The vaccine and monoclonal antibodies

Tools for preventing RSV became available in 2023. They protect two of the most susceptible age groups — infants and the elderly. The tools for each age group work differently: 

• For the elderly, the new tool is a traditional vaccine. The vaccine causes the person’s immune system to generate immunity. This is known as active immunization because the person is protected by their own immune system.
• For infants, two methods of protection became available. Both protect babies by passive immunization, meaning the protective immunity was not generated by the infant’s own immune system. Passive immunization provides short-term protection. The RSV tools to protect infants both rely on antibodies. One is a monoclonal antibody given to the baby. “Monoclonal” means that the antibodies in the preparation are identical, recognizing one part of a virus or bacteria. The second tool is vaccination of the mother during pregnancy. As her immune system responds to the vaccine, some of her antibodies go to the baby through the placenta. In both cases, the antibodies last long enough to protect infants during their first RSV season when they are most vulnerable to severe disease. Over time the antibodies fade. When older babies are exposed to RSV, they make their own immune response at a time when they are less susceptible to severe disease. 

The RSV vaccine: Who should get it?

Adults 75 years and older

Adults 75 years and older are recommended to get a single dose of an RSV vaccine. Three RSV vaccines are available for elderly adults (i.e., Abrysvo, Arexvy, or Mresvia). Additional doses in future RSV seasons are not currently recommended.

High-risk adults between 50 and 74 years of age 

Those 50 to 74 years of age with conditions that increase their risk for severe disease should get one dose of any of the three RSV vaccines. People at increased risk for severe disease include: 

• Those with chronic disease of the heart, lungs, liver or blood
• People with advanced kidney disease or diabetes with organ damage
• Individuals with immune-compromising, neurologic, or neuromuscular conditions
• Residents of nursing homes or other long-term care facilities
• Those considered medically frail
• People who have other chronic conditions that may increase the risk for severe respiratory infection 

Those 50 and older who received the RSV vaccine previously do not need another dose.

Pregnant people

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. For much of the U.S., this means getting vaccinated if that timing occurs from September through January. The seasonality of RSV varies slightly throughout the U.S. People in Alaska, Florida or outside of the continental U.S. 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 during pregnancy.

Those who received an RSV vaccine during a previous pregnancy should not get another dose during a future pregnancy. More data are needed 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 one of the monoclonal antibody products, nirsevimab or clesrovimab, after birth.

The RSV monoclonal antibody: Who should get it?

Infants younger than 8 months of age

Infants whose mothers were vaccinated against RSV between 32 and 36 weeks of pregnancy, most often do not need the RSV monoclonal antibody. But all other infants younger than 8 months of age, including those born during RSV season, are recommended to get it. RSV season for most infants would be between October and March, depending on where they live. Babies born during RSV season should get their dose within 1 week of birth, ideally before leaving the hospital. For those born between April and September, they should get vaccinated right before the start of RSV season in their area.

Infants should get one dose of RSV monoclonal antibody product, either nirsevimab (Beyfortus) or clesrovimab (Enflonsia). One dose can protect infants for five months, the length of an average RSV season.

A small number of babies whose mothers got vaccinated may still be recommended to get the monoclonal antibody, including:

• If the maternal vaccination occurred less than two weeks before delivery
• If the mother has a condition that may limit her ability to generate an adequate immune response (e.g., immune-compromising condition) or limit the transfer of antibodies across the placenta (e.g., HIV)
• If the baby has a procedure that decreases the presence of maternal antibodies
• If the baby is at increased risk for severe RSV due to medical conditions at birth or ongoing need for oxygen after hospital discharge

Infants between 8 months and 19 months of age

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.

Clesrovimab is not approved for use in infants during the second RSV season.

How is the RSV vaccine made?

Protein-based vaccines

Two of the RSV vaccines available for adults (Arexvy and Abrysvo) 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 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. Notably, the F protein in both types tends to be the same. Likewise, both vaccines have the same total quantity of F protein. For these reasons, this difference is not likely 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. Abrysvo does not contain an adjuvant.

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

mRNA vaccine

Mresvia is the third RSV vaccine. It is an mRNA-based vaccine. The mRNA in the vaccine instructs a person’s cells to produce protein F. This is the same protein that is contained in the other RSV 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 COVID-19 mRNA vaccines work as shown in this animation.

How are the monoclonal antibody products made?

Both monoclonal antibody products include an antibody that can bind to protein F on RSV. When the antibody binds to protein F, it prevents 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 in our blood. 

