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Issue 1,832: September 16, 2025
Ask the Experts: Immunize​.org Answers Questions About 2025–26 Season Influenza and RSV Immunization

As an additional service to IZ Express readers, we periodically publish special editions such as this one, providing you with Ask the Experts questions and answers from Immunize​.org experts. This issue includes 8 general Q&As 2025–26 about influenza vaccines and 10 Q&As about immunization for RSV in children and adults. A comprehensive update to the COVID-19 section of Ask the Experts is planned following publication of updated CDC recommendations and clinical considerations and will also include recommendations from major professional societies.

To find the full set of Immunize​.org’s Ask the Experts Q&As related to influenza and RSV, visit www.immunize.org/ask-experts/topic/influenza or www.immunize.org/ask-experts/topic/rsv.

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 www.immunize.org/ask-experts.

Immunize​.org’s team of experts includes Kelly L. Moore, MD, MPH (team lead); Carolyn B. Bridges, MD, FACP; Iyabode Beysolow, MD, MPH; and Jane R. Zucker, MD, MSc.
Influenza Vaccination
* Q: How serious a problem is influenza in the United States? (#IZX1)
* Q: Please summarize what’s new in CDC's 2025–26 season influenza vaccine recommendations. (#IZX2)
* Q: Are any of the available influenza vaccines recommended over others? (#IZX3)
* Q: When does CDC recommend starting influenza vaccination? (#IZX4)
* Q: For which vaccines is an egg allergy a contraindication? (#IZX5)
* Q: How long after someone is vaccinated with FluMist (LAIV) must they stay away from a severely immunosuppressed person (a person who is hospitalized in protective [reverse] isolation)? (#IZX6)
* Q: Can a clinic vaccinate children younger than age 3 years with influenza vaccine taken from a multidose vial (MDV) of influenza vaccine? The MDVs contain thimerosal as a preservative. (#IZX7)
* Q: Does ACIP recommend one influenza product over another during pregnancy? (#IZX8)

RSV Immunization
* Q: What is the ACIP recommendation for RSV vaccination of older adults? (#IZX9)
* Q: What are the three RSV vaccines and how are they different? (#IZX10)
* Q: Is there information on how well RSV vaccines have worked in the first season they were used? (#IZX11)
* Q: How serious is RSV in children? (#IZX12)
* Q: What is the recommendation for use of RSV vaccine during pregnancy? (#IZX13)
* Q: What is clesrovimab preventive antibody and how well does it work? (#IZX14)
* Q: When is the optimal timing for infants and toddlers to receive an RSV preventive antibody product? (#IZX15)
* Q: When should palivizumab (Synagis) be used now that nirsevimab and clesrovimab are recommended? (#IZX16)
* Q: Is there a strict limit to the volume of vaccine and/or monoclonal antibody that can be administered in a single limb (deltoid or thigh) at one immunization visit? (#IZX17)
* Q: What are the side effects of RSV preventive antibody products? (#IZX18)

Editorial Information (#Editorial)
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Influenza Vaccination
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Q: How serious a problem is influenza in the United States?

A: From 2010–11 through 2023–24, CDC estimates the annual influenza-related burden of 9–41 million illnesses, 4.3–18 million medical visits, 120,000–710,000 hospitalizations, and 6,300–52,000 deaths. The 2024–25 season was particularly severe. Preliminary estimates for the 2024–25 season include 47–82 million illnesses, 21–37 million outpatient visits, 610,000–1,300,000 hospitalizations, and 27,000–130,000 deaths. While an average of 129 pediatric deaths were reported during the years 2010–24, 275 influenza-related deaths were reported in 2024–25 (see www.cdc.gov/flu-burden/php/data-vis/index.html).

Rates of infection from seasonal influenza are highest among children, but the risks for complications, hospitalizations, and deaths are highest among adults age 65 years and older. CDC's estimates of hospitalizations and deaths associated with influenza during the 2023–24 season showed that adults age 65 years and older accounted for only 8% of medical visits for influenza but 68% of deaths and 50% of hospitalizations. Adults in the 15-year age span from 50 through 64 years accounted for 20% of influenza-related hospitalizations and 20% of influenza-related deaths. (www.cdc.gov/flu-burden/php/data-vis/2023-2024.html).

