Vaccine Risk Monitoring and Evaluation at the Centers for Disease Control and Prevention (2025)
Chapter: 1 Introduction
1
Introduction
On March 13, 2020, the Secretary of Health and Human Services (HHS) declared a nationwide public health emergency (PHE) due to the unfolding COVID pandemic (CDC, n.d.; NASEM, 2024). A key component of the response was the rapid development and deployment of new vaccines. While many organizations of the government were required to meet the challenges posed by the pandemic and the development of COVID vaccines, safety issues that might arise from their deployment fell in part to the Immunization Safety Office (ISO) in the Centers for Disease Control and Prevention (CDC). ISO plays a key role in monitoring and evaluating the risks of all vaccines, including COVID vaccines, once a decision has been reached that they can be administered to the public. ISO also oversees a set of complementary data collection and analysis systems, described later, crucial for updating vaccine recommendations.
The ISO requested that the National Academies of Sciences, Engineering, and Medicine convene an ad hoc committee of experts to evaluate its work on the safety of the COVID vaccines and recommend ways to improve all that it does. The task can be found in Box 1-1.
This chapter continues with a brief description of the history, structure, and function of vaccine safety assessment by ISO and others in the federal government, including during the COVID PHE. The chapter concludes with a set of guiding principles the committee used in framing its recommendations, followed by a brief description of the report structure and important terminology.
BOX 1-1
Statement of Task
An ad hoc committee of the National Academies of Sciences, Engineering, and Medicine will evaluate the Centers for Disease Control and Prevention (CDC) Immunization Safety Office (ISO) systems, methods, and processes for monitoring COVID-19 vaccine safety during the U.S. COVID-19 vaccination program and provide recommendations for sustaining, maintaining, and strengthening CDC ISO current monitoring systems moving forward.
Specifically, the committee will assess the statistical and epidemiological methods and processes employed to detect and evaluate potential safety problems with the U.S. COVID-19 vaccines; catalog the findings from safety monitoring including pertinent positive and negative findings; and evaluate CDC external communications about its safety monitoring systems, the findings of COVID-19 vaccine safety monitoring, and vaccination and clinical guidance recommendations to healthcare professionals, public health officials, and the public.a
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a Because ISO is not responsible for clinical guidance responsibilities, the committee evaluated the information ISO presented to those who do develop vaccine use recommendations. The committee did not evaluate the content of the clinical use guidelines.
VACCINE SAFETY ASSESSMENT
Vaccines have been an important public health tool for decades and were declared one of the 10 great public health achievements of the 20th century (CDC, 1999). They provide benefit to the vaccinated person and others. They prevent infections, protect against the most severe forms of disease, and prevent spread of infectious disease. The type and amount of protection depends on the target infection, specific vaccine, and recipient person or population.
Vaccines differ from pharmaceuticals in several important ways. While vaccines play a critical role in responding to immediate health threats, their greatest impact lies in the long-term prevention of infectious diseases across populations. Many vaccines are given to infants and children, so the decision to vaccinate is not theirs but that of their parent or guardian. Some vaccines are required or mandated for public school attendance, and some occupations can be subject to vaccine mandates as well.
Over the 20th century, vaccines became widely used and drove down the incidence of many once-common—and often deadly—infectious diseases in children. In recognition of their lifesaving impact, all 50 states
adopted laws requiring one or more immunizations for school entry. As the appropriate use of vaccines was widely adopted, many communicable diseases were eliminated—such as smallpox—or dramatically reduced, including measles, mumps, whooping cough, and influenza. The threat of these serious and frequently life-threatening diseases faded (or appeared to fade) in the public’s perception. Without an imminent threat of disease, fear of injury possibly related to vaccines led to a vaccine-related litigation environment (Blake, 2012; Grey, 2011).
