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Ask the Experts: RSV (Respiratory Syncytial Virus)

Results (93)

The 50-mg manufacturer-filled syringes (MFS) of nirsevimab (Beyfortus) should be reserved for infants weighing less than 5 kilograms (less than 11 pounds). The cost of a single MFS is the same, whether it is a 50-mg MFS or a 100-mg MFS. Insurers are likely to cover only the cost of one MFS dose to an infant younger than age 8 months. Only high-risk children age 8 through 19 months are recommended to receive two (100-mg) MFS doses at the same visit.

Last reviewed: August 24, 2025

This question has arisen more frequently since the introduction of the RSV preventive antibody, nirsevimab (Beyfortus, Sanofi), which has an injection volume of 1 mL for infants younger than age 8 months who weigh 5 kg or more at the time of immunization. High-risk children entering their second RSV season require a Beyfortus dose volume of 2 mL. Beyfortus is often administered at a routine visit when other infant immunizations are due.

There is no specific guidance to not exceed 1 mL in one muscle. In fact, there is no clear standard of practice and reference texts vary in guidance. Facilities or health systems may have medication policies/procedures that outline guidance for their staff. Professional judgement is needed when administering intramuscular medications or immunizations to people, including children, because muscle size varies from person to person.

CDC immunization education experts suggest a range of volume, depending upon the muscle injected. For the deltoid, the typical volume injected is 0.5 mL (maximum: 2 mL). For the vastus lateralis (the thigh): the typical volume that may be injected is 1–4 mL (maximum: 5 mL). Infants and toddlers fall at the lower end of these ranges, whereas adolescents and adults generally fall on the higher end of the range.

If more than 1 mL of volume needs to be injected into the thigh, that can be done while staying well within the acceptable range. Use of combination vaccines, when indicated and available, can decrease injection volume.

Last reviewed: January 18, 2026

Yes, nirsevimab (Beyfortus, Sanofi) or clesrovimab (Enflonsia, Merck) may be coadministered with all other recommended age-appropriate vaccines. An RSV preventive antibody product should not interfere with the immune response to routine childhood vaccines when given together or at any time before or after them. Likewise, vaccination does not interfere with the effectiveness of the preventive antibody product.

When giving several injections at a single visit, separate intramuscular vaccines by at least 1 inch in the body of the muscle, if possible, to reduce the likelihood of overlapping local injection site reactions.

Last reviewed: August 24, 2025

No, these two antibody preparations should not interfere with each other. Administer nirsevimab (Beyfortus, Sanofi) or clesrovimab (Enflonsia, Merck) as recommended.

Last reviewed: August 24, 2025

Nirsevimab (Beyfortus, Sanofi) and clesrovimab (Enflonsia, Merck) come in manufacturer-filled syringes (MFS). In most states, anyone who can administer injections can administer them. If you have questions about a specific type of healthcare worker, check the scope of practice rules in your state.

Last reviewed: August 24, 2025

Only infants younger than age 8 months 0 days are routinely recommended to receive an RSV preventive antibody product during or before their first RSV season. The recommended use of nirsevimab in older infants and toddlers age 8 months through 19 months is narrowly limited to American Indian/Alaska Native children and children with specific conditions that put them at high risk of severe lower respiratory tract disease due to RSV.

Last reviewed: August 24, 2025

The recipient should be informed of the error, and RSV vaccine should be administered as recommended. A 50-mg or 100-mg MFS dose of antibody is very small compared to the body weight of an adult and you should not assume the dose would have any protective effect. There is no defined waiting period after antibody administration for vaccine administration. Facilities that stock RSV vaccine and RSV preventive antibody products should put systems and procedures in place to prevent this type of error, including staff training and clear labeling and warnings in storage units.

CDC strongly encourages reporting of this medication error and any suspected adverse events following the error to the Vaccine Adverse Event Reporting System (VAERS) at https://vaers.hhs.gov if the antibody was administered at the same visit as vaccines. If antibody was administered alone, report the incident to MedWatch online (www.fda.gov/medwatch), by fax, by mail, or by contacting FDA at 1-800-FDA-1088.

Last reviewed: August 24, 2025

Vaccination will have the most benefit if administered in late summer or early fall, just before the RSV season. In most of the continental United States, this corresponds to vaccination during August–October.