Annual seasonal vaccination is the best way to reduce the serious burden of hospitalizations and deaths from influenza in the United States. Most people understand that the burden is highest among the oldest people, and 2024–25 vaccination rates in this age group were estimated at 70% by the end of April 2025. However, 2024–25 vaccination rates dropped off dramatically to just 48% among adults 50 through 64, despite this age group accounting for 20% of hospitalizations and deaths. ([link removed])
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Q: Please summarize what’s new in CDC's 2025–26 season influenza vaccine recommendations.

A: Yearly influenza vaccination continues to be recommended for everyone age 6 months and older. Changes relevant to clinical practice in the CDC’s published ACIP recommendations for influenza vaccination in the 2025–26 season are summarized below:
1. In September 2024, FDA approved FluMist (live-attenuated influenza vaccine, LAIV) for self-administration (for recipients age 18 through 49 years) or administration by a caregiver age18 years or older (for children and adolescents age 2 through 17 years). FluMist for self-administration or caregiver administration is available during the 2025–26 season.
2. In March 2025, FDA expanded approval of Flublok (recombinant influenza vaccine, RIV), previously approved for people age18 years or older, to children and adolescents age 9 through 17 years. Flublok is now approved for people age 9 years or older.
3. On June 26, 2025, ACIP made a new recommendation that all people eligible to receive influenza vaccine receive seasonal influenza vaccines only in single-dose formulations that do not contain thimerosal as a preservative.

The current ACIP recommendations for influenza vaccination are available here: www.cdc.gov/acip-recs/hcp/vaccine-specific/flu.html.

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Q: Are any of the available influenza vaccines recommended over others?

A: For people age 6 months through 64 years, CDC recommends any available age-appropriate influenza vaccine product.

For adults age 65 years and older, three influenza vaccines are preferentially recommended: Fluzone High-Dose (HD-IIV, Sanofi), Flublok recombinant (RIV, Sanofi), and Fluad adjuvanted (aIIV, CSL Seqirus). In June 2022, ACIP concluded that these three vaccines are potentially more effective than standard dose, unadjuvanted flu vaccines. However, if none of the three products are available, people age 65 years and older should get any other age-appropriate influenza vaccine.

Review the full explanation for the ACIP decision to prefer these products in this age group in the 2022 ACIP recommendations for influenza vaccination: www.cdc.gov/mmwr/volumes/71/rr/pdfs/rr7101a1-H.pdf.

ACIP recommends that solid organ transplant recipients (SOTRs) age 18 years through 64 years have the option of receiving HD-IIV or aIIV, both of which are licensed for people age 65 years or older. However, HD-IIV and aIIV are not preferentially recommended over age-appropriate influenza vaccine products for SOTRs.
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Q: When does CDC recommend starting influenza vaccination?

A: For most people who need only 1 dose of influenza vaccine, vaccination should ideally be offered in September and October. For people not vaccinated by the end of October, vaccination efforts should continue as long as influenza viruses are circulating and unexpired vaccine is available.

Vaccination in July and August should be avoided for most groups unless there is concern that vaccination later in the season might not be possible. Early vaccination has been associated with waning of vaccine-induced immunity and decreased vaccine effectiveness before the end of the influenza season, particularly among older adults.

Vaccination in July and August may be considered for people in their third trimester of pregnancy, to allow time for protective maternal antibodies to transfer to the fetus, providing protection during early infancy. Children younger than age 9 years who need two doses of vaccine this season should receive their first dose as soon as possible so that they can get their second dose before the end of October. Children who need only one dose can be considered for vaccination in July or August.
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Q: For which vaccines is an egg allergy a contraindication?

A: Yellow fever is contraindicated for people who have a history of a severe (anaphylactic) allergy to eggs.

ACIP and CDC no longer consider egg allergy of any severity to be a contraindication or precaution to egg-based influenza vaccines. A person with egg allergy of any severity may receive any influenza vaccine that is appropriate for the person’s age and health status. When administering an egg-based influenza vaccine to a person with egg allergy of any severity, no additional safety precautions are needed, beyond those recommended when administering any vaccine to any recipient.

For more details about giving influenza vaccine to people with a history of egg allergy, go to www.cdc.gov/acip-recs/hcp/vaccine-specific/flu.html for a link to the current ACIP recommendations for influenza vaccination.
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Q: How long after someone is vaccinated with FluMist (LAIV) must they stay away from a severely immunosuppressed person (a person who is hospitalized in protective [reverse] isolation)?