Fearing that this increased liability would drive vaccine manufacturers out of the market, imperiling public health, Congress intervened in 1986 with the National Childhood Vaccine Injury Act (NCVIA) (Blake, 2012).1 Recognizing that even the safest vaccine may produce some degree of harm in some individuals yet still serve the broader public, NCVIA limits liability for manufacturers (thus encouraging them to remain in the vaccine-making market) and ensures that injured persons have a mechanism for receiving compensation through the Vaccine Injury Compensation Program (Blake, 2012). The act also led to creating other important programs, such as the National Vaccine Program Office, Vaccine Adverse Event Reporting System (VAERS), and Vaccine Information Statements.
Questions about the safety of the COVID vaccines specifically arose for several reasons. Operation Warp Speed, a presidential initiative, led to the very rapid development and use of vaccines, two of which involved a novel technology, the mRNA platform. The vaccines were made available to the public initially through a legal provision known as “Emergency Use Authorization” (EUA) (see Box 1-2). Confusion about the EUA process made some hesitant to get vaccinated, due to the public misperception that the vaccines hadn’t been rigorously assessed for risks. The safety profile of all COVID vaccines, including the mRNA vaccines, were studied in clinical trials as required by Food and Drug Administration (FDA), and risk monitoring and evaluation was launched by ISO staff as soon as vaccines were given to the public. However, public mistrust of government pandemic restrictions and policies began to spread, and trust in safety of the vaccines faltered (Funk et al., 2023).
CDC ISO
As described in Chapter 2, ISO’s vaccine risk monitoring and evaluation involves staff and researchers, mostly from universities and health plans, who work under various contracts in support of various systems. These systems include the VSD, Clinical Immunization Safety Assessments,
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1 The National Childhood Vaccine Injury Act of 1986, Public Law 660, 99th Cong., October 14, 1986.
BOX 1-2
Emergency Use Authorization (EUA)
The EUA mechanism is a legal provision allowing the Food and Drug Administration (FDA) to authorize not yet licensed medical products or unapproved uses of approved products during a public health emergency (PHE), as declared under Section 564 of the Federal Food, Drug, and Cosmetic Act (21 U.S.C. § 360bbb-3).
EUAs can be granted when
- A PHE is declared by the HHS Secretary,
- FDA concludes that the product may be effective in diagnosing, treating, or preventing a serious or life-threatening disease,
- The known and potential benefits outweigh known and potential risks, and
- No adequate, approved, and available alternatives exist.
Key Differences from Standard Approval (BLA):
- Evidence Threshold: An EUA requires evidence from at least one well-conducted Phase 3 trial demonstrating that the product may be effective in preventing, diagnosing, or treating a serious or life-threatening disease and that the known and potential benefits outweigh the known and potential risks. For COVID vaccines, FDA required a median of at least 2 months of safety follow-up after completion of the primary series.
- By contrast, a Biologics License Application (BLA) under Section 351 of the Public Health Service Act (42 U.S.C. § 262)a requires substantial evidence of effectiveness from adequate and well-controlled Phase 3 trials, longer-term safety data (typically 6 months or more of follow-up), and comprehensive submissions demonstrating manufacturing consistency, stability, pharmacovigilance planning, and compliance with labeling and facility standards.
- Duration: EUAs are temporary and valid only during the declared emergency or until revoked. BLAs grant permanent licensure.
- Postmarket Surveillance: EUA products are subject to thorough surveillance, such as through the Vaccine Adverse Event Reporting System (VAERS) and Vaccine Safety Datalink (VSD). Full licensure involves long-term postmarketing monitoring and periodic FDA inspections.
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a Public Health Service Act, Public Law 262, 42nd U.S. Congress, June 15, 2025.
SOURCE: FDA, 2017.
VAERS (comanaged with FDA), V-safe, and a pregnancy registry. ISO data are an integral part of the deliberations of the Advisory Committee on Immunization Practices (ACIP), which makes vaccine recommendations, and FDA review of safety data related to postmarketing safety evaluations. See Figure 1-1 for the committee’s understanding of how ISO’s vaccine risk monitoring and evaluation moves from data collection to public communications, which will be described in more detail in Chapters 2 and 3.
