Each year in the United States, respiratory syncytial virus (RSV) kills up to 10,000 people, including 100 to 300 children. The Centers for Disease Control and Prevention (CDC) estimates that 1 or 2 of every 100 children will be hospitalized with a lower respiratory tract infection caused by RSV in the first six months of life, and virtually all children are infected with RSV at least once by the time they are 24 months of age. On the other end of the spectrum, most of those who die from RSV infections are the elderly. As such, a virus that infects virtually all children and kills around 10,000 elderly individuals each year is an important target for prevention.

The disease

What is RSV?

RSV is a virus in the Paramyxoviridae family, which is also the family of mumps and measles viruses. However, the three viruses are different enough that they are more like cousins than siblings.

RSV was first isolated in the mid-1950s. It replicates in the cells that line the nose, large breathing tubes (causing bronchitis), small breathing tubes (causing bronchiolitis), and lungs (causing pneumonia). Sometimes it can also cause an infection of the voice box (called croup). As the virus replicates, it disrupts this lining. When this disruption is coupled with immune responses that cause inflammation and mucus production, the infected person’s airways become narrower and fill with excess mucus, cell debris and fluid. Depending on where the virus is causing infection, symptoms can include coughing, sneezing, runny nose, wheezing, and disrupted breathing (e.g., faster breathing, shortness of breath). Breathing problems can lead to low levels of oxygen in the blood and worsening of existing lung conditions, such as asthma. Infants can experience apnea, a condition in which they temporarily stop breathing. Some people may have fever, lack of energy, or loss of appetite. Children frequently have ear infections following an RSV infection.

How do you catch RSV?

RSV spreads through respiratory secretions from close contact with infected people and from contaminated objects, such as when a person touches an object and then touches their eyes, nose or mouth. It takes about three to five days after exposure for symptoms to start, and symptoms typically last one to two weeks.

Who gets RSV?

Anyone can get RSV, and people typically experience multiple bouts of RSV throughout their lifetime. Most adults do not realize that they have RSV; instead, they describe having mild, cold-like symptoms. However, three groups are at particular risk from RSV:

• Young Infants: RSV is one of the most common respiratory infections to cause hospitalization of infants. About 7 of every 10 children will be infected by 1 year of age, and almost all will be infected by 2 years of age. Babies between 6 weeks and 6 months of age are the most likely to be hospitalized. Between 100 and 300 children die each year from RSV and its complications. In children younger than 5 years of age, RSV leads to more than 2 million outpatient visits and between 58,000 and 80,000 hospitalizations.
• Adults over 65: Each year in the U.S., between 60,000 and 160,000 adults are hospitalized, and 6,000 to 10,000 die from RSV infections. Most of those with severe disease are 65 years of age and older and suffer from chronic medical conditions.
• Individuals with chronic conditions: Older children and adults with chronic diseases of the lungs or heart and those with immune-compromising conditions are at increased risk for severe disease if infected with RSV.

The vaccine and monoclonal antibody

In 2023, tools for preventing RSV became available for two of the most susceptible age groups — infants and the elderly. However, each tool works differently. For the elderly, the new tool is a traditional vaccine. For infants, two methods of protection became available: a monoclonal antibody given directly to the infant or vaccination of the pregnant person before delivery. Both methods for protecting babies are forms of passive immunization, meaning using antibodies not generated by the infant’s own immune system to afford short-term protection. In both cases, the antibodies last long enough to protect infants during their first RSV season when they are most vulnerable to severe disease. After the antibodies fade, older babies will be exposed to RSV and make their own immune response at a time when they are less susceptible to severe disease. Keep reading to find out more about the options now available to protect the most vulnerable among us against RSV.

The RSV vaccine: Who should get it?

Adults 75 years and older are recommended to get a single dose of an RSV vaccine (either Abrysvo, Arexvy, or Mresvia).

