RS-virus is a cold virus that infects the upper and lower respiratory tract. The infection causes colds and often inflammation and narrowing of the small bronchi of the lungs with breathing difficulties and wheezing, especially in very young children. This is a common disease that affects all age groups but can affect premature babies and young children under six months as well as adults over seventy, who often have underlying heart conditions or other chronic illnesses.
Epidemiology
The virus is so common that most children under the age of two have been infected with it. Epidemics caused by the RS virus are annual, occur in winter, and usually last 2–3 months. About 20% of children under one-year-old are regularly brought to the doctor due to acute RS virus infection. Of these, it can be calculated that 2–3% may require hospitalisation.
Both adults and children can get the infection again in new outbreaks as the virus does not induce long-term immunity.
Modes of transmission and gestation period
The RS virus is mainly transmitted by direct contact between individuals but can also be transmitted by aerosol transmission when coughing or sneezing. The virus can live for several hours on toys or a tabletop and can be infected this way and enter the body through the nose, mouth, and eyes.
An infected person is most contagious in the first few days after becoming ill but can remain contagious for several weeks. Symptoms of the disease appear four to six days after infection.
Symptoms
Infections caused by the respiratory syncytial virus (RSV) can become very serious, particularly in premature infants, babies, and young children with underlying heart or lung diseases. The most common symptoms in young children include cold-like symptoms (runny and/or congested nose), a wet cough, and wheezing due to inflammation and mucus in the small airways (bronchioles). Fever may be present but is not necessarily a prominent symptom in many cases.
Symptoms of a severe infection or complications may include rapid breathing, intense coughing, high fever, difficulty breathing with chest retractions, head bobbing in rhythm with breathing and/or flaring of the nostrils, and bluish skin discoloration due to lack of oxygen. Ear infections and bacterial pneumonia are fairly common complications of RSV infection.
Patients who become severely ill from RSV may require hospitalization, often due to dehydration caused by increased fluid loss through rapid breathing and reduced fluid intake due to breathing difficulties. Some patients may need supportive treatment with oxygen or assisted ventilation, while others are admitted mainly because of complications such as secondary infections occurring alongside the RSV infection."
The infection usually peaks in 3-5 days and resolves within a week. Most children recover fully, but some develop asthma symptoms with a cold later in life.
In adults and older children, symptoms are usually mild and resemble the mild flu, i.e. runny nose, dry cough, fever, sore throat, and mild headache.
The virus can also cause severe symptoms in older people, people with underlying heart and lung disease, or those with suppressed immune systems.
Diagnosis
Diagnosis is primarily based on symptoms, medical history, and medical examination, but the virus can also be detected in mucus from the nasopharynx.
Treatment
The treatment is mainly based on supportive treatment due to the child's symptoms, e.g. fluid and oxygen administration.
Mild symptoms of the disease do not require any special treatment. Most children make a full recovery.
Vaccination and Infection Prevention
Since 2023, new preventive options against RSV (respiratory syncytial virus) infections have become available for both children and adults, with marketing authorization in Europe.
For newborns and young children:
Monoclonal antibody against RSV A single-dose monoclonal antibody (Beyfortus® / nirsevimab) can be administered from birth up to 6 months of age in healthy infants, and also in children during their first or second year of life if they have serious underlying health conditions that increase the risk of severe RSV infection. This medicine will be used in Iceland during the RSV seasons 2025–2026 and 2026–2027 to reduce hospitalisations and illness in young children, as RSV is one of the leading causes of admissions to the Children’s Hospital in Reykjavík (Barnaspítali Hringsins) between December and February each winter.
Maternal vaccination during pregnancy Vaccination with Abrysvo® during pregnancy leads to the production of antibodies in the mother, which are passed to the baby through the placenta. This method of protecting newborns and young infants has been implemented in countries such as the United States, the United Kingdom, and Sweden. However, it does not provide protection for children aged 6 to 24 months who have underlying health conditions.
RSV prevention for infants in the 2025–2026 winter season
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RS virus (respiratory syncytial virus) is a very common cold virus that spreads in large outbreaks most winters in Iceland. These outbreaks most commonly occur in November/December and typically last for about two months. However, the risk of infection lasts longer, and the period from October to March is considered the "RS season" during most winters.
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Most people who get RSV develop cold-like symptoms and possibly an annoying cough, but do not become seriously ill.
RSV can, however, cause severe illness, with intense coughing and difficulty breathing. It also increases the risk of secondary infections caused by other germs, such as pneumonia.
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Children born after the end of the last RSV outbreak are most likely to become seriously ill if they are infected with the virus.
Children with severe underlying health conditions and those with underdeveloped lungs—such as very premature infants—have for years received a medication called palivizumab during the RSV season in their first and second year of life, as they are at particularly high risk of requiring hospital care if infected.
Older children and adults with serious underlying conditions—especially immunocompromising diseases, heart failure, or chronic lung diseases—are also more likely than others their age to develop severe illness from RSV.
