Vaccines & Preventable Disease
Usually the final weeks of an immunology & genetics course, after adaptive immunity and memoryVaccination is applied immunology: every platform is a bet about which arm of the adaptive response you need, and every contraindication follows from the mechanism. The exam writes this two ways — classify the vaccine (live vs inactivated vs subunit vs toxoid vs conjugate vs mRNA, and who can't get it) and recognize the disease (a barking cough, an asymmetric flaccid paralysis, a rash that starts on the face). Know both directions cold; the vignettes are pattern recognition, and the patterns here reappear on the boards for the rest of your training.
This guide is the frame. The exam is written from your lectures— drop this topic's slides on the dashboard to get flashcards and board-style questions from your own course, and quiz them all term (one cumulative exam rewards nothing more than early, repeated self-testing). The immunology & genetics playbook has the weekly loop.
AI-authored study notes, not faculty-reviewed and not medical advice. Verify every mechanism against your course materials and faculty; programs differ in emphasis.
High-Yield Pearl
Live attenuated is the only platform that replicates — which is exactly why it gives the strongest, longest immunity and exactly why pregnancy and immunocompromise forbid it; every other vaccine question is a footnote to that trade-off.
What the exam asks
Mechanisms, patterns, and what happens when each one fails.
- Active vs passive immunity. Active = the patient's own immune system responds to antigen (infection or vaccination) — slow onset, memory, long duration. Passive = preformed antibody given or transferred (maternal IgG across the placenta, IgA in breast milk, IVIG, rabies or tetanus immune globulin, monoclonals like palivizumab/nirsevimab) — immediate protection, no memory, fades in weeks to months. Post-exposure prophylaxis for rabies and tetanus combines BOTH: immune globulin now, vaccine for later.
- Live attenuated vaccines. Weakened replicating organism → strong, durable humoral AND cellular (CD8) immunity, often lifelong after 1–2 doses; mimics natural infection including mucosal immunity for oral/nasal routes. Examples: MMR, varicella, zoster (old Zostavax — the current recombinant Shingrix is NOT live), rotavirus, intranasal influenza (LAIV), yellow fever, oral typhoid, BCG. Contraindicated in pregnancy and significant immunocompromise (the attenuated organism can revert or disseminate).
- Inactivated, subunit, toxoid, conjugate. Inactivated whole organism (injected influenza, hepatitis A, IPV polio, rabies): killed, cannot cause disease, safe in pregnancy/immunocompromise, but weaker — needs boosters and mainly drives humoral immunity. Subunit/recombinant = purified antigen (hepatitis B surface antigen, acellular pertussis, HPV virus-like particles, Shingrix). Toxoid = inactivated exotoxin (tetanus, diphtheria) — immunity is anti-TOXIN, not anti-organism. Conjugate = polysaccharide covalently linked to a carrier protein so a T-cell-dependent response (memory, class switching) forms in infants, who respond poorly to plain polysaccharide (Hib, PCV pneumococcal, MenACWY).
- Polysaccharide vs conjugate mechanism. Pure polysaccharide (PPSV23) is a T-INDEPENDENT antigen: it crosslinks B-cell receptors directly, giving mostly IgM, no germinal centers, poor memory, and essentially no response under age 2. Conjugating the sugar to a protein carrier lets B cells present carrier peptides on MHC II to helper T cells → class switching, affinity maturation, memory, and herd effects via reduced nasopharyngeal carriage. This mechanism question is a perennial favorite.
- mRNA vaccines. Lipid-nanoparticle-delivered mRNA encoding the antigen (COVID-19 spike); host cells translate it, so antigen is presented on MHC I as well as II — good antibody plus CD8 T-cell responses. No live virus, cannot integrate into the genome, safe in immunocompromise (though response may be blunted). Key adverse-event association: rare myocarditis, mostly in young males after the second dose.
- Herd immunity. When enough of a population is immune, transmission chains break and the unvaccinated (neonates, chemo patients, the vaccine-contraindicated) are protected indirectly. The threshold rises with contagiousness (roughly 1 − 1/R0): measles, the most contagious human virus (R0 12–18), needs ~92–95% coverage — which is why measles outbreaks are the first signal of falling vaccination rates.
- The classic exanthem discriminators. Measles (rubeola): cough, coryza, conjunctivitis, Koplik spots on the buccal mucosa, then a cephalocaudal maculopapular rash; complications include pneumonia, encephalitis, and years-later SSPE. Rubella: milder 3-day rash with posterior auricular/suboccipital lymphadenopathy — the danger is congenital rubella syndrome (cataracts, sensorineural deafness, PDA) from first-trimester infection. Varicella: pruritic vesicles in crops at different stages ('dewdrop on a rose petal'); the virus goes latent in dorsal root ganglia and reactivates as zoster.
