Introduction

A vaccine functions as a pharmacologic compound that strengthens an individual's immunity against a specific disease. When a pathogenic bacterium or virus enters the body, the immune system identifies it as foreign by detecting distinct protein segments of the invading organism, known as antigens. Vaccines contain a modified form of the disease-causing agent, which may include a weakened or inactivated microbe, a neutralized toxin, or a protein derived from the pathogen’s surface.[1]

Modern mRNA vaccine technology introduces messenger RNA that the human body translates into a particular viral or bacterial protein, generating the target antigen internally. This process exposes the immune system to the antigen in a controlled manner, enabling the development of antibodies and memory T lymphocytes specifically tailored to recognize and combat the pathogen. By preparing the immune system in advance, vaccination ensures a faster and more effective defense if the body encounters the infectious organism in the future. Without prior immunization, the initial exposure to the natural pathogen can overwhelm the immune response, potentially leading to severe or fatal illness before adequate protection develops.[1]

Immunization stands as one of the most powerful achievements in the history of public health, transforming once-devastating diseases into preventable conditions. Through widespread vaccination, illnesses such as smallpox, polio, and measles—once responsible for immense suffering and mortality—have been drastically reduced or eliminated in many parts of the world. This progress has led to longer life expectancy, enhanced quality of life, and strengthened societal resilience against infectious threats. Despite these profound successes, vaccination continues to face controversy in the modern age, where misinformation, skepticism, and inequitable access pose ongoing challenges to global health progress.[2][3]

The history of immunization dates back to 1796, when Edward Jenner inoculated a 13-year-old boy with cowpox material, demonstrating immunity to smallpox and laying the foundation for modern vaccines. Jenner’s work led to the creation of the first smallpox vaccine in 1798, ultimately culminating in the global eradication of smallpox by 1979.[4][5][6] Over the centuries, vaccine development has advanced through the refinement of scientific methods, the discovery of microbial pathogens, and the innovation of novel vaccine platforms, eg, mRNA technology. These achievements not only expanded the scope of preventable diseases but also reinforced the vital role of immunization in safeguarding humanity against both established and emerging infections.

In the contemporary era, the World Health Organization (WHO) has emphasized lifelong immunization as a cornerstone of the 2030 Sustainable Development Goals, aiming to reduce the burden of infectious and noncommunicable diseases by expanding vaccine access, supporting vaccine research, and integrating immunization into all stages of healthcare delivery.[7] This approach extends beyond childhood vaccination to encompass adolescents, adults, pregnant women, healthcare workers, and older adults, emphasizing the necessity of catch-up doses, booster programs, and robust vaccine data systems. The COVID-19 pandemic underscored the critical need for strong immunization infrastructures, revealing vulnerabilities but also catalyzing new strategies for outbreak response and system resilience. Building a future of sustained immunization requires not only scientific advancement but also cross-sectoral collaboration, public trust, and a shared commitment to ensuring equitable protection against vaccine-preventable diseases for all populations.[8]

Anatomy and Physiology

Vaccines primarily influence the immune system through the activity of B and T lymphocytes. Before exposure, the immune system contains populations of these lymphocytes, each capable of responding to a specific antigen. Many antigens require a coordinated response involving both B lymphocytes and T lymphocytes, a process known as T-cell-dependent immunity. Some antigens, however, can directly stimulate B lymphocytes to produce antibodies without assistance from T lymphocytes, thereby eliciting T-cell-independent immunity.

The introduction of an antigen through vaccination initiates the immune response. Macrophages first engulf and digest the foreign material, breaking it down into smaller peptide fragments. These peptides move to the macrophage surface and are displayed by major histocompatibility complex (MHC) molecules, which exist in 2 forms: MHC-I and MHC-II. Antigen presentation triggers the release of inflammatory mediators, including cytokines and interferons, which amplify the immune reaction and promote communication among immune cells.[1]

During T-cell-dependent immunity, T-helper cells recognize the presented antigen and activate corresponding B lymphocytes to proliferate and produce antibodies. As B lymphocytes proliferate, they differentiate into plasma cells that secrete specific antibodies. These antibodies perform several critical functions: neutralizing soluble protein toxins (antitoxins), disrupting bacterial membranes to facilitate intracellular digestion (lysins), preventing viral replication (neutralizing antibodies), and blocking bacterial adhesion to mucosal surfaces (antiadhesion antibodies). In response to vaccination, IgM antibodies form first, followed by a gradual transition to IgG antibodies over the ensuing weeks. Vaccines fall into several categories, including live attenuated, killed or inactivated, subunit, mRNA, and toxoid vaccines (eg, tetanus).[9]

