RSV in Infants and Young Children: What Parents Need to Know About the Most Common Cause of Baby Hospitalizations

Research-informed explainer — last updated April 12, 2026

Respiratory syncytial virus is the single leading cause of infant hospitalization in the United States, sending approximately 58,000–80,000 children under 5 to hospitals and causing 100–500 deaths annually — and until the 2023 approval of nirsevimab, there was no effective prevention available for all infants. A single injection of nirsevimab given before an infant's first RSV season reduces severe bronchiolitis hospitalizations by approximately 77%, changing the risk profile of RSV infection for the first time in the disease's history.

This article draws on research from three pediatric specialists. Dr. Octavio Ramilo, Chair of Infectious Diseases at St. Jude Children's Research Hospital, is one of the world's foremost RSV researchers — contributing the global RSV seasonality overview (483 citations, Journal of Infectious Diseases, 2018), the comprehensive review of the RSV vaccine landscape and its decades of failures (467 citations, Lancet Infectious Diseases, 2018), research on how the nasopharyngeal microbiome affects RSV disease severity in children (446 citations), and a Nature Immunology paper on the fundamental challenges of inducing immunity in infant immune systems (427 citations). Dr. William Zempsky, Professor of Pediatrics at UConn and Head of Pain and Palliative Medicine at Connecticut Children's, contributed evidence-based reviews of pain reduction during pediatric immunizations (331 citations) — relevant for the practical delivery of nirsevimab as an injection given to newborns. Dr. David Askenazi, Director of the Pediatric and Infant Center of Acute Nephrology at the University of Alabama at Birmingham, led the AWAKEN study establishing the incidence of neonatal acute kidney injury (708 citations, Lancet Child and Adolescent Health, 2017) — establishing the severity context for RSV bronchiolitis hospitalization, as severe RSV is a leading cause of acute kidney injury in hospitalized infants.

What is RSV and why is it so dangerous for infants?

Respiratory syncytial virus is a common RNA virus that causes upper and lower respiratory tract infections. In adults and older children, RSV typically causes a cold with runny nose, mild cough, and a few days of feeling unwell. In infants — particularly those under 6 months — the same virus can infect the small airways deep in the lungs, causing bronchiolitis: widespread inflammation, mucus plugging, and wheezing that can rapidly progress to respiratory failure.

By age 2, virtually all children have been infected by RSV at least once. But the first infection in the first year of life is by far the most dangerous, for two reasons:

1. Immature immunity: Ramilo's 2011 Nature Immunology paper (427 citations) characterized the fundamental immunological challenges infants face — including reduced innate immune activation, limited T-cell function, and maternally-transferred antibodies that wane by 2–6 months of age. These deficits explain why infants cannot mount an effective early immune response to RSV even after prior exposure, and why passive immunization (antibody transfer through nirsevimab) is more protective at this age than any vaccine-triggered active immunity.

2. Small airway anatomy: Infant airways are much smaller than adult airways. The same degree of mucosal swelling and secretion that causes mild congestion in an adult can reduce infant airway diameter by 75% — producing severe respiratory distress, hypoxia, and the need for supplemental oxygen or mechanical support.

RSV seasonality: when is your infant at risk?

Ramilo's 2018 global RSV seasonality overview (483 citations) documented that in temperate climates of the Northern Hemisphere (including the United States), RSV season peaks sharply in the November–March window. In tropical and subtropical climates, RSV is present year-round with less distinct seasonal variation.

The 2021–2022 respiratory season saw an unusual post-COVID pattern: RSV surged in summer and fall (July–November) as pandemic masking and isolation ended and children were exposed to RSV for the first time after an 18-month gap. This off-season surge illustrated that RSV is an opportunistic pathogen that takes advantage of immunologically naive populations. Public health authorities now adjust nirsevimab administration timing based on local and national surveillance, not a fixed calendar date.

The history of RSV prevention: why it took so long

Ramilo's 2018 Lancet Infectious Diseases review (467 citations) characterized what it called "lessons from the graveyard" of failed RSV vaccines — a history that spans 60 years:

• A 1960s formalin-inactivated RSV vaccine produced vaccine-enhanced disease — vaccinated infants who were subsequently infected had worse outcomes than unvaccinated infants, with two deaths. The mechanism was immune complex-mediated lung pathology. This catastrophic trial set the field back for decades.
• Multiple recombinant subunit and live-attenuated vaccine attempts over subsequent decades failed to produce adequate neutralizing antibody responses in infants under 6 months.
• Palivizumab (Synagis), a monoclonal antibody given monthly during RSV season, was approved in 1998 — but only for high-risk infants (premature birth, congenital heart disease, chronic lung disease) due to cost and the burden of monthly injections.

