact immune systems. RSV can cause symptoms in young children ranging from a mild cold to bronchiolitis, infection of the smaller airways of the lung leading to inflammation and mucus buildup, which can cause difficulty breathing in young children. Bronchiolitis is more common in children under 2 years of age, and although most children with bronchiolitis recover at home within 7-10 days, some may need to be hospitalized for support with their breathing or IV fluids for hydration. Children under 6 months of age are at higher risk for severe disease. RSV bronchiolitis was a major cause of pediatric hospitalizations in the winter months for many years prior to 2020.

Many respiratory viruses follow a predictable yearly pattern when cases peak and fall in the community. For example, at Children’s Hospital Los Angeles where I work, RSV is most often seen in the winter months, along with influenza, the virus that causes the flu. In the United States, these viruses spread through communities in the late fall, with case numbers peaking in the winter months, then eventually fading away by the spring. Other viruses tend to reach their peaks in the summer months.

During the COVID-19 pandemic, prevention strategies — such as masking, physical distancing and staying at home from school and work — were intended to curb the spread of COVID-19, but they also decreased the spread of these common childhood viruses, including RSV, which easily spread from person to person, especially in group settings like daycare. This disrupted typical viral patterns, and the number of positive RSV cases and pediatric hospitalizations due to RSV in the winters of 2020 and 2021 were decreased compared to prior years. It is unclear how long it will take for these viral patterns to return to their typical pre-pandemic cycles.

Now that most children are back in daycare and school at a time when masking is less widespread, RSV has had the opportunity to spread among children whose immune systems have never encountered the virus before. This phenomenon has been described as “immune debt” due to pandemic prevention measures. The first time the immune system encounters a virus, symptoms of infection can be more severe as the body is learning to fight the virus, while subsequent infections tend to cause minimal or mild symptoms as the body now has a template to target the virus effectively. Pre-pandemic, most children were exposed to RSV before 2 years of age.  The current rise of pediatric RSV cases suggests children’s immune systems are “catching up” to the typical childhood viruses and so young children, as well as older children who may not have been exposed the past few years, are at risk for infection. Additionally, mothers whose immunity to RSV has waned in recent years are less likely to pass protective antibodies to infants via breastmilk.

RSV is spread from person to person via close contact with an infected person or contact with infected secretions, such as infected droplets from a cough or sneeze. Although there is no currently available vaccine for RSV yet, good hygiene measures such as hand washing, keeping frequently used surfaces clean, and staying away from others with cold symptoms can help prevent the spread of RSV.

Many children have had or will have multiple colds this season. Hospitals are already seeing children who have tested positive for multiple viruses (such as RSV, COVID-19 and influenza) at once or in quick succession. Influenza is poised to have similar surge in cases in the upcoming months. Parents can help protect themselves and their children against influenza by getting a flu vaccine this year, which is available for children 6 months and older, in addition to handwashing and other hygiene measures.

Dr. Priya Edward is a pediatric infectious disease specialist at Children’s Hospital Los Angeles and a clinical assistant professor of pediatrics at Keck School of Medicine of USC.