Those 60 to 74 years of age who have conditions that increase their risk of experiencing severe disease are also recommended to get one dose of any of the three RSV vaccines. Conditions that increase a person’s risk for severe disease include chronic disease of the heart, lungs, liver or blood; advanced kidney disease or diabetes with organ damage; and immune-compromising, neurologic, or neuromuscular conditions. Those who reside in nursing homes or other long-term care facilities and those considered medically frail or who have other chronic conditions that may increase the risk for severe respiratory infection are also among the group of 60- to 74-year-olds who can be vaccinated.

Those 60 and older who received the RSV vaccine previously do not need another dose.

Pregnant people can get a single dose of the RSV vaccine known as Abrysvo during weeks 32 through 36 of pregnancy if that period of gestation occurs during RSV season, which for much of the United States means from September through January. As the seasonality of RSV varies slightly throughout the United States, people in Alaska, Florida or outside of the continental United States should talk with their healthcare providers about when RSV season is expected in their area. The other RSV vaccines (Arexvy and Mresvia) are not approved for use in pregnant people.

Those who received an RSV vaccine during a previous pregnancy should not get an additional dose until more data are available to evaluate the safety and effectiveness of receiving additional doses. In the meantime, infants born to a person vaccinated against RSV during a previous pregnancy should receive the monoclonal antibody product, nirsevimab, after birth.

The RSV monoclonal antibody: Who should get it?

All infants younger than 8 months of age, including those born during RSV season, are recommended to get one dose of the RSV monoclonal antibody known as nirsevimab (Beyfortus™). One dose of nirsevimab can protect infants for five months, the length of an average RSV season.

A dose of nirsevimab is also recommended for some children between 8 months and 19 months of age because they remain at high risk for severe RSV even though they are entering their second RSV season. These include:

• Babies with chronic lung disease resulting from premature birth who required medical support at any time during the six-month period before the start of their second RSV season.
• Babies who are severely immune compromised. If you are unsure if your baby is in this group, talk to your child’s healthcare provider.
• Babies with cystic fibrosis who have severe lung disease or whose weight is less than the 10th percentile compared with other babies of the same length.
• American Indian and Alaska Native babies.

How is the RSV vaccine made?

Protein-based vaccines

Two of the RSV vaccines available for adults (Arexvy made by GSK [formerly GlaxoSmithKline] and Abrysvo made by Pfizer) are made of a single surface protein from the virus, called protein F. The gene for protein F is added to cells in the lab, so that as the cells grow, the protein is made too. It is then purified to remove the growth reagents and cellular debris.

There are two key differences between the two protein-based vaccines:

1. Antigen: RSV exists in two types, called A and B. Abrysvo contains the F protein from both types, whereas Arexvy only contains a single F protein. Because the F protein in both types tends to be the same and because both vaccines have the same quantity of F protein, this difference is not anticipated to make one vaccine better than the other.
2. Adjuvant: Adjuvants help create a better immune response to a vaccine. Arexvy contains the same adjuvant as the shingles vaccine, but Abrysvo does not contain an adjuvant.

Ongoing studies will determine if these differences are relevant to each vaccine’s effectiveness.

mRNA vaccine

Mresvia, made by Moderna, is an mRNA-based vaccine. The mRNA in the vaccine instructs a person’s cells to produce the same antigen (protein F) that is contained in the protein-based vaccines. When the protein is made, our immune system recognizes it as foreign and mounts an immune response against it. This is similar to how the COVID-19 mRNA vaccines work as shown in this animation.

How is the monoclonal antibody made?

The monoclonal antibody is one that can bind to the F protein on RSV, preventing the virus from binding to and entering cells. It is a type of antibody known as immunoglobulin G (IgG). This type of antibody is commonly found circulating in our blood. The gene for the IgG antibody is added to mammalian cells, so that when the cells reproduce in the lab, they also make the antibodies, which are then purified.

Does the RSV vaccine for adults work?

In clinical trials, a single dose of either protein-based RSV vaccine for adults prevented RSV infections associated with the lower respiratory tract in about 70 to 90 of 100 vaccine recipients. The mRNA-based RSV vaccine protected about 60 to 80 of 100 vaccine recipients against lower respiratory tract infections in clinical trials.

