RSV: vaccines for older adults, pregnancy and babies

A 31-year-old woman, 33 weeks pregnant, goes to a prenatal visit in early autumn. The pregnancy is singleton and uncomplicated, and the expected delivery date falls in the middle of respiratory virus season. She brings along her grandfather, 78 years old, who has chronic obstructive pulmonary disease, mild heart failure and controlled atrial fibrillation. Both have heard about vaccines against respiratory syncytial virus and about an antibody injection for the baby. She asks what applies to each of them. The scene is fictional and was written for teaching.

This text uses the case as a clinical reasoning scenario for students and health professionals. It compares the two vaccines approved in the United States for adults, Arexvy and Abrysvo, and the monoclonal antibody nirsevimab (brand name Beyfortus), given to the baby. The text explains the indication, the dose, the gestational window, the Guillain-Barré syndrome warning and what the studies showed. It does not replace your country's vaccination schedule or your institution's protocol. If you play the case of the day on NEXO, you will recognize the challenge: deciding with clues that arrive in sequence.

Key points

• Arexvy and Abrysvo are single-dose vaccines for adults aged 60 and over and for adults aged 18 to 59 at increased risk, according to current US package inserts.
• Only Abrysvo is indicated in pregnancy, between 32 and 36 weeks, to protect the baby from birth to 6 months; Arexvy is not for use in pregnant women.
• Nirsevimab is an antibody given to the baby: 50 mg below 5 kg and 100 mg from 5 kg in the first season.
• Since January 7, 2025, the FDA requires a warning on both vaccines about a possible increased risk of Guillain-Barré in the 42 days after the dose.
• In the studies, the maternal vaccine reduced severe disease in the baby by 81.8 percent in the first 90 days; Arexvy reduced lower respiratory disease by 82.6 percent in the first season in adults aged 60 and over.
• The regulatory dates in this text were checked against primary sources from the FDA and the EMA on October 24, 2026.

Why this is a reasoning scenario

Respiratory syncytial virus weighs most heavily at the two extremes of life: in small babies and in older adults with chronic diseases. For years there was no vaccine, and protection for babies was limited to high-risk groups. Between 2023 and 2026, three tools with different logics appeared, and the indications changed several times. For anyone caring for a family like this, the question is no longer whether protection exists but which product serves whom, and when.

The scenario is good for training because it forces you to separate three people: the pregnant woman, the fetus about to be born and the grandfather. Each has a different indication, and a common mistake is swapping the products. You need to check the gestational age, the expected delivery date, the local virus season, the grandfather's diseases and any history of Guillain-Barré. The post on signs, symptoms and history helps organize this collection, and the post on clinical decision-making reminds us that prevention is also a decision.

What the scene teaches

The scene trains six habits. The first is knowing which vaccine can be used in pregnancy and which cannot. The second is respecting the 32 to 36 week window. The third is understanding that the baby can be protected by the maternal vaccine or by the antibody. The fourth is recognizing older adults at higher risk. The fifth is discussing the Guillain-Barré warning with numbers. The sixth is recording the vaccine to avoid unnecessary repeat doses.

What is not at stake

The text does not discuss treatment of an infection already established, nor does it compare costs between countries. Recommendations on who should receive each product vary between national vaccination programs, and the text focuses mainly on package inserts and studies. The post on diagnostic uncertainty helps explain small risks and uncertain benefits to the patient.

Respiratory syncytial virus at the extremes of life

In the United States, a population-based surveillance network studied adults hospitalized with laboratory-confirmed respiratory syncytial virus between 2016 and 2023 (Havers et al., 2024). Among those hospitalized, the median age was 70 years, 62.6 percent were 65 or older and 39.7 percent were 75 or older (Havers et al., 2024). The authors estimated, for each season, the number of hospitalizations, intensive care admissions and in-hospital deaths across the country (Havers et al., 2024). The burden is concentrated in the oldest, which is why American recommendations now prioritize people aged 75 or older (Havers et al., 2024).

