Bronchiolitis, RSV and Croup — In a Thin Straw, the Same Swelling Blocks More of the Way

"My Child Has a Cough That Sounds Like a Dog Barking at Night"

A two-year-old child had only a runny nose until evening, then wakes up in the middle of the night coughing with a barking sound like a dog. Their voice is hoarse, and every time they breathe in, a harsh sound comes from their throat. That same week, a notice came from daycare that a child had caught RSV, and the four-month-old younger sibling has had a runny nose for days and is breathing fast today.

The RSV written on the notice is the name of a virus. Respiratory syncytial virus is a virus that enters the cells lining the inside of the breathing passages and multiplies, and is called RSV after its English initials. A virus is a tiny pathogen that cannot multiply on its own and must enter the cells of another living thing to multiply.

There are two common illnesses that such viruses cause in young children (PMID 19135584). Bronchiolitis is an illness where a viral infection causes the finest air passages inside the chest to swell and fill with mucus, mainly affecting babies under two years old. Croup is an illness where a viral infection causes the air passages to swell, centered just below the vocal cords, producing a barking cough and a hoarse voice. What the younger sibling has started suffering from could be the former illness, and the older child's night cough is a sound commonly heard in the latter illness.

Bronchiolitis in a healthy baby is an illness that resolves on its own (PMID 24093893), and the symptoms of croup usually subside within two days (PMID 25263284). Still, some children need to be hospitalized to receive help with breathing and eating (PMID 35785792; PMID 21534520).

This article is written based on global review articles and clinical practice guidelines, and where RSV is addressed, we also look together at two Korean review articles summarizing Korea's RSV burden and the prevention approval status in Korea as of 2026. The order is why a child's breathing passages easily clog, where each of the two illnesses occurs and how they progress, what evidence exists for methods to prevent RSV, and what signs clinical guidelines pick out as requiring immediate medical attention. A child's pneumonia and asthma are connected to other pieces covering those topics.

The Thin Straw — A Child's Airway

The airway is the pathway of tubes through which air travels from the nose and mouth deep into the lungs. The lungs are organs located one on each side inside the chest, where they exchange oxygen and carbon dioxide between the inhaled air and the blood.

After passing the throat, the air branches out and enters the lungs. The bronchi are the tubes where the airway descending from the throat splits to enter both lungs and gradually becomes narrower. The bronchioles are the narrowest airways, where the bronchi split and split again right up to the alveoli. The bronchioles are the place that bronchiolitis causes to swell. The alveoli are tiny air sacs hanging like grape clusters at the end of the airway, where oxygen and carbon dioxide move back and forth with the blood.

What causes illness in this pathway is mostly viruses in children (PMID 19135584). When a virus enters the inside of the airway, the body fights back and that spot swells up. Inflammation is a response in which the body summons immune cells, swells that spot, and reacts against stimuli such as damage, infection, or substances from outside the body.

A child's airway is not just a scaled-down version of an adult's airway; differences in shape and function provide clues to understanding a child's various breathing symptoms (PMID 33179415). One of those is thickness. Even with the same amount of swelling, the proportion of blockage differs depending on the thickness of the tube. If a thin layer settles inside a thick straw, the drink still comes up somehow, but if the same thickness settles in a thin straw, the passage is almost blocked. Because an infant's airway is much narrower than an adult's, the same thickness of swelling and mucus blocks a greater share of the passage. A child's airway is a thin straw, so swelling that would be mild for an adult blocks the passage much more for a child.

This analogy has limits. The straw only tells us about thickness and does not explain why, when infected with the same virus, some babies get very sick while others just end up with a runny nose. That difference is linked to age, being born early, underlying medical conditions, and an immature immune system, as we will see later.

Bronchiolitis — When the Thinnest Branch Ends Swell Up

In high-income countries, the most common reason for babies under one year old to be hospitalized is viral bronchiolitis (PMID 35785792). It is a disease where the bronchioles swell and fill with mucus, and the age range covered by the American Academy of Pediatrics (AAP) clinical practice guideline is 1 month to 23 months of age (PMID 25349312).

