Vaccines, Sterilization and Disinfection

• Live attenuated — MMR, varicella, zoster (live formulation), rotavirus, intranasal influenza, yellow fever, oral polio, BCG. Produce strong cellular and humoral immunity, often lifelong, from a single or few doses. Contraindicated in pregnancy and in significant immunocompromise, with defined exceptions (MMR and varicella are given in HIV with adequate CD4 counts).
• Inactivated / killed — injected polio, hepatitis A, rabies, injected influenza. Safe in immunocompromise; humoral immunity only, weaker, requiring boosters.
• Subunit and toxoid — acellular pertussis, hepatitis B (recombinant surface antigen), HPV, tetanus and diphtheria toxoids (which induce antitoxin rather than antibacterial immunity).
• Conjugate vaccines link a polysaccharide capsule to a protein carrier, converting a T-independent response into a T-dependent one — which is why they work in infants and generate memory. Examples: H. influenzae type b, pneumococcal conjugate, meningococcal conjugate.
• mRNA and viral vector vaccines deliver the instructions for an antigen; they are not live and are not contraindicated in immunocompromise.
• Sterilization versus disinfection: sterilization destroys all microbial life including spores — autoclaving (steam under pressure) is the standard. Disinfection reduces organisms on surfaces; **alcohol-based hand rub does not kill C. difficile spores**, so soap-and-water handwashing and contact precautions are required there.

Antigen platforms (what is actually in the vial)

• Live attenuated: replication-competent organisms passaged to lose virulence (MMR, varicella, yellow fever, oral polio, BCG, rotavirus, intranasal influenza). Because they replicate, they present antigen endogenously and load MHC class I as well as class II.
• Inactivated whole organism: formalin- or heat-killed (rabies, hepatitis A, injected polio, injected influenza). No replication, so antigen is handled as exogenous protein and the response is predominantly MHC class II/CD4+, with little CD8+ priming apart from limited cross-presentation by dendritic cells.
• Toxoid: formaldehyde-treated exotoxin retaining immunogenic epitopes but no catalytic activity — tetanus and diphtheria. Immunity is antitoxin, not antibacterial; the organism can still colonize.
• Subunit/recombinant: hepatitis B surface antigen and HPV L1 protein expressed in yeast and self-assembled into virus-like particles; recombinant zoster glycoprotein E with adjuvant.
• Polysaccharide vs conjugate: bare capsular polysaccharide (23-valent pneumococcal, older meningococcal) cross-links B-cell receptors directly. Protein conjugation (diphtheria CRM197, tetanus toxoid, H. influenzae protein D) recruits carrier-specific T cells.
• Nucleic acid and vector: mRNA in lipid nanoparticles and replication-incompetent adenoviral vectors deliver a transgene; the host cell makes the antigen. These are not live.

Microbiology of sterilization monitoring

• Spores are the reference organism because the dipicolinic acid–calcium core and keratin-like coat make them the most heat- and chemical-resistant form of life. Vegetative bacteria, fungi, and enveloped viruses die well before spores.
• Geobacillus stearothermophilus spore strips are the biological indicator for steam autoclaving; growth after processing means sterilization failed. Per CDC/HICPAC guidance, biological indicators are the definitive check, not tape color change.
• Bacillus atrophaeus spores are used for dry heat and ethylene oxide.
• Prions are the outlier: not killed by routine autoclave cycles and requiring extended autoclaving with sodium hydroxide or hypochlorite.
• Clostridioides difficile spores resist alcohol; non-enveloped viruses (norovirus, adenovirus) resist alcohol far better than enveloped viruses.

Why the platform predicts the immune response

• Live attenuated agents replicate, so antigen is synthesized inside host cells, enters the proteasome–TAP pathway, and is displayed on MHC I to CD8+ T cells, while shed protein is taken up for MHC II/CD4+ help. The result is durable cellular and humoral immunity from few doses — and the same replication that generates that immunity causes disseminated infection when T-cell immunity is absent.
• Inactivated and subunit antigens cannot enter the cytosol. They are processed through the endosomal pathway only, yielding CD4+ help and antibody with little CTL response — hence weaker, waning immunity and the need for boosters and adjuvants (aluminum salts, AS01) to supply the danger signal that a replicating organism would otherwise provide.
• mRNA and viral vectors restore endogenous antigen synthesis without a replicating pathogen, which is why they generate strong T-cell responses yet remain safe in immunocompromise.