The risk of severe RSV infection also increases with age after 60 and becomes significantly higher after age 70. Underlying health problems, which often become more common with age, also contribute to this increased risk.
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Antibodies are produced by the immune system of humans and many animals and are one of the body’s tools for marking foreign elements—such as pathogens—for attack by other immune cells like phagocytes and killer cells. Each antibody-producing cell (a B cell) makes only one specific type of antibody, and antibodies can only bind to molecules that match their unique shape—similar to how a key fits a specific lock. When a B cell produces an antibody that successfully binds to a molecule (an antigen), it receives a signal that the antibody is useful. The cell then produces more of it, releases it into the lymphatic system and bloodstream, and may further refine the antibody to strengthen its binding. The B cell can also multiply to increase production capacity.
Antibodies of various types have been used in medicine for a wide range of purposes. For over a hundred years, antibodies extracted from human or animal blood have been used for prevention or treatment of various diseases, such as infectious diseases (e.g., diphtheria and rabies) and poisonings (e.g., snake venom). Later, they were also used to treat immune deficiencies. However, using such blood-derived antibody preparations can lead to complications—especially allergic reactions—when the antibodies come from non-human sources. These preparations usually contain a mixture of antibodies targeting various molecules (called polyclonal antibodies), which increases the risk of side effects compared to uniform antibodies that all target the same molecule. These problems are particularly relevant when repeated treatments are needed. In addition, producing blood-derived antibodies is a complex process. For these reasons, laboratory-made, uniform antibodies called monoclonal antibodies have been developed to treat various diseases where overactivity or underactivity of a specific molecule plays a key role.
Monoclonal antibodies targeting components of the immune system are increasingly used in immunosuppressive therapies for diseases like rheumatoid arthritis and similar conditions. These medications are often referred to as biologic drugs in this context. Although complex and expensive to produce, monoclonal antibodies are more reliable and safer than blood-derived products. Therapeutic monoclonal antibodies often have names ending in “-mab” (such as adalimumab, tocilizumab, palivizumab), indicating that they are monoclonal antibodies. Their effects—both beneficial and adverse—depend entirely on what they are designed to target, and they belong to many different classes of drugs.
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Monoclonal antibodies have been successfully used for many years to protect children with underlying health conditions that put them at higher risk of severe RSV disease. The first monoclonal antibody against RSV, Synagis (palivizumab), is somewhat challenging to use because it requires repeated doses throughout the RSV season, must be administered in a hospital setting, and is very expensive. Due to these factors, it is only recommended for children at high risk of severe RSV illness.
Vaccines against RSV for adults are available but cannot be given directly to young children. One such vaccine can be administered during pregnancy—similar to vaccines for pertussis and influenza—so that the mother produces antibodies that pass to the baby through the placenta. This vaccine has been available in Iceland since 2024. It may be used systematically in the future, but currently, nirsevimab is preferred because it benefits a broader range of infants and young children. The vaccine (Abrysvo), for example, cannot replace palivizumab for children with severe health issues.
Efforts to develop a vaccine providing broad protection for all infants, rather than only high-risk groups, have been ongoing for decades. An early inactivated RSV vaccine using formalin-inactivated virus particles in the 1960s actually worsened disease instead of preventing it, likely due to an overly aggressive immune response triggered by the vaccine and infected cells during subsequent RSV infections. More recent vaccine candidates that did not use formalin-inactivated virus particles have unfortunately shown similar results, making it unlikely that conventional newborn vaccination against RSV will be adopted. In contrast, antibodies against RSV do not boost the recipient’s immune response but neutralize the virus if it enters the body, thereby reducing the effects of RSV infection.
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Nirsevimab is a monoclonal antibody targeting a specific site on the surface of the RSV virus. When the antibody binds to the virus, it prevents the virus from entering the body's cells. Therefore, it needs to be present in the bloodstream before an individual is infected with RSV to be able to block the virus from entering cells and multiplying there. Antibodies on the surface of viruses mark them as foreign particles, which the immune system then clears away.
The use of nirsevimab does not completely prevent RSV infection but significantly reduces the symptoms and severity of the infection. Even children who require hospital care for RSV are less likely to need oxygen or other supportive treatments and tend to have shorter hospital stays if they have received nirsevimab compared to children who have not been given the antibody.
Nirsevimab has been in use since 2023 and has had a major impact in reducing severe illness (hospitalizations, intensive care, and even deaths) due to RSV in children under 1 year of age where it has been used. A reduction of about 80%−90% in RSV hospitalizations among infants receiving nirsevimab compared to children of the same age who did not receive it has been observed in large-scale use (e.g., in Spain, France, and Luxembourg since 2023).
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Nirsevimab is administered intramuscularly like a vaccine. Each child in the target group receives a single dose that can protect them for at least 6 months.
Children with risk factors who previously required monthly doses of palivizumab throughout the RSV season during their first and second years of life only need to receive nirsevimab once per RSV season. However, they receive a larger dose, which is given as two injections simultaneously instead of one, which is the standard dose.