The breakthrough came from structural biology: identifying the RSV fusion protein's prefusion conformation as the optimal antibody target. Nirsevimab (Beyfortus) is a long-acting monoclonal antibody engineered to bind this prefusion epitope with extended half-life — allowing a single injection to provide protection for a full RSV season.

What nirsevimab offers: the evidence

The MELODY trial randomized 1,490 healthy infants born at or after 35 weeks gestation to nirsevimab versus placebo before their first RSV season. Results through 150 days:

• Medically attended lower respiratory tract illness from RSV: 2.6% nirsevimab vs. 9.5% placebo (74.5% efficacy)
• RSV hospitalization: 0.6% vs. 2.6% (77.3% efficacy)
• No safety concerns attributable to nirsevimab beyond injection-site reactions and rash

The HARMONIE trial, conducted in real-world clinical settings in Europe, found 83.2% efficacy against RSV hospitalization in otherwise healthy infants.

The CDC now recommends nirsevimab for:

• All infants under 8 months entering their first RSV season
• Children aged 8–19 months at increased risk for severe RSV (premature birth, chronic lung disease, congenital heart disease, immunocompromised)

The nasopharyngeal microbiome: why some infants get sicker than others

Ramilo's 2016 AJRCCM study (446 citations) found that the bacterial composition of the nasopharynx during RSV infection is a major independent predictor of disease severity — independent of viral load. Infants with high abundance of Haemophilus, Streptococcus, and Moraxella in their nasopharynx had significantly higher clinical severity scores, more severe disease transcriptome profiles, and higher rates of hospitalization than those with Corynebacterium or Staphylococcus predominance.

This finding has important implications: it helps explain why two infants with identical RSV exposure have dramatically different outcomes, and it opens research avenues around microbiome-modifying strategies for high-risk infants. For parents, it reinforces that infection risk is not simply about exposure — host factors including airway microbiome composition matter.

RSV and kidney injury: a less-recognized complication

Askenazi's AWAKEN study (708 citations, Lancet Child and Adolescent Health, 2017) found that acute kidney injury (AKI) affects 30% of neonates in intensive care — a strikingly high rate that was not previously recognized. Severe RSV bronchiolitis is among the leading causes of pediatric hospitalization, and hospitalized infants who require intensive respiratory support are at elevated risk for fluid overload, medication nephrotoxicity (from aminoglycosides used for secondary bacterial infections), and hypoxia-related AKI. This complication is often subclinical but has implications for long-term kidney health.

When to seek emergency care for a baby with RSV

Most RSV infections in otherwise healthy infants older than 3 months cause a cold-like illness that resolves in 1–2 weeks with supportive care at home. Seek emergency care immediately if your infant shows:

• Breathing rate consistently above 60 breaths per minute
• Visible chest muscles pulling inward with each breath (retractions)
• Nostrils flaring with breathing
• Persistent or worsening wheezing
• Skin turning blue or gray around the lips (cyanosis)
• Not drinking — or taking less than half of normal feeds over 8 hours
• Difficulty waking or unusual lethargy

For infants under 3 months, call your pediatrician even for mild RSV symptoms — this age group can deteriorate rapidly.

About the nirsevimab injection

Zempsky's research on pain reduction during pediatric immunizations (331 citations, Pediatrics, 2007) established evidence-based strategies that apply to nirsevimab delivery: sucrose pacifiers, breastfeeding during injection, and non-nutritive sucking have all been shown in RCTs to reduce infant pain scores. These strategies are appropriate for nirsevimab administration as well and can be requested from the administering provider.

Questions to ask your doctor

• Is my baby eligible for nirsevimab, and when should the injection be given relative to RSV season in our area?
• My baby was born premature — does that change the dose of nirsevimab or the need for additional doses?
• My older child has RSV right now — what precautions can I take to reduce the risk of spreading it to my newborn?
• My infant has RSV symptoms but seems okay at home — what specific signs should prompt an emergency room visit rather than continued home management?
• Is there a maternal RSV vaccine that would have protected my baby through transferred antibodies?
• My child had severe bronchiolitis requiring hospitalization — does that increase their risk for asthma later in childhood?

The bottom line

RSV is the dominant cause of infant hospitalization in the United States, and prior to nirsevimab it had resisted all prevention efforts for 60 years. The approval of nirsevimab for all infants in their first RSV season — not just high-risk groups — represents a true public health breakthrough, reducing RSV hospitalization by approximately 77% with a single dose. For parents of newborns, asking about nirsevimab timing before RSV season arrives is one of the most impactful conversations you can have with your pediatrician. For infants with active RSV, knowing the specific warning signs that warrant emergency evaluation rather than home management can be life-saving.