The conjugate principle

• Polysaccharide alone is a T-independent type 2 antigen: it cross-links surface Ig, drives IgM from marginal-zone B cells, and produces no germinal center, no class switching, no memory, and no booster effect. Infants under 2 years respond poorly because the marginal zone is immature.
• Covalent linkage to a protein carrier allows the B cell to internalize the complex, present carrier peptides on MHC II, and receive CD40L and cytokine help from carrier-specific CD4+ T cells — converting the response to T-dependent: germinal centers, somatic hypermutation, IgG class switch, affinity maturation, memory B cells, and mucosal IgA that reduces nasopharyngeal carriage and produces herd protection.

Sterilization physiology

• Moist heat under pressure kills by protein denaturation and coagulation; steam transfers energy far more efficiently than dry air, which is why the autoclave outperforms a dry-heat oven at lower temperature. Alcohols denature protein and dissolve lipid membranes but have no sporicidal activity: alcohols cannot penetrate the spore coat and cortex, and the dehydrated core prevents the protein denaturation alcohols rely on — hence no sporicidal activity.

Reactions that are expected (not contraindications)

• Local reactogenicity: sore arm, low-grade fever, myalgia 24–48 h after mRNA, Tdap, or adjuvanted recombinant zoster vaccine. Reflects innate immune activation, not allergy.
• Measles-like rash or transient fever 7–14 days after MMR, or a sparse vesicular rash after varicella vaccine — attenuated viral replication on schedule.

Reactions that change management

• Anaphylaxis within minutes: urticaria, wheeze, hypotension. The only universal contraindication to a further dose of that vaccine.
• Febrile seizure in toddlers, slightly more common with MMRV combination than separate MMR + varicella, per ACIP.
• Myocarditis/pericarditis after mRNA COVID-19 vaccine — chest pain and troponin elevation in adolescent and young adult males, typically after the second dose; recognized by CDC and generally self-limited.
• Intussusception risk with rotavirus vaccine; prior intussusception and uncorrected intestinal malformation are contraindications.

Syndromes of the wrong host receiving a live vaccine

• Disseminated BCG (BCGosis) in an infant with SCID — failure to thrive, adenitis, disseminated mycobacterial disease.
• Vaccine-associated paralytic poliomyelitis from oral polio vaccine, classically in a hypogammaglobulinemic host; the reason the US uses IPV exclusively.
• Progressive vaccine-strain varicella in leukemia or high-dose steroid therapy.
• Congenital rubella–type concerns drive the pregnancy contraindication for MMR and varicella.

Infection control failures

• C. difficile outbreak on a ward where staff used alcohol hand rub — profuse diarrhea, leukocytosis, recent antibiotics.
• **Duodenoscope-associated carbapenem-resistant *Enterobacterales*** clusters from inadequate high-level disinfection of a semicritical device.
• Norovirus outbreak persisting despite alcohol gel, since non-enveloped viruses resist it.

Documenting immunity (serologic correlates)

• Hepatitis B: check anti-HBs 1–2 months after the final dose in healthcare personnel and dialysis patients. ≥10 mIU/mL defines a responder (CDC/ACIP). Non-responders receive a repeat series; persistent non-response after two complete series prompts testing for HBsAg and anti-HBc to exclude chronic infection.
• Measles, rubella, varicella, hepatitis A: IgG positivity is acceptable evidence of immunity. Documented age-appropriate vaccination is equally acceptable — routine post-vaccination titers are not recommended for MMR outside occupational or pregnancy screening.
• Rubella IgG is checked at the first prenatal visit per ACOG; a non-immune result means MMR postpartum, never during pregnancy.
• Tetanus/diphtheria antitoxin titers are almost never needed clinically; history of doses drives management.

BCG and tuberculosis testing

• A prior BCG recipient may have a false-positive tuberculin skin test. The interferon-gamma release assay uses ESAT-6/CFP-10, antigens absent from BCG, so IGRA is the preferred test in BCG-vaccinated persons (CDC/ATS/IDSA).

Verifying sterilization — the layered sequence

• Mechanical monitors first: the printout confirming the cycle reached 121°C at ~15 psi for the required exposure (a longer cycle at 132–135°C is used in flash/prevacuum sterilizers).
• Chemical indicators (autoclave tape, internal strips) confirm the pack was exposed to steam — they do not confirm sterility and are the classic distractor.
• Biological indicator with Geobacillus stearothermophilus spores is the gold standard, run at least weekly and with every implant load; no growth after incubation confirms sporicidal conditions.
• Bowie–Dick test daily in prevacuum sterilizers to confirm complete air removal.
• Spaulding classification determines the required level: critical items entering sterile tissue require sterilization; semicritical items contacting mucosa require high-level disinfection; noncritical surfaces require low-level disinfection.