RSV causes 60 to 80% of this disease (PMID 35785792). A lower respiratory tract infection is an infection that occurs in the bronchi and lungs below the neck. Bronchiolitis is the most common lower respiratory tract infection in babies (PMID 21486501).

There is a breathing sound commonly heard in this disease. Wheezing is a high-pitched, whistle-like breathing sound produced as air escapes through narrowed airways. Bronchiolitis is characterized by wheezing and increased mucus, and the degree of respiratory distress ranges from mild to severe (PMID 32841971).

The disease starts like a cold. After fever, runny nose, and nasal congestion last for about two to four days, coughing increases, wheezing develops, and breathing requires more effort (PMID 28084708). Respiratory symptoms are most severe between the third and seventh days after the illness begins, and whether enough water cannot be drunk at this time, leading to dehydration, is an important signal for determining care (PMID 30334624). Diagnosis is made by physical examination, a test to identify the specific virus is not strictly necessary (PMID 35785792), and chest X-rays are not always taken either (PMID 37082471).

In healthy babies, bronchiolitis is a self-limiting disease (PMID 24093893). Bronchiolitis is a disease that worsens for a few days and then gets better, and most babies recover on their own, but some are hospitalized to receive help with breathing and feeding.

What Is Done, and What Has Not Been Proven Effective

What doctors do for a baby with bronchiolitis is simple. Most recommendations in clinical practice guidelines rely on high-quality evidence, advising only that hydration and oxygen be maintained (PMID 35785792). If there is too little oxygen in the blood, extra oxygen is given, and for a baby who cannot feed enough by mouth, fluids are provided through a vein or a tube inserted through the nose into the stomach (PMID 28084708). The core of bronchiolitis treatment is to maintain oxygen and fluids, helping the baby endure the few days it takes to pass through the illness.

Many drugs have been tested, but most have not been proven effective. Let us first look at medicines used for asthma. Bronchodilators are drugs that relax the muscles surrounding the bronchi, widening narrowed airways. A Cochrane review is a synthesis of literature that gathers and examines studies using a set procedure, and when new studies accumulate, it updates and reissues its findings. In Cochrane reviews that pooled these trials, bronchodilators did not improve blood oxygen, did not reduce hospitalizations after use in outpatient settings, and did not shorten the length of stay in the hospital or the time it took to recover at home. There were side effects such as a rapid heartbeat or trembling hands. Concluding that this drug is not effective for the routine treatment of bronchiolitis, the authors added that the trials were small and had varying designs, so larger trials are needed (PMID 24937099).

The evidence also points toward steroid-type drugs, which suppress inflammation, offering no benefit for babies experiencing bronchiolitis for the first time (PMID 27549684). Antibiotics are drugs that kill bacteria or stop them from growing, and they do not work against viruses. In bronchiolitis without a concurrent bacterial infection, bronchodilators, steroids, and antibiotics did not alter the course of the disease or shorten the hospital stay (PMID 28814254). Italian guidelines noted that these drugs also carry a risk of harm (PMID 36765418).

Regarding whether to routinely use certain inhaled therapies, recommendations have differed among clinical practice guidelines worldwide, which may reflect evidence that is still shifting (PMID 31541233). The Australian and New Zealand guidelines, revised in 2025, listed guidance on the combined use of steroids and inhaled medications for bronchiolitis severe enough to require intensive care unit treatment as one of their major changes (PMID 40685806).

A gap remains between the evidence and actual medical practice. Even though review articles and guidelines have recommended supportive care over drugs, reducing unproven treatments has not yet been achieved everywhere in the world and remains a major challenge (PMID 35785792). Italian guidelines suggested that educating doctors alongside providing information to parents helps reduce inappropriate prescriptions (PMID 36765418).

RSV — A Virus Almost Every Child Catches Once

In the United States, almost all children catch RSV before they turn two or three years old (PMID 14980256). The "syncytial" in the name refers to how infected cells fuse together to form a mass, and the way this virus causes disease involves entering airway cells and merging those cells in that way (PMID 40507642). The appearance of illness ranges widely from a mild runny nose to an ear infection, croup, pauses in breathing in premature babies, pneumonia, and bronchiolitis (PMID 14980256).