The gene for the IgG antibody is added to mammalian cells. When the mammalian cells reproduce in the lab, they also make the IgG antibodies. The antibodies are then purified to be given to infants.

Does the RSV vaccine work in adults?

How well the RSV vaccines work was measured by their ability to prevent lower respiratory tract infections. Infections of the lower respiratory tract are those in the lungs and breathing tubes. These types of RSV infections tend to be more severe, leading to hospitalizations and deaths. 

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.

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 their impact on the rates of hospitalizations and deaths. Studies will also continue to monitor vaccine safety, length of protection, and if additional doses will be needed.

Does the RSV vaccine work when given to pregnant people?

In clinical trials, a single dose of the RSV vaccine given during pregnancy reduced the risk of an RSV infection leading to infant hospitalization during the first six months of life in 5 or 6 of every 10 infants born to vaccinated individuals.

Does the monoclonal antibody for children work?

Babies who get nirsevimab or clesrovimab are less likely to require medical care for RSV. In studies, medical care included going to the emergency room, being hospitalized, ending up in the intensive care unit or needing oxygen. Both antibody products are estimated to prevent about 7 or 8 of every 10 babies who receive them from having severe RSV that requires these types of medical care.

What are the side effects of the RSV vaccine?

Common side effects

• Pain
• Redness and swelling where the shot was given
• Tiredness
• Fever
• Headache
• Nausea or diarrhea
• Muscle or joint pain 

In elderly vaccine recipients: Question about Guillain-Barré syndrome (GBS)

The clinical trials of the protein-based RSV vaccines (Abrysvo and Arexvy) for older adults found a small number of people experienced neurological effects, like Guillain Barré syndrome (GBS): a form of paralysis that starts in the legs and can travel upwards affecting the muscles that help us breathe. The number of cases was too small to tell if they were related to receipt of the vaccine. 

Studies completed after the vaccines became available have confirmed that the risk for GBS is greater in adults 65 years and older after getting RSV protein-based vaccines. It is estimated that GBS occurs rarely (in fewer than 10 people per 1 million doses of vaccine). GBS has not been identified after receipt of the mRNA-based version. Vaccine safety will continue to be monitored for all three vaccines.

In pregnant vaccine recipients: Question about premature births

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 the 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). This approach reduced the potential for early births. In the first season of community use, the data were reassuring. Rates of early births were similar to what would be expected if someone was not vaccinated. Safety monitoring will continue as the vaccine is given to more pregnant people.

What are the side effects of the RSV monoclonal antibody?

Side effects following receipt of the RSV monoclonal antibody are rare. They may include:

• Injection site reactions, like redness, tenderness and swelling
• Mild rash in a small number of babies (less than 1 in 100) 

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. As such, some professionals refer to it as an immunization, meaning something that gives immunity.

When a vaccine is given, it teaches the person’s immune system to make a response to the pathogen. When someone encounters the pathogen in the future, their immune system is ready to fight it. This is known as active immunization. 

With passive immunization, the antibody is given to the person. The person’s own immune system does not generate immunity. Passive immunity is effective for a short time, but the quantity of antibodies decreases over time, so 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. It was also expensive. The new products, nirsevimab and clesrovimab, are longer lasting, so one dose will be protective throughout a typical RSV season. Fewer doses also mean it is less expensive.

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

Adults 75 years and older and those 50 to 74 years who are at high risk can get 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 U.S., this means vaccinations most often occur between September and January. Due to the seasonal variation of RSV 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?

Babies 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 receive it 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 generally need the monoclonal antibody. A few exceptions exist based on the health of the baby or the mother. Healthcare providers can help determine if your baby is among this small group.

Some high-risk babies are recommended to receive one additional dose of nirsevimab during their second RSV season. If you are not sure if your baby is in this category, talk to their healthcare provider. Of note, clesrovimab is not approved for use in these babies.

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. In rare instances (less than 10 per 1 million doses), the protein-based versions may cause GBS. 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 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, 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. Up to 80,000 babies in the U.S. are hospitalized with RSV 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
• Protein-based versions: GBS (fewer than 10 cases per 1 million doses)

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. Respiratory Syncytial Virus (RSV). Accessed Aug 3, 2025.

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

Lloyd P. Evaluation of Guillain-Barré Syndrome (GBS) following Respiratory Syncytial Virus (RSV) Vaccination Among Adults 65 Years and Older. Centers for Disease Control and Prevention. ACIP meeting Oct. 23-24, 2024.

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.

Reviewed by Paul A. Offit, MD, on August 4, 2025