Who is at higher risk among adults

The American package inserts for the two vaccines describe a group aged 18 to 59 with increased risk of lower respiratory disease from the virus (American package insert for Arexvy, 7/2026). In the studies, chronic lung disease, chronic heart disease, diabetes and other conditions were called comorbidities of interest (American package insert for Arexvy, 7/2026). The grandfather in the scene is 78 years old, with chronic obstructive pulmonary disease and heart failure, which places him in a high-risk group by age and by disease.

And in babies

In babies, the virus is a common cause of bronchiolitis and hospitalization in the first months. The studies of protection for the baby measure mainly medically attended lower respiratory disease, severe disease and hospitalization (Simões et al., 2025). So when efficacy for the baby is discussed, it is always worth asking which outcome and in what period after birth.

Applying it to the family

The pregnant woman is not the main focus of prevention: the vaccine is given to her to protect the baby in the first months. The grandfather is the adult who benefits most from the vaccine for himself. Separating who is the target of each product avoids the most frequent confusion in the consultation. The post about differential diagnosis reminds us that, if the baby falls ill, other viruses also cause bronchiolitis.

Three products, three strategies

The three tools target the same protein of the virus, the F protein, which helps the virus fuse with the cell. The difference lies in how the protection reaches those who need it.

Arexvy

Arexvy, from GSK, is a vaccine with the F protein stabilized in the pre-fusion form and an adjuvant (American package insert for Arexvy, 7/2026). It is indicated for adults aged 60 or older and for adults aged 18 to 59 with increased risk (American package insert for Arexvy, 7/2026). The package insert says it is not for use in pregnant women (American package insert for Arexvy, 7/2026). In a study of an adjuvant-free vaccine with the same antigen, given to pregnant women, there were more preterm births than with placebo, which creates uncertainty about the use of Arexvy in pregnancy (American package insert for Arexvy, 7/2026).

Abrysvo

Abrysvo, from Pfizer, is a bivalent vaccine, with pre-fusion F protein from subtypes A and B, without adjuvant (American package insert for Abrysvo, 12/2025). It has three indications in the United States: pregnant women between 32 and 36 weeks, to protect the baby from birth to 6 months; adults aged 60 or older; and adults aged 18 to 59 with increased risk (American package insert for Abrysvo, 12/2025).

Nirsevimab

Nirsevimab is a long-acting monoclonal antibody against the F protein, given by intramuscular injection to the baby (American package insert for Beyfortus, 04/2026). It does not depend on the baby's immune response, because it delivers the antibody ready-made. The American package insert indicates the product for newborns and babies born during or entering their first virus season, and for children up to 24 months who remain vulnerable to severe disease in their second season (American package insert for Beyfortus, 04/2026).

The table summarizes the main differences, according to the current American package inserts.

Product	Who receives it	Dose	Source
Arexvy	Adults aged 60 or older; 18 to 59 with increased risk; not for pregnant women	Single dose of 0.5 mL, intramuscular	(American package insert for Arexvy, 7/2026)
Abrysvo	Pregnant women at 32 to 36 weeks; adults aged 60 or older; 18 to 59 with increased risk	Single dose of 0.5 mL, intramuscular	(US prescribing information for Abrysvo, 12/2025)
Nirsevimab, first season	Newborns and infants	50 mg under 5 kg; 100 mg from 5 kg	(US prescribing information for Beyfortus, 04/2026)
Nirsevimab, second season	Children up to 24 months who remain vulnerable	200 mg, in two 100 mg injections	(US prescribing information for Beyfortus, 04/2026)

Regulatory status: FDA and EMA

Arexvy in the United States. The FDA approved Arexvy on May 3, 2023 for adults aged 60 and older (FDA safety communication, 7/1/2025). The indication was later expanded to adults aged 50 to 59 at increased risk (FDA safety communication, 7/1/2025). The FDA page lists later approval letters up to July 27, 2026, and the current indication includes adults aged 18 to 59 at increased risk (FDA page on Arexvy). The prescribing information consulted was revised in July 2026 and marks indication changes in March 2026 (US prescribing information for Arexvy, 7/2026).

