A 2-year-old boy is brought in by his parents to his pediatrician. The boy was born by spontaneous vaginal delivery at 39 weeks and 5 days after a normal pregnancy. The boy has received all age-appropriate vaccinations as of his last visit at 18 months of age. Of note, the boy has confirmed sickle cell disease and the only medication he takes is penicillin prophylaxis. The parents state that they plan on enrolling their son in a daycare, which requires documentation of up-to-date vaccinations. The pediatrician states that their son needs an additional vaccination at this visit, which is a polysaccharide vaccine that is not conjugated to protein. Which of the following matches this description?

Live attenuated influenza vaccine

A 9-month-old boy is brought to a pediatrician by his parents for routine immunization. The parents say they have recently immigrated to the United States from a developing country, where the infant was receiving immunizations as per the national immunization schedule for that country. The pediatrician prepares a plan for the infant’s immunizations as per standard US guidelines. Looking at the plan, the parents ask why the infant needs to be vaccinated with injectable polio vaccine, as he had already received an oral polio vaccine back in their home country. The pediatrician explains to them that, as per the recommended immunization schedule for children and adolescents in the United States, it is important to complete the schedule of immunizations using the injectable polio vaccine (IPV). He also mentions that IPV is considered safer than OPV, and IPV has some distinct advantages over OPV. Which of the following statements best explains the advantage of IPV over OPV to which the pediatrician is referring?

IPV is known to produce higher titers of serum IgG antibodies than OPV

IPV is known to produce higher titers of mucosal IgG antibodies than OPV

IPV is known to produce virus-specific CD4+ T cells that produce interleukins and interferons to control polio viruses

IPV is known to produce higher titers of mucosal IgA antibodies than OPV

IPV is known to produce virus-specific CD8+ T cells that directly kill polio-infected cells

A 35-year-old man is brought to the emergency department by his wife because of a 1-week history of progressive confusion, myalgia, and nausea. His wife says that he first reported headaches and fatigue 10 days ago, and since then “he has not been himself”. He has refused to drink any liquids for the last day. Two months ago, he helped his neighbor remove a raccoon's den from her backyard. He appears agitated. His temperature is 100.8°F (38.2°C). Examination shows excessive drooling. Muscle tone and deep tendon reflexes are increased bilaterally. Administration of which of the following is most likely to have prevented this patient's condition?

Inosine monophosphate dehydrogenase inhibitor

RNA-dependent DNA polymerase inhibitor

Immunoglobulin against a bacterial protein

A previously healthy 15-year-old girl is brought to the emergency department 24 hours after the onset of a severe headache. She returned from a 1-week camping trip 3 days ago; she went spelunking and swimming in a freshwater lake during the trip. She is agitated, uncooperative, and oriented only to person. Her temperature is 38.9°C (102°F), pulse is 112/min, respirations are 20/min, and blood pressure is 100/68 mm Hg. There are several crusted insect bites on her extremities. Neurologic examination shows diffuse hyperreflexia and an extensor plantar response bilaterally. Her neck is supple without lymphadenopathy. An MRI of the brain shows asymmetrical, bitemporal hyperintensities. A lumbar puncture is performed. Cerebrospinal fluid analysis shows: Opening pressure 150 mm H2O Glucose 58 mg/dL Protein 108 mg/dL Leukocyte count 150/mm3 Segmented neutrophils 15% Lymphocytes 85% Erythrocyte count 25/mm3 Which of the following is the most likely causal pathogen?