Because infections of the lower respiratory tract suggest a more severe infection, preventing these types of infections means that these vaccines will likely reduce hospitalizations and deaths associated with RSV. While rates of protection for the protein-based vaccines were similar in community evaluations based on the first year of availability, ongoing studies will continue to monitor the impact on the rates of hospitalizations and deaths, vaccine safety, how long protection lasts, and if additional doses will be needed later in life.

Does the RSV vaccine work when administered to pregnant people?

In clinical trials, a single dose of the RSV vaccine administered during pregnancy reduced the risk of an RSV infection leading to hospitalization during the first six months of life in 57 of 100 infants born to vaccinated individuals.

Does the monoclonal antibody for children work?

Yes. Babies who get nirsevimab are less likely to require medical intervention for an RSV infection, including going to the emergency room, being hospitalized, ending up in the intensive care unit or needing oxygen. The antibody product is estimated to prevent about 7 or 8 of every 10 babies who receive it from having severe RSV that requires these types of interventions.

Monoclonal antibodies are an example of protection gained through passive immunity, which is immunity gained in a way other than from our own immune systems. One common example of passive immunity that people often think of is when a baby gets antibodies from their mother through the placenta or breast milk. Other examples are “immunoglobulin treatments” given to people after they have been exposed to rabies or bitten by a snake.

What are the side effects of the RSV vaccine?

• Pain, redness and swelling where the shot is given, tiredness, fever, headache, nausea, diarrhea, and muscle or joint pain can happen after RSV vaccination.
• During the clinical trials for adults 60 and older, a small number of people experienced neurological effects, like Guillain Barré syndrome (GBS). However, the number of cases were too small to tell if they were related to receipt of the vaccine. In the first season of community use, GBS numbers were still too small to determine any relationship, so this will continue to be monitored as the vaccines are given to more people.
• During the clinical trials for pregnant people, slightly more vaccinated people delivered their babies prematurely compared to those who had not been vaccinated (control group). The numbers were too small to tell if these early births were related to receipt of the vaccine. Because of these concerns, the FDA approved the vaccine for use during a specific period later in pregnancy (32 through 36 weeks) to reduce the potential for complications related to early birth. In the first season of community use, the data were reassuring in that early births were within the expected range if someone was not vaccinated.  However, this will continue to be monitored as the vaccine is given to more people.

What are the side effects of the RSV monoclonal antibody?

• Injection site reactions including redness, tenderness and swelling, are the most common side effects. A small number of babies (less than 1 in 100) also developed a mild rash during the clinical trials.

Other questions you might have

Is the RSV monoclonal antibody a vaccine? 

No. The monoclonal antibody is not a vaccine, but it works to prevent infection by a process called passive immunization. Typically, when a vaccine is given, it will teach the person’s immune system to make a response to the infection, so that when someone encounters that infection in the future, their immune system is ready to fight it. This is known as active immunization. With passive immunization, on the other hand, the antibody is given to the person, so the person’s own immune system does not generate immunity. While passive immunity is effective for a short time, the quantity of antibodies will decrease over time, and the person will again become susceptible to the infection.

Wasn’t there another RSV product?

A monoclonal antibody, called palivizumab (Synagis), has been available since 1998. Palivizumab was used only for the highest risk babies. It was not widely used because babies needed to get a dose each month during RSV season, and it was expensive. The new product, nirsevimab, is longer-lasting, so one dose will be protective for the duration of a typical RSV season. Fewer doses also mean it will be less cost prohibitive.

When is the best time to get the RSV vaccine for adults?

Adults 75 years and older and those 60 to 74 years who are at high risk can receive the RSV vaccine at any time. However, getting the vaccine in late summer or early fall can ensure that antibody levels are highest during the RSV season that follows.

Pregnant people should receive the RSV vaccine from 32 through 36 weeks of gestation if it occurs during the months of RSV season. In most parts of the United States, this means vaccinations will occur between September and January. However, due to the seasonal variation of disease in some areas, the recommendation may vary slightly in different parts of the country. Individuals should check with their healthcare provider to find out about the timing in their area.