Almost every child catches RSV at least once before their second birthday, but only a fraction of children progress to bronchiolitis or an illness severe enough to require hospitalization, and those children are usually babies under six months of age (PMID 18625684). The weight of the illness is summarized as depending on the amount and type of the virus and the baby's immature immune system (PMID 40507642).

There are babies who have a high risk of the illness becoming severe. A premature baby is a baby born before 37 weeks of pregnancy. Babies under three months old, premature babies, babies with weakened immunity, babies with pre-existing conditions in the heart, lungs, or nerves and muscles, and babies who frequently experience pauses in breathing are counted among those at high risk for severe bronchiolitis (PMID 24093893). In particular, among babies born early, those with chronic lung disease, congenital heart disease, nerve and muscle disorders, or immune deficiencies, the rate of hospitalization from RSV has been observed to be 3 to 10 times higher (PMID 19651396).

The opposite fact carries equal weight. Although the risk of RSV becoming severe is several times greater in premature babies and babies with pre-existing conditions, the majority of babies who are hospitalized are babies who were healthy until then (PMID 19651396; PMID 35572550).

Globally, the weight of this virus is heavy. In 2019, deaths attributed to RSV among children under five years old worldwide were estimated at roughly 101,400. More than 97% of these deaths occurred in low- and middle-income countries, and the weight was especially heavy within the first six months of life (PMID 35598608). It is summarized that in South Korea too, a significant portion of hospitalizations among babies and older adults for severe lower airway infections are due to RSV (PMID 36960592).

There are also viruses that resemble RSV. Human metapneumovirus is a virus that causes infections in the airways of children and older adults. First identified in 2001, it appears to have been infecting humans from long before that, its spread pattern and symptoms are similar to RSV, and when adults catch it again, it usually passes looking like a cold or the flu (PMID 21858749).

Two Paths to Prevent RSV

The ways to prevent RSV have long been narrow. An antibody is a protein that the body produces to attach to a specific pathogen and block it. The RSV antibody shots used for a long time had to be given repeatedly every month, and the targets were fixed as babies born very early or children with long-term lung disease or heart disease affecting blood flow (PMID 28084708). For high-risk babies, this shot cut RSV hospitalizations by about half (PMID 19651396). Still, because the targets were narrow, most babies were outside protection (PMID 35572550).

Prevention used today splits into two paths. Pointed to as realistic ways to protect all babies are both the vaccine given to the mother and the long-acting antibody given to the baby (PMID 35572550).

The first path is the antibody given directly to the baby. A long-acting antibody is a preventive medicine made by creating antibodies that attach to RSV and putting them into a baby by injection, designed to stay long once given. Various long-acting antibodies have been developed (PMID 35572550), and the numbers below all came from studies measuring one such antibody. In an analysis combining randomized trials gathering high-risk babies, the group that received this antibody had 54 fewer RSV hospitalizations per 1,000 people than the group that received a dummy drug, and there was no clear difference in overall deaths and drug-related side effects. The authors wrote that more studies are needed, especially to nail down safety and cost-effectiveness (PMID 36800182).

After being approved in 2023, this antibody began to be used targeting all babies in a few high-income countries (PMID 40319903). An observational study is a research where people's lives are not meddled with, leaving them to live as they originally do and watching what happens afterwards. In an analysis gathering observational studies from actual vaccination programs, this antibody was calculated to have reduced hospitalizations caused by RSV by 80.7%, and no serious side effects were reported (PMID 40930981). Other analyses also came out with the side where hospitalizations dropped significantly, but the length of hospital stay for hospitalized babies showed no difference between babies who received it and those who did not (PMID 40319903). This percentage is a relative reduction compared to babies who did not receive it. Such studies included babies born healthy and on time (PMID 41897044), and there was no clear evidence that effectiveness differed between premature babies and these babies (PMID 41211378).