Indications

Vaccination recommendations depend on several factors, including age, medical conditions, prior vaccination history, lifestyle, and occupational exposure. The Centers for Disease Control and Prevention (CDC) advises annual influenza vaccination for all adults, pneumococcal vaccination for older adults and those with chronic diseases, and tetanus-diphtheria boosters every 10 years with at least 1 adult dose including acellular pertussis. The inclusion of acellular pertussis in adult immunization aims to reduce the transmission of whooping cough to infants and individuals unable to receive vaccines. Adults born after 1956 who never received measles, mumps, rubella, or varicella vaccines during childhood require these immunizations later in life to maintain population immunity and prevent disease resurgence. Additionally, childhood vaccinations serve as a critical defense against numerous viral and bacterial infections and their complications. Regional differences in disease exposure influence specific vaccination recommendations for children, leading to variations worldwide. 

Immunization remains a cornerstone of disease prevention, particularly for conditions, eg, influenza and pneumococcal infections, which continue to cause significant illness, hospitalization, and death each year. Adults aged 65 years and older face the greatest vulnerability, accounting for the majority of influenza-related hospitalizations and deaths despite representing a smaller portion of the population. Influenza alone contributes to hundreds of thousands of hospitalizations and tens of thousands of deaths annually in the United States. Pneumococcal disease also continues to exact a heavy toll, with tens of thousands of invasive cases and thousands of deaths recorded each year, the vast majority occurring among adults. These data highlight the critical need for consistent immunization efforts across all age groups to prevent severe disease outcomes and reduce strain on healthcare systems.

Additional vaccines recommended include human papillomavirus, hepatitis A, and hepatitis B. Meningococcal vaccination protects those at elevated risk, particularly those residing in communal living environments, eg, college dormitories or military barracks. Pneumococcal vaccination remains vital for adults with chronic medical conditions, especially pulmonary disorders, eg, chronic obstructive pulmonary disease (see Image. United States Centers for Disease Control and Prevention Recommended Vaccine Schedule).[10][11]

Passive immunization benefits individuals who are unable to produce antibodies (eg, immunocompromised patients). This approach also applies when disease progression could outpace the development of active immunity, as seen in rabies exposure. In such cases, the patient may contract the disease before the immune system generates a sufficient antibody response through active vaccination, making passive immunization a vital measure for immediate protection.[12]

Despite the proven effectiveness of vaccines, immunization coverage remains below national goals, leaving substantial portions of the population vulnerable to preventable diseases. To address this gap, quality measures, eg, the composite adult vaccination index, were developed to monitor coverage for key vaccines, including influenza, pneumococcal, herpes zoster, and tetanus-diphtheria-pertussis. Data from national surveys provide valuable insight into disparities in access and adherence, guiding public health efforts toward education, accessibility, and system-level improvement. Strengthening immunization practices requires coordination among healthcare practitioners, integration with chronic disease management, and public awareness emphasizing vaccination as a lifelong commitment rather than a childhood milestone.[13]

Contraindications

Contraindications to immunization occur infrequently, and most vaccines remain safe for the vast majority of individuals.[14] The primary contraindication is a known allergy to a vaccine or its components. Patients with compromised immune function, including those with acquired immunodeficiency syndrome or other immune deficiencies, require caution when receiving live attenuated vaccines. Live attenuated vaccines may replicate within the host, posing potential risks to individuals with weakened immune defenses. Pregnant women should avoid live vaccines unless an immediate and significant threat exists, eg, exposure during a polio outbreak, because live vaccines can potentially infect the fetus. Severely immunocompromised individuals generally should not receive live vaccines due to the heightened risk of vaccine-derived infection.[15]

Caution also applies when vaccinating children experiencing acute moderate to severe illness, whether or not fever is present. Postvaccination fever in these cases may create diagnostic uncertainty, as distinguishing between vaccine-related reactions and symptoms of the underlying illness can be challenging, complicating clinical management. According to the CDC, severe combined immunodeficiency and a prior history of intussusception represent specific contraindications to rotavirus vaccination.[16] The Advisory Committee on Immunization Practices (ACIP) further advises that patients who experienced encephalopathy within 7 days of a previous pertussis-containing vaccine should not receive additional doses of vaccines containing pertussis components.[16][17]