Abrysvo in the United States. The FDA approved Abrysvo on May 31, 2023 for adults aged 60 and older (FDA safety communication, 7/1/2025). The approval letter for pregnant women is dated August 21, 2023, and the most recent letter listed by the FDA is dated December 19, 2025 (FDA page on Abrysvo). The prescribing information consulted was revised in December 2025 (US prescribing information for Abrysvo, 12/2025).

Nirsevimab in the United States. The FDA approved Beyfortus on July 17, 2023, under BLA 761328 (Drugs@FDA, BLA 761328). The most recent supplement listed in Drugs@FDA was approved on April 7, 2026, and the prescribing information consulted is from April 2026 (Drugs@FDA, BLA 761328).

European Union. The EMA issued the marketing authorization for Beyfortus on October 31, 2022 (EMA, Beyfortus), for Arexvy on June 6, 2023 (EMA, Arexvy) and for Abrysvo on August 23, 2023 (EMA, Abrysvo). European indications have their own wording and may not match the US ones, so it is worth reading the summary of product characteristics before use.

Other countries. This text does not describe registration in other countries. Availability and recommendations depend on the local agency and vaccination program.

The Guillain-Barré warning

On January 7, 2025, the FDA announced that it had required and approved changes to the prescribing information for Abrysvo and Arexvy, with a new warning about Guillain-Barré syndrome (FDA safety communication, 7/1/2025). The text says that a post-marketing observational study suggests an increased risk in the 42 days after the vaccine (FDA safety communication, 7/1/2025). The current prescribing information includes the numbers from that study in people aged 65 and older (US prescribing information for Arexvy, 7/2026).

• Arexvy: estimated excess of 7 Guillain-Barré diagnoses per million doses in people aged 65 and older.
• Abrysvo: estimated excess of 9 diagnoses per million doses in the same age range.
• The prescribing information says that the background risk of the studied population changes the estimate and prevents direct comparison with other vaccines.
• In the studies, atrial fibrillation in the 30 days after the dose was reported more often with the vaccines than with placebo, but the data are not enough to say whether there is a causal link.

Sources for the list: (US prescribing information for Arexvy, 7/2026) and (US prescribing information for Abrysvo, 12/2025). The warning does not prohibit the vaccine, but it calls for a frank conversation, with absolute numbers, especially with anyone who has already had Guillain-Barré.

The 78-year-old grandfather: vaccine, dose and warnings

For the grandfather, the two adult vaccines are options according to the US prescribing information. Both are given as a single 0.5 mL intramuscular dose (US prescribing information for Arexvy, 7/2026) (US prescribing information for Abrysvo, 12/2025). Neither of the prescribing information documents describes a booster dose approved for adults without immunosuppression.

What the Arexvy study showed

In the phase 3 study in adults aged 60 and older, Arexvy reduced the risk of lower respiratory tract disease caused by the virus by 82.6 percent in the first season, with a median follow-up of 6.7 months (US Arexvy label, 7/2026). In the 70 to 79 age group, efficacy was 93.8 percent, and in the group with at least one comorbidity of interest it was 94.6 percent (US Arexvy label, 7/2026). In those aged 80 or older, there were few cases, and the confidence interval was very wide (US Arexvy label, 7/2026). In the second season, efficacy fell to 56.1 percent, and in the third to 48.0 percent (US Arexvy label, 7/2026).

And a second dose one year later?

The same study tested a new dose before the second season (Ison et al., 2024). The efficacy of one dose followed by revaccination, adding both seasons together, was 67.1 percent (Ison et al., 2024). The current US label still describes the single dose (US Arexvy label, 7/2026). The post about diagnostic pause reminds you to check the label before vaccinating again.

What the Abrysvo study in older adults showed

In the phase 3 study in adults aged 60 and older, with a median follow-up of 7 months, Abrysvo had efficacy of 66.7 percent against lower respiratory tract disease with two or more symptoms and 85.7 percent with three or more symptoms (US Abrysvo label, 12/2025). In the second season, efficacy was 55.7 percent and 77.8 percent for the same outcomes (US Abrysvo label, 12/2025).

