Issue 1739: February 8, 2024

Ask the Experts: Answers Questions about RSV Immunization (Part 1)

As an additional service to IZ Express readers, we periodically publish special editions such as this one, providing you with new and updated Ask the Experts questions and answers from experts. This issue includes 10 Q&As and is the first of two special editions focused on RSV disease and ACIP recommendations for RSV immunization of older adults, pregnant people, and infants. Part 2, coming next week, will include additional Q&As about RSV immunization timing, documentation, and administration errors.  
To find the full set of's Ask the Experts RSV Q&As, visit
You can find all of these questions and answers, plus more than 1,300 others about vaccines and vaccine administration, on our "Ask the Experts" main page at's team of experts includes Kelly L. Moore, MD, MPH (team lead), Carolyn B. Bridges, MD, FACP, and Iyabode Beysolow, MD, MPH.
RSV Disease
RSV Vaccines and Adults
RSV Preventive Antibody and Infants

RSV Disease

Q: What is respiratory syncytial virus (RSV)? 

A: Respiratory syncytial virus (RSV) is a common respiratory virus, usually causing mild, cold-like symptoms, such as cough, runny nose, sore throat, headache, fatigue, and fever. Though most people recover in a few days, for some people, RSV can cause serious lower respiratory tract infection (LRTI), such as bronchiolitis or pneumonia. RSV is the most common cause of hospitalization in infants in the United States. Older adults, especially those with chronic health conditions and those age 75 and older, are also more likely to develop severe RSV disease and need hospitalization. There is no specific treatment for RSV illness, only supportive care. 

RSV causes annual outbreaks of respiratory illness in people of all ages. In the continental United States, RSV typically circulates in the fall and winter months, between October and March, although its seasonality can vary locally from year to year. 

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Q: Among adults, who is most at risk for serious RSV infection?  

A: Most adults experience a mild upper respiratory tract infection with RSV, with symptoms lasting only a few days. Some adults with RSV develop lower respiratory tract disease, including pneumonia. Because public health surveillance and testing for RSV in adults been limited in the past, our estimates of the burden of RSV disease are not precise and may underestimate the true burden among adults. Among U.S. adults age 65 and older, RSV is responsible for approximately 60,000 to 160,000 hospitalizations and 6,000 to 10,000 deaths each year.

Older adults at highest risk for severe RSV infection generally have one or more chronic medical conditions, including:

  • lung diseases (including chronic obstructive pulmonary disease [COPD] and asthma)
  • cardiovascular diseases (such as congestive heart failure and coronary artery disease)
  • moderate or severe immune compromise (whether caused by disease or by treatment with an immunosuppressive medication)
  • diabetes mellitus
  • neurologic or neuromuscular conditions
  • kidney disorders
  • liver disorders
  • hematologic disorders

Other factors associated with increased risk include:

  • frailty
  • advanced age (hospitalization rates are highest among those age 75 or older)
  • residence in a nursing home or other long-term care facility

In addition to these listed factors, individual patients may have other underlying factors that a healthcare provider determines might increase the risk for severe respiratory disease.

Q: How serious is RSV in children? 

A: Almost all U.S. infants and toddlers contract RSV illness within the first two years of life. RSV causes a mild respiratory illness in most, with symptoms including cough, runny nose, fever, and fatigue. Illness is more likely to be mild if the child is older at the time of first infection. Infants with RSV infection frequently develop bronchiolitis, a lower respiratory tract disease (LRTD) that can be severe. 

RSV LRTD is the leading cause of hospitalization among U.S. infants, who may require supplemental oxygen, treatment for dehydration, or mechanical ventilation. Approximately 50,000–80,000 RSV-associated hospitalizations and 100–300 RSV-associated deaths occur each year among U.S. infants and children younger than age 5 years. The risk of severe RSV disease is increased by prematurity and lung disease, among other health conditions. However, RSV is also the leading cause of hospitalization among healthy, full-term infants. The large majority (almost 80%) of infants and children younger than age 2 who are hospitalized with RSV are otherwise healthy and have no underlying medical conditions.