If you have an opportunity to vaccinate an eligible patient and are concerned that there will not be an opportunity to vaccinate during an ideal time of year, you may administer RSV vaccine at any time of year. A meaningful degree of protection after vaccination should last at least two years.

Last reviewed: August 24, 2025

Aim for nirsevimab (Beyfortus, Sanofi) or clesrovimab (Enflonsia, Merck) administration in the first week of life for infants born shortly before or during the RSV season (typically October through March, unless directed otherwise by state or territorial public health officials). Ideally, eligible infants should be immunized before discharge from the birthing facility. Eligible infants with prolonged birth hospitalizations due to prematurity or other causes should be immunized shortly before or promptly after discharge.

Infants younger than age 8 months born outside of the RSV season, as well as older children at high risk who are recommended be protected in their second RSV season, should aim to be immunized shortly before the start of the RSV season (typically October).

If the ideal timing is missed, age-eligible infants and children who have not yet received a dose may be immunized at any time during the RSV season.

If you are located in Alaska, Hawaii, or another region of the United States with a different pattern of RSV circulation, follow the timing guidance of your state or territorial public health officials.

Last reviewed: August 24, 2025

CDC has published an immunization information statement (IIS) for RSV preventive antibody products that is the equivalent of the vaccine information statement (VIS) for vaccines. The version available at this time only names nirsevimab, but should also be used with clesrovimab until an updated version is produced. Just as with a VIS, providers should give the IIS to the parent or caregiver before immunization and document it in the medical record.

Access the current RSV preventive antibody IIS and translations in numerous languages from Immunize.org at: www.immunize.org/vaccines/vis/iis-rsv/.

Last reviewed: August 24, 2025

Yes, you should report administration of the RSV preventive antibody products (nirsevimab [Beyfortus, Sanofi] and clesrovimab [Enflonsia, Merck]) to your state immunization information system (IIS, or “registry”) as you would report vaccine administration. Contact your state immunization program if you have specific questions about reporting to your state immunization information system.

Last reviewed: August 24, 2025

It is important that all healthcare providers, both prenatal and pediatric, ensure that maternal RSV vaccination status is clearly documented and communicated. Prenatal care providers and birthing hospitals should ensure maternal RSV vaccination is reported to state immunization information systems (registries) and documented in maternal and newborn health records. It is also important to provide the pregnant person with a personal record of immunization.

Failure to document and communicate maternal RSV vaccination may result in extra work for pediatric offices and families to obtain records or in unnecessary administration of an RSV preventive antibody product.

RSV vaccination is recommended only once at this time, so the history of RSV vaccination is also important information for future pregnancies when the mother would need to be counseled that RSV vaccination is not an option and the infant should receive a preventive antibody product after delivery for RSV prevention.

Last reviewed: August 24, 2025


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Last reviewed: August 24, 2025

In clinical trials of the three licensed and recommended RSV vaccines, Arexvy (GSK), Pfizer (Abrysvo), and mResvia (Moderna), mild, local injection site reactions (redness, swelling), fatigue, muscle aches, and headache were common.

Last reviewed: August 24, 2025

In the clinical trials of both RSVPreF3 (Arexvy, GSK) and RSVpreF (Abrysvo, Pfizer) vaccines, a small number of inflammatory neurologic events, including GBS, were reported after RSV vaccination. Further FDA studies presented to the ACIP in October 2024 suggest a small increased risk of GBS of a similar magnitude for both products, approximately 7-9 excess cases per 1 million doses given. No cases of GBS or other inflammatory neurologic events were reported after mRNA RSV (mResvia) vaccination during the clinical trials. CDC and FDA continue to actively monitor the safety of all of these vaccines through national safety surveillance systems.

Estimates presented to ACIP in October 2024 comparing the benefit of RSV vaccination in the older adult population for whom it is recommended to the small increased risk of GBS following Arexvy or Abrysvo show that vaccination of a million people would prevent thousands more hospitalizations and hundreds more deaths than the small number of GBS cases vaccination may trigger. Because the benefits of preventing RSV in the recommended population far outweigh the potential risk of GBS, no changes to recommendations have been made.