Equipment

Immunizations are administered with syringes and sterile needles. The size and type of syringe and needle depend on the vaccination, the specific route of administration, and the clinician's preference. Personal protective equipment, including medical gloves, is also required. Alcohol or another topical disinfectant is used to prepare the skin before vaccine administration. A simple band-aid, gauze, and tape can cover the site following administration.[18]

Personnel

Immunizations are administered by a variety of healthcare personnel, including medical assistants, licensed practical nurses/licensed vocational nurses, registered nurses, nurse practitioners, physicians' assistants, physicians, pharmacy technicians, and pharmacists.

Preparation

Proper vaccine preparation and administration rely on strict aseptic technique and adherence to infection prevention standards. Every practice, clinic, and pharmacy must maintain adequate supplies for vaccination. Hand hygiene must precede vaccine preparation, and vials must be inspected carefully for damage, particulate matter, or contamination before use. Each vaccine must be verified for correct storage temperature and prepared in a clean, designated area separate from patient care zones and potential contaminants. Declaring the preparation area a “Quiet Zone” or “No Interruptions Area” helps minimize distractions, a known contributor to vaccine errors.[CDC. Vaccine Administration: During Vaccination. June 24, 2025]

Vaccine preparation must follow the manufacturer's directions provided in the package insert. Expiration dates on vaccines, diluents, syringes, and needles must be checked before use, and expired products must never be administered. Each injection requires a separate needle and syringe, and vaccines must be prepared only when ready for immediate administration. Only self-prepared vaccines should be administered, and all used syringes and needles must be discarded promptly in puncture-proof sharps containers within the vaccination area. Needle selection must ensure delivery of the vaccine to the correct tissue for optimal immune response and minimal local reaction. Factors influencing needle choice include administration route, patient age, weight, sex, and injection site.[CDC. Vaccine Administration: During Vaccination. June 24, 2025]

Single-dose vials contain no preservatives and are intended for use with a single patient. After withdrawing the appropriate dose, the vial and any remaining contents must be discarded. Single-dose vials must never be used for multiple patients. Manufacturer-filled syringes, sealed under sterile conditions and containing a single vaccine dose, also lack preservatives. Once the sterile seal has been broken, the vaccine must be used or discarded by the end of the workday.[CDC. Vaccine Administration: During Vaccination. June 24, 2025]

Multidose vials, which contain multiple doses and include antimicrobial preservatives, can be punctured more than once. Many vaccinations are combinations, eg, measles, mumps, rubella, tetanus, diphtheria, and acellular pertussis. Multidose vials used for multiple patients must remain in a clean, dedicated preparation area, never in patient care zones. Only the number of doses specified by the manufacturer should be withdrawn, and partial doses from separate vials must never be combined. Use of specific antiviral drugs (acyclovir, famciclovir, or valacyclovir) should be discontinued 24 hours before varicella vaccination. Avoiding antiviral use for 14 days after varicella vaccination is also recommended.[11][CDC. Vaccine Administration: During Vaccination. June 24, 2025][11]

Technique or Treatment

Immunizations are administered via 5 commonly used routes: oral, intranasal, subcutaneous, intradermal, and intramuscular. Each vaccine’s route and site are derived from clinical evidence and theoretical rationale. Varying from recommended administration routes may reduce efficacy or increase adverse effects, rendering some doses invalid and requiring revaccination.[CDC. Vaccine Administration: During Vaccination. June 24, 2025]

Oral applicators provide a single vaccine dose administered by mouth and contain no preservatives. In the United States, this method is used exclusively for the rotavirus vaccine. The live attenuated influenza vaccine, by contrast, is delivered intranasally using a manufacturer-filled sprayer that deposits the vaccine into each nostril. Subcutaneous injections deposit vaccines into the layer of fatty tissue situated between the dermis and the underlying muscle. Common vaccines given by this route include DEN4CYD, IPV, MMR, MMRV, PPSV23, and VAR. Certain vaccines (eg, MMRII, IPOL, Pneumovax 23, and ProQuad) may also be administered intramuscularly (IM) when appropriate. Intradermal injections deliver the vaccine into the space between the epidermis and hypodermis, most often on the volar surface of the forearm. The Mpox vaccine is typically administered subcutaneously, although intradermal injection may be used under Emergency Use Authorization guidelines.[CDC. Vaccine Administration: During Vaccination. June 24, 2025]