- Toxin-mediated preventables. Tetanus: Clostridium tetani tetanospasmin blocks inhibitory (GABA/glycine) neurotransmitter release → trismus, risus sardonicus, opisthotonus; wound management pairs toxoid with tetanus immune globulin when immunization is inadequate and the wound is dirty. Diphtheria: Corynebacterium diphtheriae exotoxin (ADP-ribosylates EF-2, halting protein synthesis) → gray pseudomembranous pharyngitis, 'bull neck,' myocarditis, and neuropathy; antitoxin plus antibiotics.
- Pertussis and polio. Bordetella pertussis: catarrhal (most contagious) → paroxysmal (whooping cough, post-tussive emesis, infant apnea) → convalescent stages, with marked lymphocytosis; acellular vaccine (DTaP child, Tdap adolescent/adult, and Tdap EVERY pregnancy at 27–36 weeks to protect the newborn by transplacental IgG). Poliovirus destroys anterior horn motor neurons → asymmetric flaccid paralysis with intact sensation; the US uses inactivated IPV (the live oral vaccine carries a rare vaccine-derived paralysis risk).
- Hepatitis B, HPV, and the encapsulated killers. HBV vaccine is recombinant HBsAg; immunity = anti-HBs positive with anti-HBc NEGATIVE (natural infection gives both); birth-dose plus HBIG for infants of HBsAg-positive mothers. HPV vaccine (virus-like particles, types incl. 16/18 for cancer and 6/11 for warts) prevents cervical, anal, and oropharyngeal cancer — given at 11–12, before exposure. Encapsulated bacteria (pneumococcus, meningococcus, Hib) demand vaccination in asplenia/sickle cell; meningococcal vaccines also for college freshmen in dorms, military recruits, and complement-deficient or eculizumab-treated patients.
Worth drilling · 26 concepts
Each with the one fact that identifies it. Your course's slides are the authority; use this to check your list against.
- 1Live attenuated vaccine — replicating weakened organism; avoid in pregnancy and immunocompromise
- 2Toxoid — inactivated exotoxin; the tetanus and diphtheria strategy
- 3Conjugate vaccine — polysaccharide linked to carrier protein so infants mount T-dependent memory
- 4PPSV23 — pure polysaccharide, T-independent, poor response under age 2
- 5mRNA platform — host cells translate the antigen; MHC I presentation gives CD8 responses
- 6Passive immunity — preformed antibody, immediate but temporary, no memory
- 7Herd immunity — indirect protection once coverage exceeds roughly 1 − 1/R0
- 8Koplik spots — buccal gray-white lesions that precede the measles rash
- 9Rubeola — cough, coryza, conjunctivitis, then a cephalocaudal rash
- 10Rubella — mild 3-day rash with posterior auricular lymphadenopathy; devastating in the first trimester
- 11Congenital rubella syndrome — cataracts, sensorineural deafness, patent ductus arteriosus
- 12Mumps — parotitis with orchitis risk in postpubertal males
- 13Varicella — pruritic vesicles in crops at different stages; latency in dorsal root ganglia
- 14Bordetella pertussis — paroxysmal whooping cough with post-tussive emesis and lymphocytosis
- 15Tetanospasmin — blocks GABA/glycine release, causing trismus and opisthotonus
- 16Diphtheria — gray pharyngeal pseudomembrane and 'bull neck' from an EF-2-inhibiting exotoxin
- 17Poliovirus — anterior horn cell destruction, asymmetric flaccid paralysis, sensation spared
- 18Hepatitis B vaccine — recombinant surface antigen; immunity shows anti-HBs without anti-HBc
- 19HPV vaccine — virus-like particles against 16/18 (cancer) and 6/11 (warts), given at 11–12
- 20Hib conjugate — collapsed the leading causes of childhood epiglottitis and bacterial meningitis
- 21MenACWY — required for asplenia, complement deficiency, dorm-living freshmen, and eculizumab
- 22Tdap in pregnancy — dosed at 27–36 weeks each pregnancy to shield the newborn via transplacental IgG
- 23Rabies post-exposure prophylaxis — immune globulin into the wound plus a vaccine series, active + passive together
- 24Shingrix — recombinant (non-live) zoster vaccine, safe in immunocompromise
- 25Rotavirus vaccine — live oral series with a small historical intussusception association
- 26LAIV — the intranasal influenza option; live, so not for pregnancy or immunosuppression
Practice it
Active recall over the drill list — flip and claim, match the pairs, produce the answers cold.
Every concept on the drill list, one card at a time — the name up front, the fact that identifies it on the flip. Claim each card honestly and the deck learns what to lead with next time. Progress lives in this browser only.
What everyone misses
The distinctions that lose points on this material, year after year.
- Egg allergy is not the influenza contraindication. Students reflexively pick 'egg allergy' as a reason to withhold flu vaccine; current guidance vaccinates egg-allergic patients with any age-appropriate product, no special precautions. The real contraindication to any vaccine is a severe allergic reaction (anaphylaxis) to a PRIOR DOSE or a vaccine component. For influenza specifically, a history of Guillain-Barré within 6 weeks of a prior influenza dose is the precaution to know.