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Children born between May 1, 2025, and September 30, 2025, will receive nirsevimab at primary healthcare centers, mostly in October/November during routine infant check-ups.
Children born between October 1, 2025, and approximately March 31, 2026, will receive nirsevimab in the maternity ward, neonatal unit, at the 5-day check-up, or at primary healthcare centers according to the organization of the facility where they are born or receive healthcare during the first weeks of life. The earlier in the child’s life the antibody is administered during this period, the better, as RSV is expected to have spread by mid-November at the latest. It is possible that administration will stop before the end of March if the RSV season ends early. RSV infections are rare after April 1, but if nirsevimab is available, it can be used as long as an RSV outbreak is ongoing.
Children in their first and second years of life with severe underlying health conditions and very premature infants—those groups who have previously received monthly doses of palivizumab in the hospital during the RSV season—will receive nirsevimab like other children next winter, but only once. However, children born before April 1, 2025, will require a larger dose. Most children in this group will likely receive their dose at the Children’s Hospital at Landspítali (Barnaspítali Hringsins). If doctors there assess a need for nirsevimab in individuals living outside the capital area, it may be possible to arrange for administration at local healthcare facilities.
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The RSV season of winter 2025–2026 will be the second RSV season in the lives of these children, and the medication is not intended for healthy children after their first RSV season because the risk of severe RSV infection is much lower in older children who have already been exposed to RSV compared to younger children who have never encountered the virus before.
The amount of the medication to be purchased this autumn is based on the estimated number of children born or expected to be born between May 1, 2025, and March 31, 2026. The medication cannot be purchased on the general market (manufacturer’s decision).
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Monoclonal antibodies, which mimic antibodies our bodies would naturally produce, typically do not trigger immune responses in most people, unlike traditional vaccines designed to elicit specific immune reactions in the majority of recipients.
Side effects of monoclonal antibodies generally relate to the target they bind to and where that molecule is found in the body, such as immunosuppression if the antibody targets a molecule involved in the immune system, as seen with many drugs used for autoimmune diseases. In the case of nirsevimab, it is highly specific to a foreign molecule, so there are no direct effects on the body’s own cells.
Antibodies against the drug itself (anti-drug antibodies) can be present or develop after administration. The incidence was low in the manufacturer’s pre-approval studies (about 1% in the placebo group versus 6% in the treatment group). It is not known whether these antibodies reduce the drug’s effectiveness.
Repeated dosing of the same monoclonal antibody can sometimes cause allergic reactions, but nirsevimab is usually given only once, and in rare cases two or three times. True allergy to nirsevimab is very unlikely, but if it occurs in a child with underlying health conditions (e.g., heart defects), it is important to report and assess how to manage RSV prevention in subsequent seasons.
Adverse events reported in the manufacturer’s pre-approval clinical trials were similar in frequency between placebo and nirsevimab groups. In the phase III MELODY trial involving full-term and near-term infants, adverse events thought to be related to the drug occurred in about 1% of treated children, with no serious events attributed to the drug. A rash possibly related to nirsevimab appeared in one child. General discomfort, such as irritability or injection site reactions, occurred, while fever was more common in the placebo group than in the nirsevimab group.
In Spain during the 2023−2024 season, over 277,000 doses of nirsevimab were administered, with 141 reported adverse events in 67 children (approximately 50 adverse events per 100,000 doses). Most were fever (3 reports per 100,000 doses) or lack of efficacy (i.e., RSV infection or RSV-like illness despite antibody administration; 7–8 reports per 100,000 doses). Most children with reported adverse events were not newborns and likely received routine vaccinations concurrently with nirsevimab, with fever being a common side effect of infant vaccinations. Rashes and local injection site reactions were also notable in these reports (about 7 reports per 100,000 doses). It was not clear whether guidance was given to use different limbs for nirsevimab and routine vaccines, which would help differentiate side effects caused by each.
The timing of key reported side effects was approximately as follows:
Fever: Median 2 days after the first dose of nirsevimab (consistent with typical vaccine-related fever occurring within 48 hours).
Lack of efficacy: Median measured in months after nirsevimab administration.
Rashes and local injection site reactions: Varied, with median onset ranging from 1 day to 14 days after dosing.
Plan for 2025–2026 (English summary) – no summary available yet on the impact in Finland.
For adults at increased risk of severe RSV infection:
Protein-based vaccines (Abrysvo® and Arexvy®) These are very similar vaccines that can be used by adults from the age of 50 with underlying risk factors such as chronic obstructive pulmonary disease (COPD) or heart failure. Abrysvo® is approved for use from age 18. General use from the age of 75 is recommended in several European countries, although no official guidelines have yet been issued in Iceland.
mRNA vaccine (mResvia®) This vaccine induces antibodies against the same RSV antigen as the protein-based vaccines and is approved in Europe for use from the age of 60.
After contact with an infected individual, it is essential to maintain strict hygiene and wash hands thoroughly to prevent transmission.
When such an infection is suspected or confirmed, directors of laboratories shall send information to the Chief Epidemiologist by further instructions.