Some otherwise healthy American Indian or Alaska Native (AI/AN) children experience higher rates of severe RSV disease than the general population. One study found that the incidence of RSV-associated hospitalization among children in their second RSV season in some AI/AN communities was 4 to 10 times higher than that of similar-aged children elsewhere in the United States. AI/AN children living in remote areas also may have difficulty accessing adequate medical care when they develop LRTD.

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RSV Vaccines and Adults

Q: What are the two RSV vaccines and how are they different? 

A: In May 2023, FDA licensed two RSV vaccines: RSVPreF3 (Arexvy, GSK) and RSVpreF (Abrysvo, Pfizer). They are both licensed for the prevention of RSV-associated lower respiratory tract disease (LRTD) in adults age 60 years and older in the United States. Only Abrysvo is licensed for use during pregnancy (during 32 through 36 weeks and 6 days’ gestation) for the prevention of RSV disease in infants. Both vaccines are recombinant protein vaccines that contain the prefusion form of a spike protein on the surface of the RSV virus. Because these vaccines contain only a piece of the virus, they cannot cause RSV illness.
The GSK RSVPreF3 vaccine, Arexvy, includes an AS01adjuvant, a chemical designed to enhance the immune response to vaccination. AS01 is the same adjuvant used in GSK’s recombinant zoster vaccine (Shingrix), but Arexvy contains half the amount of adjuvant as a dose of Shingrix. Pfizer’s vaccine does not contain an adjuvant. 
For further information about both vaccines, the package inserts are available from FDA: Package Insert – AREXVY ( and Package Insert – ABRYSVO (

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Q: How well do the RSV vaccines work for older adults?

A: Both RSVPreF3 (Arexvy, GSK) and RSVpreF (Abrysvo, Pfizer) vaccines were effective in preventing RSV-associated lower respiratory tract disease (LRTD) in clinical trial participants, over the course of two RSV seasons.

The global clinical trials for RSVPreF3 (Arexvy, GSK) vaccine involved nearly 25,000 participants and some participants were followed for 2 RSV seasons. One dose had an efficacy of 82.6% during the first RSV season and 56.1% during the second season in preventing symptomatic, laboratory-confirmed RSV-associated LRTD. 

The RSVpreF (Abrysvo, Pfizer) vaccine global clinical trials involved nearly 37,000 participants, and some participants were followed for 2 RSV seasons. Efficacy of 1 dose in preventing symptomatic, laboratory-confirmed RSV-associated LRTD was 88.9% during the first RSV season and 78.6% during a partial second season.

Few people enrolled in the clinical trials were either frail or of advanced age (80 or older), and none lived in long-term care facilities. People with immunocompromising conditions were excluded from clinical trials. For this reason, the clinical trials did not measure how well the vaccines would work in the people at highest risk of serious RSV disease. 

Q: What is the ACIP recommendation for RSV vaccination of older adults?

A: ACIP recommends use of either RSVpreF (Abrysvo, Pfizer) or RSVPreF3 (Arexvy, GSK) as a single dose in adults age 60 years and older, using shared clinical decision-making (SCDM). In SCDM-type recommendations, the choice to vaccinate is an option guided by a patient’s individual risk for severe disease and the patient’s values and preferences. SCDM also takes into account the provider’s clinical discretion concerning the patient and the vaccine.
ACIP recommends the following factors be considered during the shared clinical decision-making process: whether the patient has one or more risk factors for severe RSV disease; a patient’s risk of exposure to RSV (e.g., are they around young children, or living in a long-term care facility?); a patient’s preferences for RSV vaccination, including an understanding of safety, side effects, and costs; and the clinical discretion of the healthcare provider.
The risk of hospitalization with RSV disease rises with increasing age after age 60. Other factors that increase risk are overall frailty, residence in a long-term care facility or nursing home, and one or more chronic medical conditions. A separate question provides details on risk factors.
CDC has developed a simple job aid for providers to assist in these discussions:
See the full 2023 ACIP recommendation for older adults here:

Q: What is the recommendation for use of RSV vaccine during pregnancy? 