Protection outside the studies

In a US test-negative design study, in the 2023 to 2024 season, vaccine effectiveness against hospitalization for the virus was 80 percent in immunocompetent adults aged 60 and older (Payne et al., 2024). Effectiveness was similar between 60 and 74 years and from 75 years on (Payne et al., 2024). A living systematic review gathered real-world data on the vaccines for older adults, nirsevimab and the maternal vaccine (Lee et al., 2025).

Along with the flu vaccine

In a phase 3 study in Australia, the immune responses to Abrysvo and the flu vaccine were noninferior when the two were given on the same day (Athan et al., 2024). In a study with Arexvy and the adjuvanted flu vaccine, giving them together was well tolerated (Clark et al., 2024). These data help when grandfather needs both vaccines at the start of autumn.

Applying it to grandfather

Grandfather has an age and conditions that justify the vaccine according to both labels. Before giving it, you need to ask whether he has already received any vaccine against the virus, whether he has had Guillain-Barré or a severe allergy to a vaccine, and whether he uses immunosuppressants. Controlled atrial fibrillation does not appear as a contraindication in the labels, but it is worth knowing that the studies recorded more episodes in the vaccinated group (US Arexvy label, 7/2026). The post about interprofessional clinical reasoning shows how pharmacy and nursing help check the vaccination history.

The pregnant woman at 33 weeks and the baby

The pregnant woman is at 33 weeks, within the 32 to 36 week window of the US Abrysvo label (US Abrysvo label, 12/2025). Arexvy is not for her, because the label says it is not for use in pregnant women (US Arexvy label, 7/2026). The maternal vaccine is given as a single 0.5 mL dose (US Abrysvo label, 12/2025).

What the maternal vaccine study showed

In the phase 3 study, the vaccine given to the mother had an efficacy of 81.8 percent against severe lower respiratory tract disease in the infant with medically attended care in the first 90 days, and 69.4 percent at 180 days (Simões et al., 2025). Against medically attended lower respiratory tract disease of any severity, efficacy was 57.1 percent at 90 days and 51.3 percent at 180 days, and the success criterion was not met at 90 days (US Abrysvo label, 12/2025). In the final analysis, efficacy against hospitalization for the virus was 69.7 percent at 90 days and 55.3 percent at 180 days (Simões et al., 2025).

Why the window starts at 32 weeks

In the analysis of the same study, preterm births were 5.7 percent in the vaccine group and 4.7 percent in the placebo group, with a relative risk of 1.20 and a confidence interval that includes the null value (Madhi et al., 2025). The label says the data are not enough to prove or to rule out a causal relationship, and calls for vaccination between 32 and 36 weeks to avoid the possible risk before 32 weeks (US Abrysvo label, 12/2025). In the study, the vaccine was given between 24 and 36 weeks, and fewer than half of the participants were vaccinated within the window approved today (Jasset et al., 2025).

The timing within the window

A study of antibody transfer across the placenta showed that vaccinating at least 5 weeks before delivery gave a much more efficient transfer to the umbilical cord than vaccinating 3 to 4 weeks before (Jasset et al., 2025). For the pregnant woman in the scene, vaccinating at 33 weeks generally leaves enough time before a term delivery. If the baby is born earlier than expected, the neonatology team should check whether maternal protection had time to form.

When to think about nirsevimab

Nirsevimab is given to the baby. In the first season, the dose is 50 mg if the baby weighs less than 5 kg and 100 mg from 5 kg (US Beyfortus label, 04/2026). Babies born during the season can receive the antibody from birth; those born outside it receive a dose before the season starts (US Beyfortus label, 04/2026). Children who remain vulnerable in the second season receive 200 mg, in two injections of 100 mg (US Beyfortus label, 04/2026). The label also provides for an extra dose after cardiac surgery with extracorporeal circulation (US Beyfortus label, 04/2026).