- Zostavax vs Shingrix. MCQs bank on you tagging every zoster vaccine as live. The old Zostavax was live attenuated; the current recommended Shingrix is a recombinant subunit with adjuvant — non-live, preferred, and usable in immunocompromised adults. If the stem says 'recombinant zoster vaccine,' immunocompromise is NOT a contraindication.
- Toxoid immunity doesn't stop colonization. Tetanus and diphtheria vaccines raise antibody against the TOXIN, not the organism — a fully vaccinated person can still carry or be infected by C. diphtheriae; they just don't get toxin-mediated disease. Distractors that promise 'sterilizing immunity' or 'prevents carriage' from a toxoid are wrong.
- Anti-HBs vs anti-HBc serology. Vaccination produces anti-HBs ONLY (the vaccine is pure surface antigen). Anti-HBc positive means real exposure to the virus. The stem 'anti-HBs positive, anti-HBc negative' = vaccinated; adding anti-HBc flips the answer to resolved natural infection. This one serology pattern is asked constantly.
- Mild illness and household contacts are not contraindications. A low-grade fever or URI does NOT defer vaccination, and live vaccines (MMR, varicella) may be given to healthy household contacts of immunocompromised patients — the exception being that oral polio (not used in the US) sheds dangerously. Pregnancy in the PATIENT contraindicates live vaccines; pregnancy in a housemate does not.
- Conjugate vs plain polysaccharide in infants. Choosing PPSV23 for a 6-month-old is the classic trap: infants cannot mount a T-independent response, so plain polysaccharide is useless there. Conjugates (PCV, Hib, MenACWY) work in infancy precisely because the carrier protein recruits T-cell help. If the stem hinges on age under 2, the answer is the conjugate and the mechanism is T-dependent memory.
- Passive immunity questions disguised as vaccine questions. A neonate protected against measles for the first several months, or a needlestick managed with HBIG, is PASSIVE immunity — immediate, antibody-mediated, temporary, no memory. Students pick 'active' because a vaccine appears somewhere in the stem. Ask what is actually protecting the patient right now: transferred antibody or their own response.
Clinical correlations
Where this topic shows up again — in clinic, on rotations, and on the boards.
- The unvaccinated child with drooling and stridor. Sudden high fever, tripod positioning, drooling, muffled voice → epiglottitis, classically H. influenzae type b in the under-immunized. The Hib conjugate vaccine made this rare — vaccination history is the first question in the airway-emergency vignette, and the mechanism link is conjugation enabling infant immunity to a capsular polysaccharide.
- Rusty nail, unclear tetanus history. Dirty or puncture wound + fewer than 3 documented toxoid doses (or unknown status) = tetanus immune globulin PLUS Td/Tdap; clean minor wound = toxoid alone if >10 years since the last dose (>5 years for dirty wounds). The logic is pure immunology: passive antibody covers the gap while active immunity builds.
- Asplenic and sickle cell patients. No spleen → no efficient clearance of encapsulated organisms → overwhelming post-splenectomy infection. These patients get pneumococcal (PCV then PPSV23), meningococcal (ACWY and B), and Hib vaccination, ideally 2 weeks BEFORE elective splenectomy, plus prophylactic penicillin in children. Any 'splenectomy planned' stem is a vaccination question in disguise.
- Whooping infant, coughing parent. Infants too young to complete DTaP die of pertussis caught from adults whose immunity has waned — hence Tdap every pregnancy (transplacental IgG) and cocooning of close contacts. An infant with paroxysmal cough, apnea, and lymphocytosis needs a macrolide, and so do the household contacts regardless of symptoms.
- Pregnant woman and the rubella titer. Non-immune rubella titer found on prenatal labs: you CANNOT give MMR during pregnancy (live vaccine) — vaccinate immediately postpartum instead, and counsel avoidance of exposure meanwhile. The stakes are congenital rubella syndrome; the same logic defers varicella vaccine but allows Tdap and inactivated influenza during pregnancy.
- Measles outbreak thinking. Measles is airborne, contagious 4 days before the rash, and has an R0 of 12–18, so a single case in a low-coverage community is an emergency. Post-exposure: MMR within 72 hours for eligible contacts, immune globulin within 6 days for infants, pregnant women, and the immunocompromised — active if you can, passive if you can't.
- Shingles in a young adult. Zoster in a patient under 50 without known immunosuppression should prompt you to consider HIV testing — reactivation of latent varicella from dorsal root ganglia is held in check by T-cell immunity, so early or recurrent zoster is a T-cell red flag. Dermatomal vesicular rash that respects the midline is the pattern; antivirals within 72 hours.
- HPV and cancer prevention. HPV vaccination at 11–12 (before sexual debut) prevents cervical, anal, penile, and oropharyngeal cancers driven by types 16/18, whose E6/E7 oncoproteins degrade p53 and Rb — the one vaccine question that doubles as a genetics/oncology question. Vaccination does not replace cervical cancer screening.