IM injections penetrate the muscle via the skin and subcutaneous tissue, with the injection site determined by age, weight, and sex. Vaccines routinely administered intramuscularly include COVID-19, DTaP, Hib, HepA, HepB, HPV, meningococcal, pneumococcal, and Tdap vaccines. During IM administration, the vaccine must be injected into the thickest portion of the target muscle belly at a 90-degree angle.[19] When multiple vaccines are delivered to the same arm, injection sites should be separated by at least 1 inch to prevent overlap of local reactions. Combination vaccines, eg, DTaP-IPV-HepB or DTaP-IPV/Hib, can reduce the number of injections. Vaccines known to cause more pain, eg, MMR and HPV, should be administered last, and those more likely to cause local reactions, eg, tetanus toxoid-containing vaccines, should be given in separate limbs when possible.[CDC. Vaccine Administration: During Vaccination. June 24, 2025]

Pain control remains a key component of vaccination practice, especially for children, since injection pain can heighten anxiety for both patients and caregivers. Evidence-based techniques to ease distress include the use of topical anesthetics, breastfeeding or providing sweet-tasting liquids during the procedure, and performing the injection swiftly without aspiration. Aspiration extends the needle’s time in tissue and increases discomfort through unnecessary movement. Distraction methods, eg, guided breathing, also help reduce pain perception. Fear of injections frequently deters adults—including healthcare workers—from receiving vaccines, yet applying the same comfort strategies used in pediatrics can effectively reduce anxiety and enhance vaccine acceptance.[CDC. Vaccine Administration: During Vaccination. June 24, 2025]

Complications

Most vaccines carry a small risk of symptoms after administration, including fever, fatigue, and myalgia.[20] These symptoms are caused by the immune response being mounted against the antigen introduced by the vaccine and usually last a day or 2. Swelling and redness may occur at the injection site, and localized muscle soreness may also occur. More serious complications are possible, though extremely rare. Anaphylaxis may occur after vaccination or due to one of its ingredients. Another very rare complication of vaccination is Guillain-Barré syndrome, a potentially life-threatening condition affecting the peripheral nervous system. The mechanism by which vaccination causes Guillain-Barré is not known. Symptoms include ascending paresthesias and paralysis, with paralysis of respiratory muscles possible.[11]

Clinical Significance

Vaccination remains a cornerstone of public health, serving as one of the most effective methods for preventing disease and promoting longevity and quality of life. Rising influence from antivaccination movements has contributed to the resurgence of diseases once considered eradicated or nearly eliminated, leading to significant outbreaks across various regions. Despite clear evidence supporting their value, preventive interventions, eg, vaccines, often receive insufficient emphasis from both primary care clinicians and patients.[21]

Many parents lack awareness of the recommended vaccination schedule for children, leaving them unaware when their child falls behind on required immunizations. Missed medical appointments further contribute to gaps in vaccination coverage. To address this issue, patient recall and reminder systems have been implemented and have demonstrated substantial success in improving compliance and maintaining timely vaccination rates across populations.

Enhancing Healthcare Team Outcomes

Immunization is a cornerstone of modern preventive medicine and public health. Vaccines have virtually eliminated or controlled many infectious diseases that once caused significant global morbidity and mortality. Despite overwhelming evidence supporting vaccine safety and effectiveness, vaccine hesitancy and misinformation continue to threaten public trust and population health. Ongoing education is essential for clinicians to understand immunologic mechanisms, recognize contraindications, and apply evidence-based vaccination practices across the lifespan.

Effective immunization delivery relies on strong interprofessional collaboration. Physicians, advanced practitioners, and nurses play key roles in screening for vaccine eligibility, administering vaccines, and monitoring for adverse effects. Pharmacists contribute through vaccine dispensing, patient counseling, and reinforcement of adherence to immunization schedules. Coordinated communication among all healthcare team members ensures accurate documentation, consistent messaging, and timely follow-up. By sharing responsibility for education and prevention, interprofessional teams improve patient-centered care, strengthen vaccine confidence, and enhance population-level outcomes and safety.

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Disclosure: Jacob Ginglen declares no relevant financial relationships with ineligible companies.

Disclosure: Michael Doyle declares no relevant financial relationships with ineligible companies.