Tick-borne encephalitis virus (TBEV)

Viral vaccines for USMLE Step 3: High-Yield Microbiology Lessons

Vaccine Types - The Immunity Arsenal

Live-Attenuated Vaccines
- Mechanism: Weakened pathogen that replicates in the host.
- Immunity: Strong, lifelong; humoral (IgA/IgG) & cell-mediated.
- Cons: Risk of disease in immunocompromised; reversion to virulence.
- 📌 "LIVE! See SMALL YELLOW CHICKENS get vaccinated with SABIN and MMR!" (Smallpox, Yellow Fever, Chickenpox, Sabin Polio, MMR). Also: Rotavirus, Influenza (intranasal).
Killed-Inactivated Vaccines
- Mechanism: Pathogen is killed/inactivated; cannot replicate.
- Immunity: Mainly humoral (IgG); less robust, requires boosters.
- Pros: Safer, no reversion risk.
- Examples: Rabies, Influenza (shot), Polio (Salk), Hepatitis A (RIP-A).
Subunit/Recombinant Vaccines
- Mechanism: Includes only specific antigenic components (e.g., surface protein).
- Immunity: Good humoral response; requires adjuvants.
- Pros: Very safe, minimal side effects.
- Examples: Hepatitis B, HPV, Pertussis (acellular).

⭐ Live attenuated vaccines induce both humoral and cell-mediated immunity, mimicking natural infection. Inactivated vaccines primarily stimulate a humoral (antibody) response and may require boosters.

Types of Viral Vaccines

Live vs. Inactivated - A Tale of Two Titers

Immune response to inactivated vs. live attenuated vaccines

Feature	Live Attenuated Vaccines	Inactivated (Killed) Vaccines
Organism	Weakened virus, can replicate	Killed virus, cannot replicate
Immunity	Strong, long-lasting; Humoral & Cellular (CD4+/CD8+)	Weaker, shorter; Mainly Humoral (IgG)
Boosters	Generally not required	Multiple boosters needed
Safety	⚠️ Risk of reversion to virulence; Contraindicated in pregnancy & immunodeficiency	Safer, no risk of causing disease
IgA Response	Robust secretory IgA response	Poor IgA response
Examples	MMR, Varicella, Rotavirus, Sabin polio, Intranasal flu	Hepatitis A, Rabies, Salk polio, Injected flu

📌 Mnemonic (Live): "RNA Virus Inside Me" (Rotavirus, Varicella, Influenza-intranasal, MMR)

Modern Vax - Subunit & Nucleic Acid

Subunit/Recombinant Polysaccharide/Conjugate
- Contain only specific purified macromolecules (e.g., proteins, polysaccharides) of a pathogen.
- Antigens are often produced via recombinant DNA technology.
- Examples: Hepatitis B (HBsAg), HPV (L1 protein), acellular Pertussis, N. meningitidis, S. pneumoniae, H. influenzae type b.
- Safe for immunocompromised patients; require adjuvants and boosters.
Nucleic Acid Vaccines
- mRNA Vaccines:
  - Formulation: mRNA encoding a viral antigen within a lipid nanoparticle (LNP).
  - Mechanism: Host cells translate mRNA → produce viral protein → presented on MHC class I & II.
  - Examples: COVID-19 (Pfizer, Moderna).
- Viral Vector Vaccines:
  - Mechanism: A modified, non-replicating virus (e.g., adenovirus) delivers DNA encoding the antigen.
  - Examples: COVID-19 (J&J, AstraZeneca), Ebola.

⭐ mRNA vaccines are potent because they generate both a strong humoral (B-cell) and cell-mediated (T-cell) immune response, as the host cell synthesizes the antigen endogenously.

mRNA, DNA, and Viral Vector Vaccine Mechanisms

High‑Yield Points - ⚡ Biggest Takeaways

Live attenuated vaccines (e.g., MMR, Sabin polio) induce potent humoral and cellular immunity but are contraindicated in immunocompromised patients and pregnancy.

Killed/inactivated vaccines (e.g., Salk polio, Hepatitis A) are safer but less immunogenic, requiring boosters and inducing mainly humoral responses.

Subunit (HBV, HPV) and mRNA (COVID-19) vaccines are non-infectious, presenting only specific viral antigens or their genetic code.

The Sabin (live) polio vaccine provides stronger mucosal IgA immunity than the Salk (killed) version.

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Viral vaccines