For many years, vaccine risk monitoring and evaluation at CDC was the responsibility of the National Immunization Program (NIP). ISO was moved out of NIP in 2005 in response to concerns that the work of evaluating the risks from vaccines could not be done objectively if the staff were co-housed with the offices that promoted the use of vaccines (CDC, 2006). Between 2005 and 2010, ISO was in the CDC Office of the Chief Scientist, Office of the Director. In 2010, ISO was relocated to the CDC Division of Healthcare Quality Promotion (DHQP), National Center for Emerging Zoonotic Infectious Diseases (NCEZID), where it remains. NCEZID is organizationally distinct from the National Center for Immunization and Respiratory Diseases, with each reporting to the CDC director.
During the COVID PHE, monitoring and evaluating COVID vaccine risks postauthorization were conducted by ISO, contract staff, and other CDC staff on temporary detail, but due to temporary restructuring in response to the emergency declaration and creation of a government-wide response, these were carried out by the Vaccine Task Force (Gee, 2024). It was established at CDC; its incident manager reported to the CDC COVID agency response incident manager, who reported to the CDC director. ISO staff not deployed elsewhere performed data monitoring and evaluation work not as the “Immunization Safety Office” but within that task force (Su, 2024). Communications were coordinated through the Office of the Director, Department of Health and Human Services (HHS) Secretary, and White House (Jernigan, 2025). Chapter 2 discusses the data systems deployed to study the COVID vaccines, and Chapter 3 describes the communication processes.
See Box 1-3 for a list of ISO’s official functions, as described in the Federal Register, which is consistent with the discussions the committee had with CDC in public sessions (Gee, 2024; Jernigan, 2025; Su, 2024). ISO does not have a dedicated website describing its structure, mission, and functions, although, as described in Chapter 3, voluminous CDC webpages address vaccine safety. For fiscal year (FY) 2020, before the public health declaration, the ISO budget was $22.5 million; staffing comprised 30 full-time employees (FTE) and eight contract staff. During the PHE (part of FY2020–part of FY2024), the range of annual funding, including supplemental funding, was $42–$114 million, and staffing levels were 37–47 FTE, with 122 contract staff. In FY2025, ISO annual funding is $51.5 million
with staff of 43 FTE and 21 contract staff.2 The funding derived from the NCEZID emerging infections program; NCIRD pandemic influenza program and Vaccines for Children Program; and targeted COVID funding.
Other Vaccine Safety Work in the Federal Government
Immunization work at CDC is but one piece of the larger federal government response to vaccine-preventable diseases generally and vaccine safety specifically (Gee et al., 2024) (see Table 1-1). Regulatory responsibility sits in FDA, which approves or authorizes vaccines and can require postmarket surveillance of manufacturers. ISO and FDA scientists collaborate and communicate frequently (Gee, 2024), and VAERS, described in detail in Chapter 2, is the shared responsibility of FDA and CDC. Researchers at the Veterans Health Administration, Centers for Medicare & Medicaid Services, Indian Health Service, and Department of Defense conduct vaccine safety studies of importance to their mission; the populations of interest to them (e.g., veterans of all ages; elderly people and those with disabilities; and active duty military and some beneficiaries) are complementary to those studied in ISO’s systems, as described in Chapter 2. Many other countries also have robust vaccine safety or pharmacovigilance units and publish data from those monitoring and evaluation systems, which complement or supplement the information generated in the United States.
COMMITTEE APPROACH TO THE STATEMENT OF TASK
The committee comprises expertise in administration of government public health, research and regulatory administration; pharmacoepidemiology; biostatistics; health literacy; health communication; clinical medicine and pharmacy; public health law; and public health practice (see Appendix A). It met numerous times in full and in small groups beginning in May 2024 and held five sessions open to the public. During three of these sessions, CDC presented information on the charge, the systems it uses to monitor and assess vaccine safety, and its communication process regarding vaccine safety. In addition, the committee held a public comment session at which 13 persons made 3-minute public comments in response to questions posed by the committee in advance:
- What criteria should CDC use when deciding how vaccine safety and potential harms from vaccines should be studied?
