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Advisory Committee on Immunization Practices (ACIP)
Centers for Disease Control and Prevention (CDC)
Influenza Immunization Workgroup
Terms of R eference
DRAFT: December 18 , 2025
PURPOSE
This document defines the activities, membership, and administrative requirements associated with the establishment of a n Influenza Immunization Workgroup under the Advisory
Committee on Immunization Practices, Centers for Disease Control and Prevention (ACIP, CDC). ACIP utilizes subgroups of the Committee, known as Workgroups (WGs), to review
relevant published and unpublished data , and clinical and scientific knowledge, and develop
options for presentation to the full ACIP parent committee during its public meetings to facilitate
discussion, deliberation and development of recommendations. ACIP WGs are intended to enhance the effectiveness of ACIP. The d irection, focus, and pace of both ACIP and the
individual WGs are guided by CDC and HHS policies and priorities, and by the need for expert input to inform development of CDC immunization policy. ACIP WGs serve a key scientific
role in support of immunization recommendations. The Influenza Immunization WG has been
specifically established to review data , as well as clinical and scientific knowledge on existing
and developmental- stage influenza immunization s, to help develop influenza immunization
policy options for ACIP consideratio n to formulate recommendations to the Director of the CDC.
For purposes of this document, “i mmunization ” refers to vaccines and other antibody protective
products to prevent disease, e.g., immunoglobulins.
BACKGROUND
Influenza is a disease caused by the influenza A and B virus es. There are other upper respiratory
viruses and pathogens such as respiratory syncytial virus (RSV), rhinoviruses, mycoplasma,
Sarbecoviruses, beta coronaviruses , and human metapneumoviruses that are associated with
clinical signs and symptoms similar or identical to influenza. Both i nfluenza virus es and these
other pathogens contribute to the overall burden of influenza- like illness (ILI). F or some
patients, influenza and other ILI can be severe, causing significant morbidity and mortality , and
are a frequent contributing cofactor to morbidity and mortality in some populations with
additional risk factors . Currently , there are multiple vaccine manufacturers employing a wide
range of manufacturing, formulation , and adjuvant technologies to produce i nfluenza vaccines
for the United States .
The specific virus components for the 2025–2026 season include:
An A/Victoria/4897/2022 (H1N1)pdm09-like virus (for egg- based vaccines) or an
A/Wisconsin/67/2022 (H1N1)pdm09- like virus (for cell- or recombinant -based vaccines)
An A/Croatia/10136RV/2023 (H3N2)-like virus (for egg- based vaccines) or an A/District of
Columbia/27/2023 (H3N2)- like virus (for cell- or recombinant- based vaccines)
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A B/Austria/1359417/2021 (B/Victoria lineage) -like virus
For those manufacturing platforms employing either chick embryo or mammalian cell -based
antigen production, the seed virus representing the annually selected influenza strain(s) requires
adaptation from the circulating version infecting humans to support efficient reproduction in the
relevant manufacturing platform supporting viral replication. In some cases, this adaptation may
introduce partial antigenic mismatch relative to the parental human circulating viruses. For other
platforms such as baculovirus- based manufacturing, it may be that this form of seed virus protein
sequence drift relative to parental strain is minimized, but these platforms may yield antigens
that diverge in criti cal glycosylation patter ns relative to parental human circulating viruses. The
significance, or lack thereof , of influenza antigens glycosylated with insect patterns rather than
mammalian patterns is not well characterized.
New formulations , compositions, or technologies for use in influenza vaccine s are actively being
developed, with a specific emphasis on a universal influenza vaccin e. These may require
regulatory (by the appropriate entities) and ACIP safety and efficacy and/or effectiveness review
in the foreseeable future.
According to the 21st Century Cures Act (PL 114- 255), ACIP shall also, as appropriate, consider
new vaccines or new indications at its next regularly scheduled meeting after licensure . If the
Committee defers making a recommendation, it will provide an update on the status of its
review. Ad ditionally, ACIP shall make recommendations in a timely ma nner for vaccines that
are designated as breakthrough interventions or could be used in a public health emergency. The purpose of this WG is to review available data, as well as clinical and scientific knowledge,
to support the development of vaccine and related product clinical use recommendations for ACIP parent committee consideratio n, which may then elect to advise the Director of the CDC
concerning those recommendations .
