Immunization is a cornerstone of global public health, but its long-term success relies on effective monitoring of vaccine safety. Adverse Events Following Immunization (AEFI), defined by the World Health Organization (WHO) as any medical occurrence after vaccination, remain a key challenge in maintaining public trust. This narrative review synthesizes current evidence on AEFI, detailing the WHO classification framework, which includes vaccine product–related, quality defect–related, immunization error–related, immunization anxiety–related, and coincidental events. Biological and immunological mechanisms, from innate inflammatory responses to rare immune-mediated syndromes, and their clinical manifestations were examined. Global surveillance systems show persistent reporting gaps, with only 43% of countries meeting WHO targets for serious AEFI. Clinical manifestations range from common local reactions to severe adverse events, highlighting the need for standardized assessment and management. Strengthening pharmacovigilance, healthcare worker training, and evidence-based public health policies is essential to ensure vaccine safety, sustain public trust, and maintain the success of immunization programs worldwide.

Vaccination represents one of the most successful public health interventions in history, preventing millions of deaths annually and substantially reducing the global burden of infectious diseases [1]. Despite these achievements, the effectiveness of immunization programs is intrinsically linked to public confidence in vaccine safety. Concerns regarding adverse events, whether substantiated or perceived, can significantly influence vaccine acceptance and coverage [2]. The World Health Organization defines an Adverse Event Following Immunization (AEFI) as “any untoward medical occurrence which follows immunization and which does not necessarily have a causal relationship with the usage of the vaccine.” This definition emphasizes the distinction between temporal association and causality, a critical concept in vaccine safety assessment [3;2]. Historically, safety controversies such as those related to whole-cell pertussis vaccines or influenza vaccines have had measurable impacts on immunization uptake, reinforcing the need for transparent and scientifically grounded safety monitoring systems [4;5;6]. This narrative review aims to synthesize existing literature on the classification, biological mechanisms, clinical manifestations, and public health management of AEFIs. By integrating clinical and policy perspectives, the review seeks to inform clinicians, public health practitioners, and policymakers navigating the complex and evolving landscape of vaccine safety.

This narrative review was informed by a comprehensive search of electronic databases, including PubMed/MEDLINE and Scopus, covering literature published up to December 2023. Search terms included combinations of “adverse events following immunization,” “vaccine safety,” “AEFI surveillance,” and “pharmacovigilance.” Grey literature from authoritative organizations, including the WHO and the United States Centers for Disease Control and Prevention (CDC), was also reviewed. Priority was given to systematic reviews, meta-analyses, large observational studies, vaccine safety surveillance reports, and international guidelines. Given the narrative design, the review does not aim to provide quantitative synthesis but rather to contextualize and interpret existing evidence relevant to vaccine safety monitoring and management.

AEFIs are heterogeneous in nature, and standardized classification is essential for surveillance, investigation, and response. The WHO classifies AEFIs according to their presumed cause, facilitating consistent reporting and causality assessment [7;8;9].

3.1 Vaccine Product–Related Reactions

Vaccine product–related reactions arise from the inherent properties of the vaccine or its components, including antigens, adjuvants, preservatives, and stabilizers. Live attenuated vaccines may occasionally produce mild symptoms resembling the natural infection, while inactivated vaccines are more commonly associated with transient local or systemic inflammatory responses. These reactions are typically predictable and reflect expected immune activation [9;10].

3.2 Vaccine Quality Defect–Related Reactions

Quality defect–related reactions result from issues in vaccine manufacturing, storage, or transportation that compromise vaccine integrity. Cold chain failures, contamination, or deviations in production standards may reduce potency or increase reactogenicity. Such events underscore the importance of rigorous quality assurance and regulatory oversight throughout the vaccine supply chain [5;11].

3.3 Immunization Error–Related Reactions

Immunization error related reactions are preventable events caused by inappropriate handling, preparation, or administration of vaccines. Examples include incorrect dosing, use of non-sterile equipment, or administration via an improper route. These reactions highlight the critical role of healthcare worker training and adherence to standardized immunization practices [8;12].