- How can CDC’s research on vaccine safety and potential harms from vaccines be improved?
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2 Personal Communication, J. Gee, Centers for Disease Control and Prevention, August 5, 2025.
BOX 1-3
Key Functions of the Immunization Safety Office (ISO)
The ISO “assesses the safety of new and currently available vaccines received by children, adolescents and adults using a variety of strategies:
- conducts ongoing surveillance for the timely detection of possible adverse events following immunization (AEFI) in collaboration with the Food and Drug Administration, through implementation and management of the Vaccine Adverse Event Reporting System, the national reporting system that acts as an early-warning system to detect health conditions that might be associated with an immunization;
- coordinates, further develops, maintains and directs activities of the Vaccine Safety Datalink (VSD), a collaborative effort with integrated healthcare organizations able to perform rapid epidemiologic research on potential causality for AEFI using the VSD and other data sources, provide national estimates of incidence of AEFI, and determine background rates of health conditions;
- leads the nation in developing biostatistical methods for research of AEFI using large linked databases and other data sources, and shares methods for use by other agencies and public and private entities;
- conducts clinical research to identify causes of adverse events after immunization, specific populations susceptible to specific adverse events, and prevention strategies through the DHQP supported Clinical Immunization Safety Assessment network, a national network of medical research centers, and through other research efforts;
- applies findings from epidemiologic and clinical studies to develop strategies for prevention of AEFI;
- provides global consultation and leadership for the development, use, and interpretation of vaccine safety surveillance systems, and for the development of shared definitions of specific health outcomes through participation in the Brighton Collaboration and other international organizations
- provides data for action to HHS, the Federal Advisory Committee on Immunization Practices (ACIP), the FDA’s Vaccine and Related Biological Products Advisory Committee, Health Resources and Services Administration’s Advisory Commission on Childhood Vaccines, and international collaborators including the WHO Global Advisory Committee on Vaccine Safety; and
- provides timely, accurate communication and education to partners and the public on vaccine safety issues.”
SOURCE: CDC, 2023.
TABLE 1-1 Vaccine Offices in the Department of Health and Human Services
| Agency/Department | Office | Function | Relevant Advisory Committee |
|---|---|---|---|
| CDC/HHS | ISO | Studies risks from vaccines | None |
| CDC/HHS | Immunization Services Division | Supports vaccine benefit assessment, promotion programs, and the coordination of national vaccine policy recommendations | ACIP |
| CBER/FDA/HHS | OVRR/OBP; Office of Biostatistics and Pharmacoepidemiology | Approves or authorizes vaccines for use; can mandate postmarketing studies by manufacturers; has joint responsibility for VAERS | Vaccines and Related Biologic Products Advisory Committee |
| OASH/HHS | National Vaccine Program, Office of Infectious Disease and HIV/AIDS Policy | Coordinates vaccine-related work across HHS (and the federal government) | National Vaccine Advisory Committee |
| HRSA | Division of Injury Compensation Programs | Along with Department of Justice, administers VICP and CICP | Advisory Committee on Childhood Vaccines |
NOTES: Due to pending reorganization of HHS, this is accurate as of April 2025. ACIP = Advisory Committee on Immunization Practices; CBER = Center for Biologics Evaluation and Research; CDC = Centers for Disease Control and Prevention; CICP = countermeasure injury compensation program; FDA = Food and Drug Administration; HHS = Department of Health and Human Services; HRSA = Health Resources and Services Administration; ISO = Immunization Safety Office; OASH = Office of the Assistant Secretary for Health; OBP = Office of Biotechnology Products; OVRR = Overview of the Office of Vaccines Research and Review; VAERS = Vaccine Adverse Event Reporting System; VICP = Vaccine Injury Compensation Program.
