U01.02.027 Vaccination

Understanding the types of vaccines and their mechanisms is critical for mastering immunology and infectious disease questions on the USMLE Step 1. This guide breaks down vaccine types, their characteristics, and key examples to aid your preparation.

Overview of Vaccine-Induced Immunity

Vaccination induces an active immune response—either humoral (antibody-mediated) or cellular—to specific pathogens. This process helps protect against infectious diseases by priming the immune system for future exposure.

Types of Vaccines

Vaccine Type	Description	Pros	Cons	Examples
Live Attenuated	Microorganism loses pathogenicity but retains capacity for transient growth within the host.	Strong, often lifelong immunity	Risk of reversion to virulent form. Often contraindicated in pregnancy and immunodeficiency.	MMR, Varicella, Yellow fever, Polio (Sabin), BCG, Smallpox, Rotavirus.
Killed/Inactivated	Pathogen is inactivated by heat/chemicals. Mainly induces a humoral response.	Safer than live vaccines	Weaker immune response, requires boosters.	Hepatitis A, Influenza, Rabies, Polio (SalK).
Subunit	Contains only antigens that best stimulate the immune system.	Lower chance of adverse reactions	Expensive, weaker immune response.	HBV, HPV, acellular pertussis, Neisseria meningitidis, Streptococcus pneumoniae.
Toxoid	Denatured bacterial toxin with an intact receptor-binding site.	Protects against bacterial toxins	Antitoxin levels decrease over time, requiring boosters.	Clostridium tetani, Corynebacterium diphtheriae.

Detailed Insights

1. Live Attenuated Vaccines

Mechanism: Retains the ability for transient growth, mimicking natural infection and inducing both humoral and cellular immunity.
Clinical Pearl: MMR and varicella vaccines are safe for HIV-positive patients if CD4 count ≥ 200 cells/mm³.
Mnemonic: “Attention Teachers! Please Vaccinate Small, Beautiful Young Infants with MMR Regularly!”

2. Killed/Inactivated Vaccines

Mechanism: Pathogen is inactivated while maintaining antigenic properties to stimulate a humoral response.
Clinical Pearl: Booster shots are often necessary to maintain immunity.
Mnemonic: “A TRIP could Kill you” (A for Hepatitis A, T for Typhoid, R for Rabies, I for Influenza, P for Polio [SalK]).

3. Subunit Vaccines

Mechanism: Contains selected antigens that elicit immune responses.
Clinical Pearl: These vaccines are particularly useful for patients at risk of severe adverse reactions.
Examples: HBV (HBsAg), HPV (types 6, 11, 16, 18).

4. Toxoid Vaccines

Mechanism: Uses denatured toxins to create antibodies, providing immunity without disease risk.
Clinical Pearl: Booster shots ensure long-term protection.
Examples: Diphtheria and tetanus toxoids.

Points to Remember

Mechanism Matters: Focus on whether the vaccine induces humoral, cellular, or both types of responses.
Contraindications: Understand which vaccines are contraindicated in pregnancy or immunocompromised patients.
Boosters: Know which vaccines require boosters and why.
Mnemonics Are Key: Use memory aids like “Attention Teachers!” and “A TRIP” to recall vaccine examples easily.

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