A: People should avoid contact with any person who is severely immunosuppressed for at least 7 days after receiving FluMist (LAIV). There are no restrictions on being in contact with any other patients.
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Q: Can a clinic vaccinate children younger than age 3 years with influenza vaccine taken from a multidose vial (MDV) of influenza vaccine? The MDVs contain thimerosal as a preservative.

A: Multidose vials of inactivated influenza vaccine contain a small amount of thimerosal to prevent bacterial and fungal growth in the vial.

Thimerosal-containing vaccines from multidose vials are safe to use in children. No scientific evidence indicates that the small amount of thimerosal in vaccines causes adverse events unless the patient has a severe allergy to thimerosal. In June 2025, CDC’s ACIP voted to no longer recommend use of influenza vaccines in MDVs containing thimerosal as a preservative. In addition, a few states have enacted legislation that restricts the use of thimerosal-containing vaccines in children. To find out if your state has such restrictions, check with your state immunization program (see www.immunize.org/official-guidance/state-policies/state-resources/ for program contact information).

The American Academy of Pediatrics (AAP) continues to recommend annual influenza vaccination of all children, beginning at age 6 months, using any available age-appropriate presentation, including MDVs containing thimerosal as a preservative. See the AAP policy statement on influenza vaccination for the 2025–26 season ([link removed]) .
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Q: Does ACIP recommend one influenza product over another during pregnancy?

A: Pregnant patients may receive any age-appropriate inactivated or recombinant influenza vaccine. The ACIP recommends use of influenza vaccines in single-dose vials or manufacturer-filled syringes. FluMist (LAIV) should not be given during pregnancy because it is a live virus vaccine.

In June 2025, ACIP voted to no longer recommend the use of influenza vaccines from multidose vials containing thimerosal as a preservative for any person. There is no evidence that the low doses of thimerosal contained in vaccines cause harm, unless the recipient has a rare severe allergy to thimerosal.

The American College of Obstetricians and Gynecologists (ACOG) strongly recommends vaccination with an inactivated or recombinant influenza vaccine during pregnancy ([link removed]) , and makes no specific comment on thimerosal. The American Academy of Pediatrics (AAP) recommends seasonal vaccination of all children, beginning at age 6 months, including pregnant teens. AAP states, “Any licensed influenza vaccine appropriate for age and health status can be administered, as soon as possible in the season, without preference for one product or formulation.” See the AAP policy statement on influenza vaccination for the 2025–26 season ([link removed]) .
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RSV Immunization
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Q: What is the ACIP recommendation for RSV vaccination of older adults?

A: ACIP routinely recommends a single dose of any of the three licensed RSV vaccines for all adults age 75 years and older. ACIP also recommends a single dose of any RSV vaccine for adults age 50 through 74 years who are increased risk for serious RSV infection due to specific high-risk conditions, frailty, or high-risk living arrangements (e.g., residents of long-term care facilities). Refer to the question and answer ([link removed]) about high-risk conditions for which CDC recommends vaccination among adults age 50 through 74 years for the detailed list.

In April 2025, ACIP voted to lower the age for initiating vaccination of high-risk adults from age 60 years to age 50 years. The recommendation was accepted and made official in the summer of 2025. The indications for those age 50 through 59 years are the same as those published for people age 60 through 74 years. See the detailed MMWR publication of the ACIP recommendation for older adults, including those at high risk beginning at age 60 years here: www.cdc.gov/acip-recs/hcp/vaccine-specific/rsv.html.
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Q: What are the three RSV vaccines and how are they different?

A: There are three RSV vaccines licensed and recommended in the United States. Each is listed below, along with their FDA-licensed indications:
* Abrysvo (RSVpreF, Pfizer, originally licensed May 2023): For all adults age 60 years or older, adults age 18 through 59 years with high-risk conditions for severe RSV disease, and during pregnancy from 32 through 36 weeks’ gestation (download the package insert from FDA: www.fda.gov/media/168889/download)
* Arexvy (RSVpreF3, GSK, originally licensed May 2023): For all adults age 60 years or older, adults age 50 through 59 years with high-risk conditions for severe RSV disease (download the package insert from FDA: www.fda.gov/media/167805/download)
* mResvia (mRNA RSV, Moderna, originally licensed May 2024): For all adults age 60 years or older, adults age 18 through 59 years with high-risk conditions for severe RSV disease (download the package insert from FDA: www.fda.gov/media/179005/download)

All three may be used as recommended by CDC and ACIP as a single dose for all adults age 75 years or older and for adults age 50 through 74 years with high-risk conditions for severe RSV disease. 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.