The second path is the vaccine given to the mother. A maternal RSV vaccine is given to mothers in late pregnancy so that antibodies made by the mother's body cross over to the baby and protect it for several months after birth. These antibodies cross through the placenta during pregnancy, protecting the baby during the first few months when immunity is still undergrown (PMID 32806663; PMID 32849319). In a 2024 Cochrane review gathering randomized trials, this vaccine cut the occurrence of babies being hospitalized for test-confirmed RSV in half, and its certainty of evidence was high. If 22 out of 1,000 babies of mothers who received the dummy vaccine were hospitalized, it is 11 fewer babies in mothers who received the vaccine (PMID 38695784).

The same review saw that there were no safety concerns for fetal growth restriction and congenital anomalies, and wrote that for other safety outcomes, the certainty of evidence was low so conclusions should be made cautiously. Regarding premature birth, it said there might be a signal to look into further. Premature births were slightly more numerous in the vaccinated side, but that range included "no difference" and the certainty of evidence was very low, and this signal came mainly from one trial not yet fully published. All included trials were funded by pharmaceutical companies (PMID 38695784). In another meta-analysis from 2025, the rate of premature birth did not differ clearly between the vaccine and the dummy vaccine (PMID 41455011). Data measuring the effectiveness of this vaccine in actual vaccination programs did not yet exist as of March 2025 (PMID 40930981).

One long-acting antibody given to the baby and the maternal vaccine given to the mother both have evidence of reducing baby hospitalizations caused by RSV, and the premature birth signal of the maternal vaccine is still being looked into further while remaining low in certainty. In 2023, the U.S. Centers for Disease Control and Prevention (CDC) and its Advisory Committee on Immunization Practices (ACIP) recommended protecting babies with one of the two methods, and wrote that both are not necessary for most babies (PMID 37824423). How the numbers of how many people decreased per 1,000 and how many percent decreased differ is covered in "If the Risk Was Cut in Half, How Many Fewer People Got Sick? — Relative and Absolute Risk".

In South Korea too, RSV is counted as a major threat to babies and young children, and a 2025 South Korean review wrote that its economic burden necessitates prevention with good cost-effectiveness (PMID 40183652). As of 2026, both paths have received authorization in South Korea. One long-acting antibody given to babies received authorization from the Ministry of Food and Drug Safety (MFDS) in April 2024 and domestic prescription began in February 2025, and the maternal RSV vaccine received Ministry of Food and Drug Safety authorization in August 2026.

Croup — A Disease Where the Area Just Below the Vocal Cords Swells

About 3 out of every 100 children suffer from croup each year, mostly between the ages of 6 months and 3 years (PMID 25263284). The larynx is the gateway to the airway, housing the vocal cords that produce sound. In croup, the swelling is centered inside this larynx, right below the vocal cords, and the inflammation can extend to the airway and bronchi further down (PMID 35337542). Inflammation right below the vocal cords blocks the airway in the neck—namely, the upper airway—creating a barking cough, a hoarse voice, and a rough sound when breathing in (PMID 41839076).

That rough sound also has a name. Stridor is a harsh, high-pitched sound made when inhaling through a narrowed airway near the throat. If the wheezing in bronchiolitis is the sound of air escaping through a narrow airway, stridor in croup is the sound of air entering through a narrowed area near the throat. To use a straw as an analogy, croup is a disease where the narrow neck of the straw swells. The area just below the vocal cords in a child's larynx has long been considered a narrow spot (PMID 33179415), and exactly where it is narrowest is being reassessed through new imaging studies (PMID 30861424).

Most causes of croup are also viruses. Parainfluenza virus is a respiratory virus that has a name similar to influenza but belongs to a different group. This virus causes 75% of croup cases (PMID 25263284).

Croup mostly passes mildly. It begins like a cold with a low fever and a runny nose, followed by a barking cough and varying degrees of shortness of breath, and in most cases, even the cough subsides within two days; 1 to 8% of children are hospitalized, and among the hospitalized children, fewer than 3% need a breathing tube inserted (PMID 21534520). If severely blocked, it can rarely progress to respiratory failure (PMID 25263284).