A: Pregnant people between 32 and 36 weeks and 6 days’ gestation during the months of September through January in the United States should get one dose of the Pfizer RSVpreF vaccine (Abrysvo) to protect their babies during their first RSV season after birth. Only Abrysvo is FDA-approved and recommended for pregnant people. Arexvy (by GSK) is not approved and should not be given during pregnancy.
In most of the continental United States, maternal RSV vaccine is recommended only September through January. Those who provide health care to pregnant people who live in areas with different patterns of RSV seasonality, such as Alaska, Hawaii, parts of Florida, or other jurisdictions outside the continental United States, should follow guidance from local or regional public health authorities about the timing of RSV vaccination during pregnancy in their regions. 

RSV Preventive Antibody and Infants

Q: How is nirsevimab (Beyfortus, Sanofi) different from a vaccine? 

A: Vaccination involves active immunization, where an antigen is administered to a recipient to activate the recipient’s immune system and generate an immune response (which includes developing antibodies). Active immunization may require up to 2 weeks to have its full protective effect, and sometimes a series of vaccinations is required. Protection may last for months or be life-long, depending upon the type of immune response triggered. The effectiveness of a vaccine depends on the recipient’s immune system.
Nirsevimab (Beyfortus, Sanofi) is an injectable, long-acting monoclonal antibody product that gives the recipient direct, immediate protection through passive immunization. The antibodies of nirsevimab circulate in the bloodstream and recognize and attach to the RSV virus if encountered, leading to elimination of the virus. These antibodies protect the patient for at least 5 months until they gradually break down and disappear. The highest risk of severe RSV infection and hospitalization for children is during their first RSV season as an infant. Nirsevimab will not prevent children from getting RSV infections in future seasons, but the general risk of hospitalization due to RSV in childhood is far lower after infancy.

Q: How well does nirsevimab work?

A: Clinical trials of nirsevimab in infants less than 8 months old born during or entering their first RSV season showed that giving nirsevimab reduced the risk of RSV-associated lower respiratory tract infection (LRTI) requiring a medical visit or hospitalization by approximately 80 percent and reduced the risk of ICU admission for this reason by 90 percent.
Clinical trials did not study hospitalization rates among older infants and toddlers at high risk of severe RSV disease in their second RSV season. Instead, a study was done to measure blood levels of nirsevimab given to infants and toddlers at increased risk for severe RSV disease (certain preterm infants and those with serious heart or lung disease). The blood levels of nirsevimab were equivalent to the levels in healthy infants in the clinical trial who received nirsevimab in their first RSV season. Based on this finding, it is estimated that their protection from serious infection would also be similar.

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Q: What is the ACIP recommendation for nirsevimab?

A: ACIP recommends one dose of nirsevimab (Beyfortus, Sanofi) preventive antibody for all infants less than 8 months and 0 days of age who are born during or entering their first RSV season.
ACIP also recommends one dose of nirsevimab for the following specific groups of infants and children age 8 through 19 months who are entering their second RSV season and at increased risk for severe RSV disease:

  • Children age 8 through 19 months at high risk and ineligible for palivizumab:
    • American Indian or Alaskan Native children entering their second RSV season. This is especially important for those who are in remote regions or in communities known to have increased rates of severe RSV disease in children. 
  • Children age 8 through 19 months at high risk and eligible for palivizumab (Synagis, AstraZeneca):
    • Children with chronic lung disease of prematurity who require medical support during the six months before the start of their second RSV season
    • Children who are severely immunocompromised
    • Children with evidence of severe cystic fibrosis (previous hospitalization for pulmonary exacerbation in the first year of life or abnormalities on chest imaging that persist when stable) or that have weight-for-length that is less than the 10th percentile
If nirsevimab is given to a child eligible for palivizumab, do not give palivizumab. If nirsevimab is unavailable, CDC recommends that palivizumab (a short-acting RSV monoclonal antibody) should be administered per AAP recommendations (see Because palivizumab is effective for 30 days, if nirsevimab becomes available during the RSV season, do not administer nirsevimab within 30 days of a dose of palivizumab. Do not give palivizumab during the season after administering nirsevimab.
The full ACIP recommendation is available at
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