Last reviewed: August 24, 2025

In clinical trials, the vast majority of infants had no side effects detected after nirsevimab (Beyfortus, Sanofi) or clesrovimab (Enflonsia, Merck) administration. For both products, the proportion of infants experiencing side effects was very similar in the intervention and placebo groups.

The most common side effects noted during the clinical trials of nirsevimab were rash occurring within 2 weeks after injection (seen in 0.9% of nirsevimab recipients versus 0.6% of placebo recipients) and injection site reactions (including redness, pain, swelling) occurring within 7 days after injection (0.3% of nirsevimab recipients versus 0% of placebo recipients).

Similar reactions were seen with clesrovimab. Rash was seen within two weeks after injection (2.3% of clesrovimab recipients vs. 1.9% of placebo recipients). Redness or swelling at the injection site each were reported in fewer than 4% of clesrovimab recipients, with a similar proportion of placebo recipients also reporting these side effects. See the product package inserts for more details.

Last reviewed: August 24, 2025

Adverse reactions occurring after administration of nirsevimab (Beyfortus, Sanofi) alone or clesrovimab (Enflonsia, Merck) should be reported to MedWatch online (www.fda.gov/medwatch), by fax, by mail, or by contacting FDA at 1-800-FDA-1088.

Adverse reactions occurring after the coadministration of nirsevimab or clesrovimab with a vaccine should be reported to the Vaccine Adverse Event Reporting System (VAERS) https://vaers.hhs.gov, and reports should specify that the patient received nirsevimab or clesrovimab on the VAERS form.

Last reviewed: August 24, 2025

Yes. V-safe is a vaccine safety monitoring system that lets recipients of certain new vaccines share with CDC how they feel after vaccination by receiving and responding to a series of periodic text messages. This is a voluntary system, and individual recipients must register to participate V-safe. V-safe is available to Arexvy (GSK), Abrysvo (Pfizer), and mResvia (Moderna) recipients. For information, or to register, visit CDC’s V-safe website at: www.cdc.gov/vaccine-safety-systems/v-safe/index.html.

Last reviewed: August 24, 2025

Nirsevimab (Beyfortus, Sanofi) and clesrovimab (Enflonsia, Merck) are contraindicated in persons with a history of severe allergic reactions (e.g., anaphylaxis) after a previous dose of the same product or to a product component. As with any injection, when administering nirsevimab or clesrovimab to children with increased risk for bleeding, providers should follow ACIP’s general best practice guidelines for immunization of children with bleeding disorders: www.cdc.gov/vaccines/hcp/imz-best-practices/special-situations.html#cdc_report_pub_study_section_8-vaccinating-persons-with-increased-bleeding-risk.

Last reviewed: August 24, 2025

RSV vaccines are contraindicated for and should not be administered to persons with a history of severe allergic reaction, such as anaphylaxis, to any component of the vaccine. People experiencing moderate or severe acute illness with or without fever should delay RSV vaccination until they are improved, as a precaution.

Last reviewed: August 24, 2025

Both protein-based vaccines (Abrysvo, Pfizer; Arexvy, GSK) are stored in the refrigerator. RSVPreF3 (Arexvy) is supplied as a single-dose vial of lyophilized antigen component (powder) and a single-dose vial of adjuvant suspension component (liquid). Both the liquid and the powder should be stored refrigerated between 2°C and 8°C (36°F and 46°F) in the original package in order to protect vials from light. Do not freeze. Discard if either component has been frozen.

After reconstitution, administer immediately or store in the refrigerator between 2°C and 8°C (36°F to 46°F) or at room temperature (defined for this vaccine as up to 25°C [77°F]) for up to 4 hours before use. Discard reconstituted vaccine if not used within 4 hours.

Last reviewed: August 24, 2025

Both protein-based RSV vaccines (Abrysvo, Pfizer; Arexvy, GSK) are stored in the refrigerator. RSVpreF (Abrysvo) is supplied in a kit that includes a vial of lyophilized antigen component (a white powder), a prefilled syringe with a sterile water diluent, and a vial adapter. Store at refrigerated temperatures between 2°C and 8°C (36°F and 46°F) in the original carton. Do not freeze. Discard if frozen. Follow reconstitution guidelines at www.fda.gov/media/168889/download.