In accordance with the ACIP Charter, t he Influenza Immunization WG will prepare information
for the ACIP parent committee members to enable them to:
• Advise on population groups and/or circumstances in which one or more influenza
vaccine s or related agent s are recommended.
• Provide recommendations on contraindications and precautions for the use of influenza
vaccine s and related agents , and provide information on recognized adverse events.
• Provide recommendations that address the general use of influenza v accines and
immune -globulin preparations as a class of biologic agents, use of specific antibody
products for prevention of influenza infection- related infectious diseases, and special
situations or populations that may warrant modification of the routine recommendations.
• Support c ommittee deliberations on the use of immunization to control disease , including
consideration of disease epidemiology and burden of disease, immunization safety,
immunization efficacy and effectiveness, the quality of evidence reviewed, economic
analyses, and implementation issues.
• Revise or withdraw , as warranted, their recommendation(s) regarding a particular
influenza immunization or related product as new information on disease epidemiology,
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immunization effectiveness or safety, economic considerations, or other data that
becomes available.
• Differentiate and provide differentiated data summaries regarding the use of specific
influenza vaccines in various risk groups, and facilitate modified use recommendations as appropriate or necessary concerning specific influenza vaccines or related products and technologies.
The WG will also assist and support A CIP, i n accordance with Section 1928 of the Social
Security Act, [42 U.S.C. Section 1396s ], to be able to establish and periodically review and, as
appropriate, revise the list of immunization s for administration to children and adolescents
eligible to receive immunizations through the Vaccines for Children Program, along with
recommended schedules concerning the appropriate dose and dosing interval, and contraindications to administration of pediatric immunization s.
The WG will engage external subject matter experts, as needed, to support development of
materials for presentation to the WG and/or parent committee for its deliberations .
TOPICS UNDER DISCUSSION BY THE WORKGROUP
In accordance with the ACIP Charter and in a multi -year effort, the Influenza Immunization WG
members will work with expert consultants as appropriate and in accordance with FACA
statutory requirements and policies, to prepare information for presentation to and deliberation
by the parent ACIP committee in developing their recommendations . In particular, the following
topics and related activities relevant to influenza immunization for effective control of i nfluenza
A and/or B- related disease in the civilian population of the United States will be considered as a
framework for and/or undertaken by the Influenza WG in multi -year effort s:
1) Conduct and/or review r isk-benefit and cost -benefit data for, and/or analyses of , existing and
newly -licensed influenza vaccines, and their administration schedules, to inform the parent
committee’s deliberations on vaccine use recommendations according to age-group, major
risk factors , and health status.
2) Identify critical gaps in existing scientific and clinical knowledge and CDC monitoring
methods related to the safety , efficacy /effectiveness, immunogenicity, and long- term immune
system impacts of influenza vaccines to inform the parent committee’s deliberations on
development of policy recommendations and to identify further analyses and research to be
recommended to CDC for its consideration and CDC potential advice to other federal
agencies and the scientific community to conduct.
3) Review and summarize data, clinical and scientific knowledge related to short and long- term
adverse events associated with influenza vaccines to present to the parent committee for its
deliberations in identify ing precautions and contraindications recommendations.
4) Review and summarize data for presentation to the parent committee on the long- term
immunological effects of repeated annual influenza vaccination as a function of risk group
and strain variability or type (drift/shift).
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DESCRI PTION of WORKGROUP ACTIVITIES
The following activities provide a framework for the Influenza Immunization WG multi- year
efforts , which may involve data requests from the FDA and other f ederal agencies and private
partners , for development of presentations to the parent ACIP for its deliberations as it develops
recommendations to the CDC Director:
1. Review and summarize existing data including published and unpublished research and
clinical knowledge related to the safety, effectiveness, and immunogenicity of influenza
vaccines authorized or approved in the U nited S tates.
2. Summarize literature review s of the epidemiology of influenza disease and infection .
3. Assess the benefit -risk balance for administration of influenza immunization products co -
administered with other vaccinations.
4. Identify areas where additional data and research are needed to inform influenza
immunization recommendations to be developed by ACIP.
5. Annually review and develop updat es of U.S. seasonal influenza immunization policy
recommendations.