There are evidence-based medications for croup. In a 2018 Cochrane review, children who received a steroid medication compared to a placebo had milder symptoms two hours later, the rate of revisiting the hospital or being readmitted dropped to roughly half, and the hospital stay was shorter. Serious side effects were rare (PMID 30133690). The 2023 edition, which gathered more trials, also evaluated that most trials had a high or unclear risk of bias, and rated the certainty of the effect of reducing symptom scores as low. At the same time, it concluded that the evidence remains unchanged that steroids reduce symptoms two hours later, shorten hospital stays, and lower the revisit rate (PMID 36626194). There is evidence that steroid medications reduce symptoms, return visits, and hospital stays in croup.

Methods such as exposing the child to cold air or cool mist are summarized as not being supported by published evidence (PMID 24596395). Humidification therapy has also not proven its benefits (PMID 21534520). If croup recurs twice or more in a year, it serves as an occasion to look into the structure of the airway or other conditions (PMID 35337542).

Because there are diseases that look similar to croup but can block the airway much more severely, doctors first rule out those conditions when examining a child suspected of having croup (PMID 41839076). One of them is a disease that occurs on the flap at the entrance of the airway. The epiglottis is a leaf-shaped flap standing behind the root of the tongue and above the entrance of the larynx. Epiglottitis is a disease where the epiglottis swells significantly due to infection and can block the entrance of the airway. In the past, the mortality rate was high, and while the incidence has recently decreased and the affected age has shifted older (PMID 40588280), it is still a condition that doctors rule out first when evaluating croup (PMID 41839076).

Signs That Require Immediate Medical Attention

There are signs that distinguish between when croup is considered mild and when it is considered moderate or worse. Mild croup may involve a rattling sound, but the skin between the ribs does not pull inward when breathing in, whereas moderate or worse croup shows an increase in the effort required to breathe (PMID 24596395). Retractions are a sight where the flesh between the ribs or below the chest caves inward every time a person breathes in, which is a sign that breathing is difficult.

There are signs that medical guidelines classify as reasons for immediate medical attention. These include retractions where the space between the ribs sinks deeply with every breath, pauses in a baby's breathing for a stretch of time, lips or facial color turning bluish, an inability to drink breast milk or water, and looking very ill and limp. Such signs are classified as reasons to seek medical care urgently on their own, without waiting to know the name of any disease.

Several of these carry weight in review articles and guidelines as well. Babies who frequently stop breathing are counted as being at high risk for worsening bronchiolitis (PMID 24093893), and RSV can also trigger pauses in breathing in premature infants (PMID 14980256). A lack of water in the body is a core signal that determines the care for bronchiolitis (PMID 30334624). When deciding whether a baby with bronchiolitis can be safely sent home from the emergency room, doctors repeatedly check breathing speed, the effort required to breathe, oxygen in the blood, and whether the baby can eat by mouth (PMID 31557431).

These signs appear in only a fraction of cases. Bronchiolitis in a healthy baby is an illness that resolves on its own (PMID 24093893), and croup subsides within two days in the vast majority of cases, with only a fraction of children being hospitalized (PMID 21534520). Most cases improve over the course of a few days.

Diagnosis is made through a physical examination. Bronchiolitis is diagnosed through a physical exam (PMID 35785792), and when croup is suspected, doctors first rule out more severe causes such as epiglottitis or a foreign object stuck in the airway (PMID 41839076). Danger signs are indicators that call for prompt medical attention, while what disease that breathing sound and cough point to, and how severe it is, are sorted out by a doctor's examination.

A Week When a Cold Sweeps the Daycare

We return to the home from the opening scene. The four-month-old younger sibling started suffering with a runny nose a few days ago, and now their breathing has become fast. Bronchiolitis starts like this with fever, a runny nose, and nasal congestion for about two to four days, followed by an increase in coughing, wheezing, and working harder to breathe in that order (PMID 28084708). The two-year-old older child's barking night cough can subside within two days, as most croup does (PMID 21534520).

The same virus circulated in the same house, but the two children fell ill differently. As we saw earlier, only some children progress to severe illness, and they are usually babies under six months of age (PMID 18625684), while the weight of the illness depends on the amount and type of virus and an immature immune system (PMID 40507642). The straw analogy simply adds one more picture here, which is thickness, and that analogy does not determine whether the younger sibling will become severe.