When is the best time for infants to get the monoclonal antibody?

Children younger than 8 months of age should get the monoclonal antibody before the start of RSV season in their area. In most cases, this means by October, though seasonality changes by geography. Infants born at the beginning of or during RSV season should ideally receive this preventative medication during the first week of life, either before going home from the hospital or during an early well-baby visit.

If I got the RSV vaccine during pregnancy, does my baby need the monoclonal antibody?

Infants born to a person who received the RSV vaccine at least two weeks prior to delivery do not need the monoclonal antibody. Some high-risk babies are recommended to receive a second dose during their second RSV season. If you are not sure if your baby is in this category, talk to their healthcare provider.

Can the RSV monoclonal antibody be given at the same time as vaccines?

Yes, the monoclonal antibody can be given at the same time as routinely recommended vaccines. It will not interfere with the immune response to vaccines, nor will the vaccines affect the ability of this medication to protect the baby from RSV.

Relative risks and benefits

Do the benefits of the RSV vaccine outweigh its risks?

Older and high-risk adults: The RSV vaccine for adults can cause mild side effects. On the other hand, RSV typically hospitalizes between 60,000-160,000 adults and kills up to 10,000 people each year. In the first year after receiving the vaccine, the risk of RSV associated lower respiratory tract disease, particularly severe lower respiratory tract disease, was greatly decreased.

Pregnant people: The RSV vaccine for pregnant people can cause mild side effects. The vaccination benefits the baby from the time of delivery through their first RSV season. Because RSV is one of the leading causes of hospitalization in young babies, with up to 80,000 related hospitalizations each year, early protection against RSV is beneficial.

Babies: The RSV monoclonal antibody for infants can cause mild side effects, including injection site reactions and, for some, a rash. On the other hand, RSV is one of the leading causes of hospitalization in this age group, with up to 80,000 babies hospitalized each year.

Disease risks

• Mild upper respiratory symptoms, including runny nose, congestion, cough
• Fever
• Lack of energy
• Loss of appetite
• Bronchitis (infection of the large breathing tubes)
• Bronchiolitis (infection of the small breathing tubes)
• Pneumonia (infection of the lungs)
• Croup (infection of the voice box)
• Apnea (temporary stopping of breathing)
• Worsening of asthma or other chronic conditions
• Disease can lead to hospitalization and death

Vaccine risks (adults)

• Pain, redness and swelling at the injection site
• Fever or muscle aches

Monoclonal antibody risks (infants)

• Pain, redness and swelling at the injection site
• Rash (less than 1 of 100 babies)

References

American Academy of Pediatrics. Respiratory syncytial virus. In: Kimberlin DW, Barnett ED, Lynfield R, Sawyer MH, eds. Red Book: 2021 Report of the Committee on Infectious Diseases. 32nd ed. American Academy of Pediatrics; 2021:628-636.

Centers for Disease Control and Prevention. (Aug. 4, 2023). Respiratory Syncytial Virus (RSV). Accessed Aug 22, 3023.

Hammitt LL, Dagan R, Yuan Y, et al. Nirsevimab for Prevention of RSV in Healthy Late-Preterm and Term Infants. N Engl J Med. 2022;386(9):837-846. doi:10.1056/NEJMoa2110275.

Kampmann B, Madhi SA, Munjal I, et al. Bivalent Prefusion F Vaccine in Pregnancy to Prevent RSV Illness in Infants. N Engl J Med. 2023; 388:1451-1564. doi: 10.1056/NEJMoa2216480

Melgar M, Britton A, Roper LE, et al. Use of Respiratory Syncytial Virus Vaccines in Older Adults: Recommendations of the Advisory Committee on Immunization Practices — United States, 2023. MMWR Morb Mortal Wkly Rep 2023;72:793–801.

Orenstein W, Offit PA, Edwards KM and Plotkin, SA. “Respiratory Syncytial Virus Vaccines and Monoclonal Antibodies.” In Plotkin’s Vaccines, 8th Edition. 2024, 998-1004.