6. Review and summarize the existing clinical and scientific information concerning the role of
innate, adaptive innate, adaptive cellular , and adaptive humoral immune responses associated
with different influenza vaccines, adjuvants, and correlates of protection associated with
influenza vaccination.
7. Review and summarize the existing clinical and scientific information , and gaps in the
existing knowledge, including from the FDA and other federal agencies, as well as academic
studies, concerning the impact of repeated influenza vaccination including immunological
effects such as immune imprinting, “ original antigenic sin ,” and related phenomena.
8. Review and s ummarize the existing clinical and scientific information , and identify and
address gaps in the existing knowledge regarding both antigenic sequence mismatch , as well
as glycosylation mismatch , in existing immunization products and their health impacts to
inform immunization recommendations.
9. Review and summarize the existing scientific knowledge and gaps, regarding the cumulative
short- and long- term impact of repeated annual seasonal influenza vaccination (including
boosting in young children’s first season), including non- specific effects (e.g., IgG4 class
switching, immune imprinting, viral evolution under leaky immunizations ) to help inform
immunization recommendations.
10. Examin e the impact of Influenza A and B immunization on influenza and all-cause deaths,
hospitalizations, and disability to inform immunization recommendations .
11. Analyze existing data and scientific knowledge regarding cardiovascular, thrombotic,
neurological , immunological, and othe r serious adverse events potentially caused and averted
by influenza immunization .
12. Review, analyze, and summarize available data concerning the safety and effectiveness of influenza immunization of the elderly , including the interaction between influenza vaccines
and immunosenescence, and both innate and adaptive immunity.
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13. Review and s ummarize available data, information , and gaps regarding long- term repeated
Influenza vaccination effects from scientific literature and clinical experi ence associated with
influenza immunization products and influenza infection to inform policy recommendations.
14. Map existing influenza immunization policies in countries around the world and how they
compare to U .S. vaccine policy.
15. Analyze existing data and scientific knowledge related to the safety of influenza
immunization during pregnancy, including both potential teratogenic effects and potential
beneficial effects for the newborn .
MEMBERSHIP
Workgroup Leadership: The Influenza Immunization WG is chaired by one of the ACIP, CDC
parent committee members appointed to serve as Special Government Employees. The
Workgroup Lead (WGL ) is a federal employee, identified by the Immediate Office of the
Director in consultation with the appropriate CDC program . The WG Chair , in consultation with
the WGL, ACIP CDC Designated Federal Offi cial (DFO), determine s the WG ’s membership and
work priorities and deliverables to the full committee . The DFO may further assign some of the
DFO- related roles and responsibilities , as appropriate, to the WGL.
Workgroup Membership: The Influenza Immunization WG is comp osed of experts who are
appointed based on their professional, scientific, technical, or other expertise. They are experts
who are regarded as an authority or a practitioner of unique competence and skill by other
persons in the ir profession, or occupation. Upon request, HHS f ederal agencies name d in the
ACIP charter may also appoint members to serve on WG s. The Influenza Immunization WG will
be composed of members from a variety of disciplines. The WG will engage with the following
disciplines on WG activities :
• Public health science and practice ;
• Public health policy development, analysis, and implementation, including development and
execution of immunization programs for children and adults;
• Clinical and medical practice , and patien t-care experience;
• Epidemiology;
• Molecular biology;
• Immunology;
• Virology;
• Diagnostics and correlates of protection;
• Drug and vaccine safety;
• Bioethics; and
• Consumer perspectives and/or social and community aspects of immunization programs
Due to the complexity and variability of the information to be gathered, additional external
subject matter experts may also be invited to provide data and presentations to the WG and
answer questions during Influenza Immunization WG meetings on an ad -hoc basis. Such
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additional external subject matter experts will not be members of the WG and will not participate
in any deliberations or WG discussions.
MEETINGS, ADMINISTRATION, and TIMELINES
1. Administrative Oversight: The WGL will work with the WG Chair to arrange meetings,
document meeting proceedings, and report to the ACIP on the Influenza Immunization WG’s
activities and findings.
2. Meeting frequency and location : The Influenza Immunization WG will meet on an as needed
basis as determined by the WG Chair and WGL . All Influenza Immunization WG meetings are
convened virtual ly via teleconference.