Acute vaccine reaction

• Anaphylaxis: epinephrine IM into the anterolateral thigh — 0.3 mg (1:1000) in adults; in children 0.01 mg/kg (1:1000), maximum 0.3 mg (max 0.5 mg in adults) — repeated every 5–15 minutes as needed, with airway support and IV fluids. Antihistamines and glucocorticoids are adjuncts only and never replace epinephrine.
• Vasovagal syncope after adolescent vaccination: supine positioning and 15-minute observation — distinguish from anaphylaxis by bradycardia and absent urticaria.

Post-exposure prophylaxis by exposure (CDC/ACIP)

• Rabies: wound washing, rabies immune globulin infiltrated into the wound plus a 4-dose vaccine series in the previously unvaccinated; previously vaccinated persons receive vaccine boosters only and no RIG.
• Hepatitis B: nonimmune person with percutaneous exposure to HBsAg-positive source → HBIG plus initiate vaccine series. Neonate of an HBsAg-positive mother → both within 12 hours of birth.
• Tetanus wound management: clean minor wound → Td/Tdap if last dose >10 years; dirty or puncture wound → Td/Tdap if >5 years, and add tetanus immune globulin if fewer than three prior doses or unknown history.
• Measles: MMR within 72 hours for susceptible immunocompetent contacts; immune globulin for infants, pregnant women, and the immunocompromised.
• Varicella: vaccine for susceptible immunocompetent contacts; varicella-zoster immune globulin for neonates, pregnant women, and immunocompromised contacts.

Special populations

• Pregnancy: Tdap every pregnancy (preferably 27–36 weeks) for neonatal pertussis protection, plus inactivated influenza and RSV/COVID-19 products per ACOG and ACIP. No live vaccines.
• Asplenia/complement deficiency/eculizumab: pneumococcal, meningococcal ACWY and B, and Hib.

Decontamination choices

• Sporicidal need (C. difficile, Bacillus): soap-and-water handwashing plus sodium hypochlorite surface disinfection; alcohol rub is inadequate.
• Semicritical endoscopes: high-level disinfection with glutaraldehyde, ortho-phthalaldehyde, or peracetic acid; heat-labile critical items go to ethylene oxide or hydrogen peroxide gas plasma.

• The single most tested decision is live vs non-live. Pregnant patient, symptomatic HIV with low CD4, SCID, active leukemia, high-dose steroids, or a recent transplant → no MMR, varicella, zoster live formulation, yellow fever, oral polio, BCG, rotavirus, or intranasal influenza. mRNA and viral vector vaccines are not live and are safe in these hosts.
• Household contacts of an immunocompromised patient may generally receive live vaccines — MMR, varicella, and rotavirus are all acceptable, and withholding them is the classic distractor. The exceptions are oral polio vaccine (not used in the US) and LAIV/intranasal influenza in contacts or caregivers of severely immunosuppressed patients in a protective environment (e.g., HSCT recipients), who should receive inactivated influenza vaccine instead, per ACIP.
• Conjugate = works in infants. If the stem says a 2-month-old or an asplenic child, the answer is the conjugate (protein-linked) product, because it converts a T-independent polysaccharide response into a T-dependent one with class switching and memory.
• Toxoid vaccines produce antitoxin, not antibacterial immunity: a fully immunized patient can still carry C. diphtheriae or have C. tetani in a wound but will not develop the toxin-mediated disease.
• Autoclave 121°C at ~15 psi kills spores; alcohol does not. The best next step in a C. difficile room is soap-and-water handwashing, gown-and-gloves contact precautions, and hypochlorite cleaning — alcohol gel is the trap. Non-enveloped norovirus behaves the same way.
• ***Geobacillus stearothermophilus* spore strips are the gold-standard proof of sterilization**; autoclave tape only proves steam exposure.
• Egg allergy no longer bars influenza vaccination per ACIP; yellow fever vaccine remains the egg-related concern.
• BCG causes a false-positive tuberculin skin test, not a false-positive IGRA — order the interferon-gamma release assay.