Chapter 17: Immunization – Principles and Applications

Immunization: Principles and Historical Context

Overview of Immunization

Immunization is a critical strategy in microbiology and public health, designed to protect individuals and populations from infectious diseases. It involves the administration of antigens or antibodies to induce immunity, either actively or passively.

• Active Immunization: Administration of antigens so the patient actively mounts an adaptive immune response.

• Passive Immunotherapy: Transfer of antibodies formed by an immune individual or animal to provide immediate, short-term protection.

Brief History of Immunization

The practice of immunization has evolved over centuries, beginning with observations of disease resistance and culminating in modern vaccine technology.

• Early observations in China: Children who recovered from smallpox did not contract the disease again. This led to variolation, the deliberate infection with material from smallpox scabs.

• 1796: Edward Jenner developed the process of vaccination using cowpox to protect against smallpox.

• 1879: Louis Pasteur developed a vaccine against Pasteurella multocida.

• Discovery that vaccines protect through the action of antibodies led to the development of antibody transfer techniques.

Despite advances, some developing nations lack access to vaccines, and effective vaccines for certain pathogens remain unavailable. Concerns about vaccine safety can also hinder development and uptake.

Impact of Immunization on Disease Prevalence

Vaccination programs have dramatically reduced the incidence of many infectious diseases, as illustrated by the decline in polio and measles cases following vaccine introduction.

• Example: The introduction of polio and measles vaccines in the United States led to a rapid and sustained decrease in reported cases.

Types of Vaccines and Their Characteristics

Attenuated (Modified Live) Vaccines

Attenuated vaccines use pathogens with reduced virulence, a process known as attenuation. These vaccines can cause mild infections but generally stimulate strong, long-lasting immune responses and may provide contact immunity.

• Risk: Modified microbes may retain enough residual virulence to cause disease in susceptible individuals.

Inactivated (Killed) Vaccines

Inactivated vaccines are safer than live vaccines and come in two forms:

• Whole agent vaccines: Contain inactivated but whole microbes.

• Subunit vaccines: Contain antigenic fragments of microbes.

These vaccines often require multiple doses and may include adjuvants—chemicals that enhance antigenicity.

Toxoid Vaccines

Toxoid vaccines use chemically or thermally modified toxins to stimulate active immunity, particularly for bacterial diseases. They primarily induce antibody-mediated immunity and require multiple doses due to limited antigenic determinants.

Combination and Recombinant Vaccines

• Combination vaccines: Simultaneously administer antigens from several pathogens.

• Recombinant gene technology: Utilizes recombinant DNA techniques to improve vaccine efficacy, safety, and cost-effectiveness.

Vaccine Manufacture and Administration

Production Methods

Vaccines are mass-produced by growing microbes in culture vessels. Viruses are often cultured in chicken eggs, which can be problematic for individuals with egg allergies.

CDC-Recommended Immunization Schedule

The Centers for Disease Control and Prevention (CDC) provides guidelines for vaccine administration across different age groups to maximize public health benefits.

Principal Vaccines to Prevent Human Diseases

The following table summarizes key vaccines, their target diseases, types, and methods of administration, as recommended by the CDC.

Vaccine	Disease Agent	Disease	Vaccine Type	Method of Administration
Hepatitis B	Hepatitis B virus	Hepatitis B	Inactive subunit (recombinant yeast)	Intramuscular
Rotavirus	Rotavirus	Gastroenteritis	Attenuated, recombinant	Oral
DTaP	Diphtheria toxin, Tetanus toxin, Bordetella pertussis	Diphtheria, Tetanus, Whooping cough	Toxoid, Toxoid, Inactivated subunit	Intramuscular
Hib	Haemophilus influenzae	Meningitis, pneumonia, epiglottitis	Inactivated subunit	Intramuscular
Pneumococcal (PCV)	Streptococcus pneumoniae	Pneumonia	Inactivated subunit	Intramuscular
Polio	Poliovirus	Poliomyelitis	Inactivated (attenuated also available)	Subcutaneous or intramuscular (attenuated: oral)
Influenza	Influenzaviruses	Flu	Inactivated subunit	Intramuscular or oral
MMR	Measles, Mumps, Rubella viruses	Measles, Mumps, Rubella	Attenuated	Subcutaneous
Varicella-zoster	Chickenpox virus	Chickenpox, shingles	Attenuated	Subcutaneous
Hepatitis A	Hepatitis A virus	Hepatitis A	Inactivated whole	Intramuscular
HPV	Human papillomaviruses	Genital warts, cervical cancer	Inactive recombinant	Intramuscular
Meningococcal	Neisseria meningitidis	Meningitis	Inactive	Subcutaneous or intramuscular

Vaccine Safety and Limitations

Potential Problems with Immunization

• Mild toxicity (e.g., soreness at injection site, fever)

• Risk of anaphylactic shock (severe allergic reaction)

• Residual virulence from attenuated viruses

• Unsubstantiated allegations linking vaccines to autism, diabetes, and asthma

Extensive research has not confirmed a causal relationship between vaccines and these chronic conditions.

Passive Immunotherapy

Principles and Applications

Passive immunotherapy involves the administration of antiserum containing preformed antibodies, providing immediate but temporary protection against infection or disease.

• Limitations include the risk of allergic reactions (serum sickness), rapid degradation of antibodies, and lack of long-term protection.

• Development of hybridomas (cells that produce monoclonal antibodies) has helped overcome some limitations.

Summary Table: Active vs. Passive Immunity

Characteristic	Active Immunization	Passive Immunotherapy
Source of Immunity	Antigen exposure (vaccine)	Preformed antibodies (antiserum)
Onset of Protection	Delayed (days to weeks)	Immediate
Duration of Protection	Long-term (memory cells)	Short-term (antibodies degrade)
Memory Response	Yes	No