The time when the younger sibling's breathing is hardest may be between the third and seventh day after the illness starts, and whether they drink enough milk at this time serves as an important signal (PMID 30334624). Retractions, breathing pauses, blue lips, inability to eat much, and appearing very sick and limp are classified by clinical guidelines as signs requiring immediate medical evaluation, as we saw earlier. Even without those signs, an examination distinguishes whether the younger sibling has bronchiolitis and how severe it is (PMID 35785792).

Even if hospitalized, what is done centers on maintaining oxygen and hydration, and bronchodilators and steroids have not been shown to have benefits (PMID 35785792). Most babies get better after passing a few days (PMID 24093893).

Will It Turn Into Asthma Later?

A common question from parents of babies who have had bronchiolitis is whether the child will develop asthma later. An association between the two is observed. The link between severe RSV disease and later wheezing and asthma is well established (PMID 36073299).

In a 2000 analysis pooling controlled studies following babies who had been hospitalized with RSV, 40% of the children were reported to have wheezed up to age 5, which was higher than the 11% in the control group. Between ages 5 and 10, the difference was not clear, and recurrent wheezing also stopped showing a clear difference after about age 5. The authors concluded that it seemed unlikely RSV bronchiolitis causes allergic-type asthma (PMID 10914957). A 2008 review similarly summarized that the association is visible within the first 10 years but is not clear after age 13 (PMID 18625684).

Most of this association comes from observational studies, and appearing together is not proof that one causes the other. A 2022 review noted that a causal relationship in which RSV causes wheezing and asthma has not yet been proven (PMID 36073299). Whether prevention reduces later asthma is also unsolved. A 2008 review suggested that preventing severe first RSV infections could reduce childhood asthma risk (PMID 18625684), but a 2022 review stated that the effect of prevention on later wheezing and asthma is unclear (PMID 36073299), and a 2024 review also noted that more research is needed to clarify this (PMID 39265601). An association is observed between bronchiolitis and later wheezing and asthma, but whether that association is causal and whether prevention reduces asthma remain unresolved.

Recurrent wheezing and asthma itself are picked up in another installment of this series, "What Is Asthma? — An Airway That Tightens Too Easily, and the Inflammation That Keeps It Swollen".

Where to Look Next

How this article set aside children's pneumonia, where colds, the flu, and pneumonia reach in the airway, and where antibiotics work are covered in "How Do a Cold, the Flu, and Pneumonia Differ? — An Infection That Stays in the Entryway, and One That Reaches the Inner Room". How the branches of the airway stretch all the way to the alveoli can be seen in "How Do We Breathe? — An Upside-Down Tree, and the Leaves Where Air Meets Blood".

How to read the numbers of prevention—namely, how many people per 1,000 the statement that hospitalizations were cut in half translates to—can be seen in "If the Risk Was Cut in Half, How Many Fewer People Got Sick? — Relative and Absolute Risk". The disease where the thin walls between alveoli grow thick is covered in another part of this series, the one dealing with pulmonary fibrosis.