After reconstitution, administer immediately or store at room temperature (defined for this vaccine as 15ºC to 30ºC [59°F to 86°F]) for up to 4 hours. Do not store reconstituted Abrysvo vaccine in the refrigerator or freezer. Discard reconstituted vaccine if not used within 4 hours.

Last reviewed: August 24, 2025

The mRNA RSV vaccine (mResvia, Moderna) is supplied as a manufacturer-filled syringe (MFS) that contains a frozen suspension that must be thawed before administration either in the refrigerator or at room temperature. Vaccine should not be refrozen once thawed. Do not shake any mRNA vaccine because shaking can damage the potency of the vaccine by breaking up the small bubbles of fat (lipid) that contain the mRNA.

  • Freezer storage: The frozen vaccine in the plastic MFS can be stored in the freezer between -40°C to -15°C (-40°F to 5°F) until the expiration date printed on its packaging.
  • Refrigerator storage: The thawed plastic MFS may be stored refrigerated between 2°C to 8°C (36°F to 46°F) for up to 30 days before being used. If stored in the refrigerator, place a label on the vaccine indicating when it was placed in the refrigerator and its ‘use by date’, which is 30 days later. Discard an MFS that has been in the refrigerator beyond its 30-day use-by date.
  • Room temperature storage: The thawed plastic MFS may be stored between 8°C to 25°C (46°F to 77°F) for a total of 24 hours after removal from refrigerated conditions. Discard the MFS if not used within this time. Do not return an MFS to the refrigerator after it has been thawed at room temperature.
  • Refer to the mResvia package insert for specific instructions and additional details on storage and thawing: www.fda.gov/media/179005/download.
Last reviewed: August 24, 2025

Nirsevimab (Beyfortus, Sanofi) comes in two single-dose manufacturer-filled syringe (MFS) formulations: 50-mg and 100-mg. Nirsevimab should be stored in the refrigerator between 36°F to 46°F (2°C to 8°C). It may be kept at room temperature 68°F to 77°F (20°C to 25°C) for a maximum of 8 hours. After removal from the refrigerator, nirsevimab must be used within 8 hours or discarded. Store nirsevimab in the original carton to protect it from light until time of use. Do not freeze. Do not shake. Do not expose to heat.

Last reviewed: August 24, 2025

Clesrovimab (Enflonsia, Merck) comes in a manufacturer-filled syringe (MFS) as a 105-mg, 0.7 mL dose. The MFS should be refrigerated at 36°F to 46°F (2°C to 8°C) in the original carton to protect from light until time of use.

Clesrovimab may be kept at room temperature between 68°F to 77°F (20°C to 25°C) for a maximum of 48 hours. After removal from the refrigerator, ENFLONSIA must be used within 48 hours or discarded.

Clesrovimab should not be used if frozen. The MFS should not be shaken.

Last reviewed: August 24, 2025

No. Seasonal influenza vaccines are reformulated each year; for this reason, all unused seasonal influenza vaccines expire and should be discarded no later than the end of June each season. RSV products (vaccines and preventive antibody) do not change each season and products in your storage unit now may not expire until sometime during or after the next season.

Unless state or territorial public health authorities advise otherwise based upon local conditions, RSV vaccine should not be given during pregnancy after January. Likewise, nirsevimab (Beyfortus, Sanofi) or clesrovimab (Enflonsia, Merck) should not be given to infants after March or before October unless specifically instructed by public health authorities because of differences in local RSV circulation patterns.

Vaccination of eligible adults age 50 years or older will have the most benefit if RSV vaccine is administered in late summer or early fall, just before the typical RSV season. However, eligible adults may be vaccinated at any time of year if there is a need to vaccinate at other times of year. Vaccination is expected to provide meaningful protection from serious RSV disease for 2 years or longer. Only one dose of RSV vaccine is recommended at this time: revaccination is not recommended but will be considered in the future.

Properly store and clearly label unexpired products remaining in your storage unit during periods when they are not in use and train staff on the timing of these products, especially for vaccination during pregnancy and use of RSV preventive antibody products. Consider storage and labeling that minimizes the risk of mishandling or administration outside the times when the product is recommended to be used.

Last reviewed: August 24, 2025


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