3. Meeting structure : In addition to the WGL, at least two ACIP parent committee member s (one
of whom serves as the Influenza Immunization WG Chair) must be present at each meeting for a
quorum. An agenda, relevant publications, and background documents will be circulated as
read-ahead mat erial before each meeting.
4. Conflicts of Interest : WG members will complete an ACIP WG Agreement and C onflict of
Interest Certification process before participation on the WG. The WGL will screen for conflict
of interest declarations and share any conflicts with the ACIP Secretariat. The ACIP Secretariat /DFO, in collaboration with WGL will work with the Office of Business Initiatives
(OSBI) ethics officials and the Office of General Counsel (OGC), as needed, to resolve any
conflicts that the WGL identifie d. WG members will consent to abide by several guiding
principles and disclose interests (e.g., employment, special interests, grants, or contracts) that a
reasonable person could view as conflicts or potential conflicts of interest with their Influenza
Immunization WG participation. Members will also disclose any potential conflicts of interest
before each meeting. If an Influenza Immunization WG member indicates a potential or actual
conflict of interest to the WGL , the WGL or designee will forward any conflicts to the DFO to
determine whether the individual must recuse themselves from participating in WG discussions
that implicate such a conflict -of-interest concern. If needed, the DFO will engage OSBI and
OGC to assist with making COI determination.
5. Confidentiality : The discussions by the Influenza Immunization WG may include information
that is unpublished, protected, privileged, proprietary, or confidential. WG deliberations, including policy options under consideration by the WG , are also considered confidential.
Information of this nature must not be disseminated, distributed, or copied to, and/ or described or
discussed with, persons not authorized to receive such information. When these types of
information are distributed, the person/s presenting will identify the information as such, so all members are duly informed; and written materials shall be clearly marked as such. Unlike ACIP
parent committee meetings, which are open to the public, Influenza Immunization WG
teleconferences are not subject to the open meeting requirements of the Federal Advisory Committee Act or the GSA Final Rule; data presented during these meetings/teleconferences are often proprietary and should not be distributed to people other than approved Influenza Immunization WG members.
6. CDC Staff Involvement: CDC staff do not serve as members of the Influenza Immunization WG
but may provide administrative support and technical expertise to ACIP WGs, bringing subject matter expertise and current professional focus in areas relevant to the goal s of the Influenza
Immunization WG. C onsultation or information al presentations by CDC staff will be transparent
and evident to minimize the risk of, or the appearance of, undue influence that would
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compromise the independence of the WG . The DFO , and WGL of Influenza Immunization WG,
in consultation with the Chair of the Influenza Immunization WG, will monitor the interaction
between the WG and the agency staff to ensure that the WG activities and work products are
appropriate and that there is not undue influence by the CDC or by any special interest group on
the activities or work products of the WG .
7. Timelines: ACIP WGs are established when needed and terminated once the activities and work
products stated in the terms of reference have been completed and the WG ’s charge has been
fulfilled .
8. Workgroup Meeting Summaries: Meeting minutes will be created to capture the information
gathered during each Influenza Immunization WG meeting and teleconference.
9. Workgroup findings : The Influenza Immunization WG will present findings (briefing
documents, background materials, and presentations) to ACIP parent committee for
consideration and deliberation in a public meeting. Final versions of all slides presented at the ACIP parent committee meeting will be posted on the ACIP website following the meeting and
included in the committee’s official records.
10. Workgroup Record Keeping: All CDC FACA committees, subcommittees, and WGs are subject
to the Federal Records Act. All records will be uploaded to the Federal Advisory Committee
Management Portal. The summary report and other WG documents will become part of the ACIP’s official records as required by GENERAL RECORDS SCHEDULE 6.2: Federal Advisory Committee Records.
RECORDKEEPING and REPORTING
The WG Chair and /or WG L will present findings/outcomes/observations to the ACIP parent
committee for discussion, deliberations, further development of recommendations, and vote in an
open public forum . Approved ACIP recommendations adopted by the CDC Director wil l be
posted on CDC’s ACIP website and also published in the Morbidity and Mortality Weekly Report (MMWR). In addition, approved ACIP recommendations will be included in the ACIP meeting minutes and annual report.