Source Papers

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• "Croup." (2014) — PMID 25263284 — A systematic review covering sudden nighttime barking cough, inspiratory stridor, hoarseness, and dyspnea; affects about 3% of children annually, mostly between 6 months and 3 years of age. Parainfluenza accounts for 75% of cases, symptoms typically resolve within 48 hours, and respiratory failure is rare.
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• "Respiratory Syncytial Virus: A Narrative Review of Updates and Recent Advances in Epidemiology, Pathogenesis, Diagnosis, Management and Prevention." (2025) — PMID 40507642 — Invasion and fusion of airway epithelial cells form syncytia; severity depends on viral load, strain differences, and host immune immaturity. Prevention relies on maternal vaccination and monoclonal antibodies.
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• "Special populations." (2009) — PMID 19651396 — RSV hospitalization rates are 3 to 10 times higher in young infants under 3 months, premature infants, and those with chronic lung disease, congenital heart disease, or immunodeficiency. Conventional antibody prophylaxis reduces hospitalizations in high-risk groups by about 50%.
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• "Respiratory syncytial virus infection and the need for immunization in Korea." (2023) — PMID 36960592 — In Korea, RSV accounts for a substantial proportion of severe lower respiratory infection hospitalizations in infants and older adults, highlighting the potential for vaccines to reduce this burden.
• "Human metapneumovirus." (2011) — PMID 21858749 — First identified in 2001, with epidemiology and symptoms similar to RSV; causes conditions ranging from mild upper respiratory infections to bronchiolitis, croup, and pneumonia in young children, and reinfection in adults resembles a cold or the flu.
• "Use of the Pfizer Respiratory Syncytial Virus Vaccine During Pregnancy for the Prevention of Respiratory Syncytial Virus-Associated Lower Respiratory Tract Disease in Infants: Recommendations of the Advisory Committee on Immunization Practices - United States, 2023." (2023) — PMID 37824423 — The U.S. CDC and ACIP recommend protecting infants through either maternal vaccination or infant monoclonal antibodies, noting that most infants do not require both.
• "Monoclonal Antibody for the Prevention of Respiratory Syncytial Virus in Infants and Children: A Systematic Review and Network Meta-analysis." (2023) — PMID 36800182 — A network meta-analysis of randomized trials in high-risk infants showed that long-acting antibodies reduced RSV hospitalizations by 54 per 1,000 infants compared to placebo, with no difference in overall mortality or drug-related adverse events. Conclusion: further research is needed to confirm safety and cost-effectiveness.
• "Real-world effectiveness and safety of nirsevimab, RSV maternal vaccine and RSV vaccines for older adults: a living systematic review and meta-analysis." (2025) — PMID 40930981 — A collection of real-world studies showed that long-acting antibodies reduced RSV hospitalizations by 80.7% and intensive care admissions by 75.6%, with no serious adverse events reported. Real-world effectiveness data for the maternal vaccine were lacking (as of March 2025), indicating a need for further evidence on maternal vaccination.
• "Real-world effectiveness of nirsevimab against respiratory syncytial virus disease in infants: a systematic review and meta-analysis." (2025) — PMID 40319903 — Across observational studies from 5 countries, infants aged 0 to 12 months showed significantly lower odds of RSV hospitalization, intensive care admission, and lower respiratory infection, with no difference in length of stay.
• "Systematic Review: Efficacy, Safety Profile, and Cost-Effectiveness of Nirsevimab Versus Palivizumab for RSV Prevention in Children Under 24 Months." (2026) — PMID 41897044 — A systematic review of 41 studies showed that the new long-acting antibody reduced RSV lower respiratory infections and hospitalizations in both clinical trials and real-world studies, including healthy term and late-preterm infants, with adverse event rates similar to placebo for both antibodies.
• "Nirsevimab for preventing respiratory syncytial virus lower respiratory tract infections in infants: a systematic review and meta-analysis." (2025) — PMID 41211378 — Across 16 real-world observational studies, nirsevimab reduced RSV lower respiratory tract infection hospitalizations, intensive care admissions, and respiratory support. Conclusion: there is no clear evidence of differing efficacy between premature infants and healthy term infants.
• "Benefits and Risks of IgG Transplacental Transfer." (2020) — PMID 32806663 — Maternal IgG is the sole antibody that crosses the placenta, protecting infants from infections while their immune systems are immature.
• "Maternal Immunization: Nature Meets Nurture." (2020) — PMID 32849319 — Maternal antibodies generated by vaccination during pregnancy cross the placenta to provide passive protection against infections during the first few months of life.