What the nirsevimab studies showed

In a study in premature infants from 29 to less than 35 weeks, nirsevimab reduced medically attended lower respiratory tract infection by the virus by 70.1 percent up to 150 days after the dose (US Beyfortus label, 04/2026). In an open-label study with more than 8,000 babies, the reduction in hospitalization for lower respiratory tract infection by the virus was 83.2 percent during the season (US Beyfortus label, 04/2026). In an analysis of another phase 3 study, the antibody reduced infections by the virus without increasing infections by other respiratory viruses (Arbetter et al., 2025). In the systematic review of real-world data, the pooled effectiveness against hospitalization for the virus was 80.7 percent (Lee et al., 2025).

Maternal vaccine or antibody?

Both strategies protect the baby in the first months, but by different paths: one passes antibodies from the mother across the placenta, the other delivers the antibody directly to the baby. The choice between them, and the situation in which the baby of a vaccinated mother should also receive the antibody, follow the recommendation of each country's national program. What belongs in the consultation is recording the date of the maternal vaccine, because that is what the delivery team will use to decide. The post on simulation of care helps train this explanation for the family.

How NEXO helps train this reasoning

In NEXO, the case of the day is fictional, with a real clinical presentation and up to six progressive clues, and you propose the diagnosis by searching for the ICD-10 code. Prevention of respiratory syncytial virus is a good exercise because a gestational age, a vaccine date or a history of Guillain-Barré changes the approach. The post on case-based learning explains why this format fixes knowledge.

Three exercises with cases

Change the gestational age. Say what to do if the pregnant woman were at 28 weeks.

Change the delivery. Say what to check if the baby were born 10 days after the maternal vaccine.

Change the grandfather's history. Say how to talk with him if he had had Guillain-Barré a few years ago.

The limits on using the game are described on the Economy page of NEXO. If you research the teaching of clinical reasoning, the page for Researcher explains how to get in touch. Professionals who write or review cases find information on Curator and on Creators, and institutions can learn about the proposal on the partnerships page.

Frequently asked questions

Can a pregnant woman receive Arexvy?

No. The US package insert says that Arexvy is not for use in pregnant women (US package insert for Arexvy, 7/2026).

In which week is the maternal vaccine given?

Between 32 and 36 weeks of pregnancy, according to the US package insert for Abrysvo (US package insert for Abrysvo, 12/2025).

Do older adults need a booster every year?

The US package inserts describe a single dose (US package insert for Arexvy, 7/2026) (US package insert for Abrysvo, 12/2025). One study tested revaccination after one year (Ison and colleagues, 2024), but that is not part of the current package insert.

What is the dose of nirsevimab?

In the first season, 50 mg below 5 kg and 100 mg from 5 kg; in the second, 200 mg (US package insert for Beyfortus, 04/2026).

Is the risk of Guillain-Barré high?

The package inserts cite an estimated excess of 7 to 9 diagnoses per million doses in people aged 65 or older (US package insert for Arexvy, 7/2026) (US package insert for Abrysvo, 12/2025).

Final checklist

• I confirmed the gestational age and that the pregnant woman is between 32 and 36 weeks.
• I did not prescribe Arexvy during pregnancy.
• I recorded the date of the maternal vaccine for the delivery team.
• I explained that the baby can also be protected by nirsevimab, according to the local program.
• I checked whether the grandfather has already received the vaccine against the virus.
• I asked about Guillain-Barré, severe allergy to the vaccine and immunosuppression.
• I explained the Guillain-Barré warning with absolute numbers.
• I arranged the flu vaccine and the record of both doses.

Want to practice reasoning with progressive clues? Play the case of the day.