3.4 Immunization Anxiety Related Reactions

Immunization anxiety–related reactions are psychogenic responses associated with fear or stress related to vaccination rather than the vaccine itself. Manifestations include vasovagal syncope, hyperventilation, and, in some cases, mass psychogenic illness. These events are particularly relevant in school-based or mass vaccination settings [8;14].

3.5 Coincidental Events

Coincidental events are medical conditions that occur following vaccination by chance alone, without a causal relationship. Differentiating coincidental events from vaccine-related reactions is a central challenge in vaccine safety surveillance and a key function of structured causality assessment [12].

Local adverse reactions such as pain, erythema, and swelling are primarily mediated by innate immune responses at the injection site. Activation of pattern recognition receptors and subsequent cytokine release contribute to inflammation, which may be enhanced by adjuvants designed to improve immunogenicity [11]. Systemic symptoms such as fever, fatigue, and malaise reflect the activation of adaptive immune responses and cytokine-mediated signaling. In rare cases, hypersensitivity reactions may occur, including IgE-mediated anaphylaxis triggered by vaccine excipients such as gelatin or polyethylene glycol [15;16;17]. The risk and nature of AEFIs are influenced by host-related factors, including age, sex, genetic predisposition, and underlying medical conditions. Understanding these factors is increasingly important in the context of personalized medicine and risk stratification [11,14].

The majority of AEFIs are mild, self-limiting, and resolve without intervention. Local reactions, including pain and redness at the injection site, are the most frequently reported [18]. Systemic reactions such as fever, headache, and myalgia are also common. Studies of inactivated COVID-19 vaccines in pediatric populations, for example, report systemic reactions in approximately 16.7% of recipients following the first dose [3;18]. Serious AEFIs are rare but clinically significant. Anaphylaxis is an acute, life-threatening hypersensitivity reaction that typically occurs within minutes of vaccination [15;9]. Neurological events, such as febrile seizures, may occur infrequently following certain childhood vaccines, while Guillain–Barré syndrome has been associated with a very small increased risk following specific influenza vaccines [2;9]. Special considerations have emerged during the COVID-19 vaccination era, including thrombosis with thrombocytopenia syndrome linked to adenovirus-vector vaccines and myocarditis associated with mRNA vaccines [19;9;20].

Effective pharmacovigilance underpins vaccine safety assurance. Passive surveillance systems, including national spontaneous reporting mechanisms, are essential for signal detection but are limited by underreporting and variable data quality. As of 2022, only 43% of countries met the WHO target for reporting serious AEFIs [22]. Active surveillance systems, such as the Vaccine Safety Datalink, utilize linked healthcare databases to enable near real-time monitoring and robust epidemiological analysis [23]. Causality assessment is a structured process used to determine whether a vaccine is likely responsible for an adverse event [12]. The WHO causality assessment algorithm evaluates temporal relationships, biological plausibility, and alternative explanations. Application of this framework has demonstrated good inter-rater reliability among trained assessors [12].

Preparedness for managing severe AEFIs, particularly anaphylaxis, is essential at all vaccination sites. Equally important is transparent, timely, and empathetic risk communication to address public concerns and mitigate vaccine hesitancy [22]. Sustainable vaccine safety systems require ongoing investment in infrastructure, cold chain maintenance, and healthcare worker training. Strengthening reporting practices and fostering a culture of safety are critical components of effective immunization programs [19].

Adverse Events Following Immunization are an inherent aspect of vaccination and, more broadly, of medical interventions [3]. The overwhelming majority of AEFIs are minor and self-limiting, while serious events remain exceedingly rare. The central challenge for public health lies in maintaining vigilant, equitable, and responsive vaccine safety systems. Future efforts should prioritize closing global surveillance gaps, integrating digital technologies for rapid signal detection, and advancing research into host-specific risk factors to further enhance vaccine safety and public confidence.