- When should new findings or safety signals be shared with researchers, health professionals, and the public?
- How should CDC communicate new findings about vaccine safety or harms to the public?
- How can CDC improve Americans’ confidence in the safety of vaccines though research and messaging?
See Box 1-4 for a summary of salient points.3 Additionally, the committee held an open, public session with invited experts to discuss the challenges
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3 Appearance in this list does not mean the committee endorses the points made.
BOX 1-4
Salient Points from Public Comment
Monitoring and Evaluation
- CDC should improve vaccine safety research by adhering to its own protocols (such as proper proportional reporting ratio calculations) and funding further studies. Safety signals should be shared promptly, with full transparency. To rebuild public trust, CDC should support its claims with data and continue data collection even when findings challenge prior assumptions.
- Independent monitoring is crucial. Vaccine side effect reports should be collected and reviewed by a body separate from agencies responsible for promoting vaccination to ensure objectivity and public trust.
- The Vaccine Safety Datalink lacks public access, and public and independent researchers cannot easily access the data. Making it publicly available would enhance transparency and scientific scrutiny.
- CDC’s ISO team should be commended for their efforts during the pandemic, but there is a clear need for a faster, more robust, and independent system to evaluate vaccine safety. This includes better funding, coordination, and a separation between risk assessment and risk management. Historically, a lack of conclusive science on vaccine reactions highlights the need for more research, including understanding biological mechanisms. While public health authorities and providers are generally well served, public needs remain unmet, contributing to distrust and polarization.
- Continued use of both active and passive surveillance systems (like Vaccine Adverse Event Reporting System, Vaccine Safety Datalink, and V-safe) is essential for capturing a wide range of vaccine safety data, aided by standardized definitions (e.g., from the Brighton Collaboration).
Communications
- While simple guidance may work for some, more detailed and nuanced recommendations are necessary for those seeking deeper understanding.
- Communication must be swift, comprehensive, and proactive, guiding health care providers in recognizing and responding to potential harms.
- While VAERS is a commonly cited, passive reporting system often criticized for its limitations, the United States also operates more robust vaccine safety tools that are far less known to the public.
- The United States has some of the most advanced systems in the world for monitoring vaccine safety, including tools that analyze electronic health records and provide expert clinical guidance. CDC also works with global collaborators to investigate potential risks such as myocarditis. Despite these efforts, public awareness remains low.
- CDC should do more to communicate the existence and value of its surveillance systems to build trust, promote transparency, and counter misinformation.
- When safety signals emerge, CDC must communicate rapidly and transparently, clarifying the distinction between evolving scientific data and policy decisions. This open, value-aware approach—tailored to different communities and partnered with trusted local leaders—can build trust, address ethical concerns, and respect varying perspectives on autonomy and public health.
- CDC should be clearer about both what it knows and what it doesn’t know regarding vaccines to avoid misinformation and confusion. In addition, adverse events need more precise and transparent reporting; terms like “mild” or “moderate” can be misleading without concrete definitions. Acknowledging data gaps—rather than simply offering reassurances—can help maintain trust in CDC.
- Expressing benefits while downplaying risks of medical products erodes public trust. When official messaging contradicts personal experiences, it damages CDC’s credibility.
SOURCE: Public Comment, October 11, 2024.
of studying and communicating vaccine risks with specific populations. See Appendix B for agendas of those meetings. Videos of all presentations are available on the project website (https://www.nationalacademies.org/our-work/review-of-cdc-covid-19-vaccine-safety-research-and-communications).
The committee commissioned a series of confidential key informant interviews (see Appendix C). It further oversaw the creation of case studies of COVID vaccine risks to illustrate the timeline of COVID vaccine risk information generation and communication (see Appendix D). It used the information from public meetings, key informant interviews, and case studies, in addition to CDC’s website and published research, to inform its targeted analyses of ISO COVID vaccine risk monitoring and evaluation and communication.