Arexvy and Abrysvo are recombinant protein vaccines that contain the prefusion form of the spike protein found on the surface of the RSV virus. The mResvia mRNA vaccine contains mRNA that encodes the prefusion form of the RSV F glycoprotein. The mRNA vaccine temporarily enables some of the vaccine recipient’s own cells to produce the prefusion form of the spike protein, causing the immune system to respond by generating antibodies to it. Because none of these vaccines contain live virus, they cannot cause RSV illness.

The GSK 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. Abrysvo and mResvia vaccines do not contain an adjuvant.
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Q: Is there information on how well RSV vaccines have worked in the first season they were used?

A: Yes.

In studies of the real-world effectiveness of Arexvy during the 2023–24 RSV season, the first since licensure of Arexvy, Arexvy was approximately 77% effective in preventing RSV-associated emergency department encounters and 83% effective in preventing RSV-associated hospitalizations in adults age 60 and older.

In studies of the real-world effectiveness of Abrysvo during the 2023–24 RSV season, the first RSV season since licensure, Abrysvo was approximately 79% effective in preventing RSV-associated emergency department encounters and 73% effective in preventing RSV-associated hospitalizations in adults age 60 and older.

The Moderna mResvia vaccine was first used during the 2024–25 season. Real-world effectiveness data are not yet available.

For additional details, see the CDC MMWR published in August 2024: www.cdc.gov/mmwr/volumes/73/wr/pdfs/mm7332e1-H.pdf.
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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 may also have difficulty accessing adequate medical care when they develop LRTD.
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Q: What is the recommendation for use of RSV vaccine during pregnancy?

A: Pregnant patients 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 patients. Arexvy (by GSK) and mResvia (Moderna) are 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 patients 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 state or territorial public health authorities about the timing of RSV vaccination during pregnancy in their regions.

RSV vaccine is only recommended as a single dose; revaccination is not recommended. However, RSV vaccination during one pregnancy is not expected to protect infants resulting from future pregnancies. Therefore, a patient who is currently pregnant and received RSV vaccine before the current pregnancy should be counseled that their infant should be immunized with an RSV preventive antibody for protection against RSV disease after birth.
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Q: What is clesrovimab preventive antibody and how well does it work?

A: Clesrovimab (Enflonsia, Merck) is a preventive antibody product approved in mid-2025 to be administered intramuscularly as a single 105 mg dose to infants from age 0 days to age 8 months and 0 days either before or during their first RSV season. It is recommended for use as an option for prevention of serious lower respiratory tract infection with RSV in infants. There is no preference between clesrovimab and nirsevimab (Beyfortus, Sanofi) for eligible infants. However, only nirsevimab is approved for high-risk children age 8 through 19 months entering their second RSV season.

Clinical trials demonstrated clesrovimab had benefits similar to nirsevimab, with a 60% reduced risk of medically attended lower-respiratory tract illness (MALRI) caused by RSV and an 84% reduced risk of hospitalization with RSV LRI during the 150 days after administration.
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Q: When is the optimal timing for infants and toddlers to receive an RSV preventive antibody product?

A: 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.
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Q: When should palivizumab (Synagis) be used now that nirsevimab and clesrovimab are recommended?

A: Palivizumab (Synagis, AstraZeneca) is a short-acting monoclonal antibody product that is administered monthly. Synagis is no longer recommended for use by the American Academy of Pediatrics (AAP). The manufacturer is discontinuing the product in December 2025.
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Q: Is there a strict limit to the volume of vaccine and/or monoclonal antibody that can be administered in a single limb (deltoid or thigh) at one immunization visit?

A: 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 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.
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Q: What are the side effects of RSV preventive antibody products?

A: 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.
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Editorial Information
Editor-in-Chief
Kelly L. Moore, MD, MPH
Managing Editor
John D. Gr
äbenstein, RPh, PhD
Associate Editor
Sharon G. Humiston, MD, MPH
Writer/Publication Coordinator
Taryn Chapman, MS
Courtnay Londo, MA
Style and Copy Editor
Marian Deegan, JD
Web Edition Managers
Arkady Shakhnovich
Jermaine Royes
Contributing Writer
Laurel H. Wood, MPA
Technical Reviewer
Kayla Ohlde

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IZ Express is supported in part by Grant No. 1NH23IP922654 from the National Center for Immunization and Respiratory Diseases, CDC. Its contents are solely the responsibility of Immunize​.org and do not necessarily represent the official views of CDC.

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