• "Respiratory syncytial virus vaccination during pregnancy for improving infant outcomes." (2024) — PMID 38695784 — A Cochrane review of 6 randomized trials involving 17,991 pregnant women found that lab-confirmed RSV hospitalizations had an RR of 0.50 (high certainty, 22 vs. 11 fewer per 1,000). Preterm birth had an RR of 1.16, which included "no difference," had very low certainty, and was driven by a single unpublished trial; all trials were funded by pharmaceutical companies. Conclusion: reduced hospitalizations, while preterm birth represents a safety signal requiring further investigation.
• "Maternal RSV vaccine: a systematic review and meta-analysis of immunogenicity and perinatal safety." (2025) — PMID 41455011 — In 6 randomized trials comprising 17,212 participants, vaccination reduced RSV lower respiratory illness within 180 days by 49%, with no clear difference in preterm birth rates and no increase in serious adverse events.
• "RSV Prevention Strategies in Korean Children: A Review of Current Approaches and Emerging Options." (2025) — PMID 40183652 — A Korean review noting that RSV poses a major threat to Korean infants and young children, and its economic burden demonstrates the need for cost-effective prevention (cited here for the burden aspect).
• "Croup: Rapid Evidence Review." (2026) — PMID 41839076 — Inflammation of subglottic structures obstructs the upper airway, causing a barking cough, hoarseness, and inspiratory stridor. More severe causes such as epiglottitis or foreign bodies must be ruled out. Imaging and laboratory tests are generally unnecessary, and steroids serve as first-line treatment to reduce symptoms and the need for higher-level care.
• "The pediatric airway: Historical concepts, new findings, and what matters." (2019) — PMID 30861424 — New imaging techniques challenge the traditional concept that the upper airway is funnel-shaped with the cricoid ring as the narrowest point, and clinical studies in children supporting that concept are limited.
• "Croup: an overview." (2011) — PMID 21534520 — Accounts for up to 15% of emergency department visits for pediatric breathing problems; presents with a barking cough following low-grade fever and rhinorrhea, mostly resolving within two days. Parainfluenza is most common, humidification has no proven benefit, and hospitalizations occur in 1 to 8% of cases, with intubation in fewer than 3% of hospitalized children.
• "Glucocorticoids for croup in children." (2018) — PMID 30133690 — A Cochrane review of 43 trials involving 4,565 children showed symptom improvement within 2 hours lasting 24 hours or longer, reduced revisits and rehospitalizations (RR 0.52), about 15 hours shorter hospital stays, and rare serious adverse events.
• "Glucocorticoids for croup in children." (2023) — PMID 36626194 — An updated Cochrane review of 45 trials involving 5,888 children noted that 98% had high or unclear risk of bias and low certainty for symptom score reduction. Conclusion: the evidence for reduced 2-hour symptoms, length of stay, and revisits remains unchanged.
• "Viral croup: diagnosis and a treatment algorithm." (2014) — PMID 24596395 — Mild croup presents with stridor without intercostal retractions, while moderate or worse cases show increased work of breathing. Cold air and cool mists are not supported by published evidence, whereas steroids can reduce symptom severity regardless of baseline severity.
• "Recurrent Croup." (2022) — PMID 35337542 — Croup occurring more than twice a year is regarded as a symptom of structural or inflammatory airway abnormalities and serves as an indication to investigate underlying causes.
• "Croup and Epiglottitis." (2025) — PMID 40588280 — Mild croup is a self-limiting disease, whereas epiglottitis historically carried a high mortality rate, though its incidence has recently declined and shifted to older ages.
• "Acute bronchiolitis: assessment and management in the emergency department." (2019) — PMID 31557431 — Bronchodilators and steroids are ineffective; combination therapy and high-flow nasal cannula oxygen show potential, and safe discharge decisions require repeated assessment of respiratory rate, work of breathing, oxygen saturation, and oral intake.
• "Respiratory syncytial virus, recurrent wheeze and asthma: A narrative review of pathophysiology, prevention and future directions." (2022) — PMID 36073299 — The association between severe RSV illness and subsequent recurrent wheezing and asthma is well established, but causality remains unproven, and the impact of prevention on later wheezing and asthma is unclear.
• "Long-term effects of respiratory syncytial virus (RSV) bronchiolitis in infants and young children: a quantitative review." (2000) — PMID 10914957 — Across 6 controlled studies, wheezing rates at up to 5 years were 40% vs. 11%, and at 5 to 10 years were 22% vs. 10% (not statistically significant), with no difference in recurrent wheezing after 5 years. Conclusion: RSV bronchiolitis appears unlikely to cause allergic-constitution asthma.
• "Early-life respiratory syncytial virus disease and long-term respiratory health." (2024) — PMID 39265601 — Addresses the association between early RSV infection and long-term respiratory sequelae, the high efficacy of preventive products, and the need for further research on whether prevention prevents long-term respiratory disease.