References

1. Arbetter D, Gopalakrishnan V, Aksyuk AA, et al. Lower Respiratory Tract Infections Following Respiratory Syncytial Virus Monoclonal Antibody Nirsevimab Immunization Versus Placebo: Analysis From a Phase 3 Randomized Clinical Trial (MELODY). Clin Infect Dis. 2025. PMID 39656748.
2. Athan E, Baber J, Quan K, et al. Safety and Immunogenicity of Bivalent RSVpreF Vaccine Coadministered With Seasonal Inactivated Influenza Vaccine in Older Adults. Clin Infect Dis. 2024. PMID 37992000.
3. Clark R, Davies S, Labrador J, et al. Safety and Immunogenicity of Respiratory Syncytial Virus Prefusion F Protein Vaccine when Co-administered with Adjuvanted Seasonal Quadrivalent Influenza Vaccine in Older Adults: A Phase 3 Randomized Trial. Clin Infect Dis. 2024. PMID 39099085.
4. European Medicines Agency. Abrysvo | European Medicines Agency (EMA). Authorization issued on 23/08/2023. Accessed on 24/10/2026. Official source.
5. European Medicines Agency. Arexvy | European Medicines Agency (EMA). Authorization issued on 06/06/2023. Accessed on 24/10/2026. Official source.
6. European Medicines Agency. Beyfortus | European Medicines Agency (EMA). Authorization issued on 31/10/2022. Accessed on 24/10/2026. Official source.
7. GlaxoSmithKline. AREXVY (Respiratory Syncytial Virus Vaccine, Adjuvanted) for injectable suspension, for intramuscular use. Prescribing Information. Revised in 7/2026. Accessed on 24/10/2026. Official source.
8. Havers FP, Whitaker M, Melgar M, et al. Burden of Respiratory Syncytial Virus-Associated Hospitalizations in US Adults, October 2016 to September 2023. JAMA Netw Open. 2024. PMID 39535791.
9. Ison MG, Papi A, Athan E, et al. Efficacy and Safety of Respiratory Syncytial Virus (RSV) Prefusion F Protein Vaccine (RSVPreF3 OA) in Older Adults Over 2 RSV Seasons. Clin Infect Dis. 2024. PMID 38253338.
10. Jasset OJ, Lopez Zapana PA, Bahadir Z, et al. Enhanced placental antibody transfer efficiency with longer interval between maternal respiratory syncytial virus vaccination and birth. Am J Obstet Gynecol. 2025. PMID 39515450.
11. Lee B, Trusinska D, Ferdous S, et al. Real-world effectiveness and safety of nirsevimab, RSV maternal vaccine and RSV vaccines for older adults: a living systematic review and meta-analysis. Thorax. 2025. PMID 40930981.
12. Madhi SA, Kampmann B, Simões EAF, et al. Preterm Birth Frequency and Associated Outcomes From the MATISSE (Maternal Immunization Study for Safety and Efficacy) Maternal Trial of the Bivalent Respiratory Syncytial Virus Prefusion F Protein Vaccine. Obstet Gynecol. 2025. PMID 39746206.
13. Payne AB, Watts JA, Mitchell PK, et al. Respiratory syncytial virus (RSV) vaccine effectiveness against RSV-associated hospitalisations and emergency department encounters among adults aged 60 years and older in the USA, October, 2023, to March, 2024: a test-negative design analysis. Lancet. 2024. PMID 39426837.
14. Pfizer. ABRYSVO® (Respiratory Syncytial Virus Vaccine) for injection, for intramuscular use. Prescribing Information. Revised in 12/2025. Accessed on 24/10/2026. Official source.
15. Sanofi Pasteur. BEYFORTUS (nirsevimab-alip) injection, for intramuscular use. Prescribing Information. Revised in 04/2026. Accessed on 24/10/2026. Official source.
16. Simões EAF, Pahud BA, Madhi SA, et al. Efficacy, Safety, and Immunogenicity of the MATISSE (Maternal Immunization Study for Safety and Efficacy) Maternal Respiratory Syncytial Virus Prefusion F Protein Vaccine Trial. Obstet Gynecol. 2025. PMID 39746212.
17. U.S. Food and Drug Administration. ABRYSVO. Product page with approval letters. Accessed on 24/10/2026. Official source.
18. U.S. Food and Drug Administration. AREXVY. Product page with approval letters. Accessed on 24/10/2026. Official source.
19. U.S. Food and Drug Administration. Drugs@FDA: FDA-Approved Drugs. BLA 761328, Beyfortus. Accessed on 24/10/2026. Official source.
20. U.S. Food and Drug Administration. FDA Requires Guillain-Barré Syndrome (GBS) Warning in the Prescribing Information for RSV Vaccines Abrysvo and Arexvy: FDA Safety Communication. Published on 7/1/2025. Official source.