The committee encountered challenges in information-gathering. As part of Project Clean Slate (CDC, 2024), CDC reorganized its website, making it difficult to identify webpages that were active during the PHE, the period of most of the committee’s analysis. This was particularly important for assessing ISO’s communications. Other information, such as the presentation on communications and information on ISO budgets, was provided much later than requested. The HHS pause in communications in early 2025 (Fink, 2025) resulted in no participation by ISO staff in the key informant interviews.
The committee considered several important issues about vaccine risk monitoring and evaluation out of scope for its task. Postmarketing safety requirements of vaccine manufacturers are the purview of FDA, and recommendations for vaccine use are the purview of the ACIP and the CDC director. While the committee discusses the implication of the administrative placement of ISO, conclusions and recommendations on that topic were not part of the statement of task. Finally, the committee was not asked to recommend a funding level for the office, although such an important office needs full financial support. Many of these issues are discussed as context for the committee’s conclusions and recommendations. It reiterates that matters of vaccine use and clinical guidance recommendations are out of scope for ISO and therefore for this report.
The Committee’s Guiding Principles
Reliable and impartial vaccine risk monitoring and evaluation are essential to health professionals, policy makers, and the public for informed decision making. ISO is a source of much of that information. Since all of ISO’s products and communications are based on data collection and statistical analysis, the committee carefully studied the latest edition of Principles and Practices for a Federal Statistical Agency (NASEM, 2025) and adapted that for its recommendations. That report states that federal statistical agencies, which includes CDC’s National Center for Health Statistics, and
other designated offices “…provide objective and impartial information that informs policy makers and the public, they should not advocate policies or take partisan positions that would undercut public trust and credibility of the statistics they produce.” The committee is not recommending that ISO be designated a Federal Statistical Agency, because that would exceed its remit and is not necessary to achieve the goals it has for vaccine risk monitoring and evaluation, which is that this office continues to merit the nation’s trust. However, adherence to the principles that these agencies follow (see Chapter 4) provides a firm foundation for increased effectiveness and trust that are required for such an important mission:
- Relevance to Policy Issues and Society,
- Credibility Among Data Users and Stakeholders,
- Trust Among the Public and Data Subjects,
- Independence from Political and Other Undue External Influence, and
- Continuous Improvement and Innovation.
REPORT STRUCTURE AND TERMINOLOGY
Beginning in early 2025, HHS announced plans for a major reorganization. As best as possible, the committee uses the name of an office, agency, or department as it was during the period of review, that is, before the reorganization. Should an office change names or organizational placement, the committee intends that its statements, including conclusions and recommendations, still apply to the relevant office.
In this report, the committee chooses to use “vaccine risk” to describe serious, untoward effects of vaccines. This would not include more minor, common, and predictable side effects, such as time-limited fever, malaise, arthralgia, and myalgia, although ISO systems such as VAERS capture these. Additionally, although the committee recognizes that some of ISO’s work is hypothesis-driven research, ISO also performs routine monitoring. Despite ISO’s name, the committee uses “vaccine risk monitoring and evaluation” to describe ISO’s work instead of “immunization safety.” To some, “safety” reflects a balance of risks and benefits, and to others, it implies 100 percent safe or risk free. Furthermore, safety concerns include things like manufacturing errors or contaminations, which are FDA regulatory concerns.
The report contains a summary, appendixes, and four chapters: this introductory Chapter 1, Chapter 2 on ISO’s data monitoring and evaluation programs, Chapter 3 on ISO communications, and Chapter 4, which offers three conclusions and five recommendations. Three appendixes include substantive information that the committee used as part of its analysis: the independently authored report from confidential key informant interviews (Appendix C), the committee-directed case studies (Appendix D), and a catalog of ISO COVID vaccine publications (Appendix E).
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