Vaccines and infection control

Active and passive immunity

• Vaccination stimulates artificial active immunity: slow onset (weeks), long-lasting, with memory. Passive immunity is immediate but lasts only weeks to months, with no memory; animal sera risk anaphylaxis and serum sickness.
• In Leavell and Clark's levels of prevention, immunisation is specific protection, part of primary prevention.

Types of vaccine

• Why conjugates work in infants: a pure polysaccharide cross-links B-cell receptors but cannot be presented on MHC class II, so there is no T-cell help, no class switching and no memory, and infant B cells respond poorly to such antigens (which is why infants are vulnerable to encapsulated bacteria). Linking it to a protein lets the B cell present carrier peptides to helper T cells, giving IgG, affinity maturation, memory and reduced carriage (herd immunity).
• How toxoids protect: they induce IgG antitoxin that neutralises toxin before it binds its target. Antitoxin levels wane over years, so boosters are needed (Td for adults, especially pregnant women).
• OPV versus IPV: OPV replicates in the gut and induces mucosal secretory IgA, which stops the wild virus multiplying in the intestine and so blocks person-to-person transmission; it is cheap, oral and spreads to contacts. IPV gives excellent serum IgG (protects the individual from paralysis) but weak gut immunity, and it cannot revert.
• HPV vaccine for adolescents before sexual debut prevents cervical cancer (HPV 16, 18) and genital warts (6, 11).
• No licensed vaccine yet exists for HIV or hepatitis C.

Pakistan EPI schedule (outline)

• Td for women of childbearing age and in pregnancy prevents neonatal tetanus; rubella vaccination of girls and young women before pregnancy prevents congenital rubella. The toxoids in the EPI schedule are tetanus and diphtheria.
• BCG protects best against TB meningitis and miliary TB in young children.
• Measles vaccine is given at 9 months, not at birth, because maternal IgG neutralises the live vaccine virus in early infancy.
• Cold chain: most vaccines at +2 to +8 °C; OPV tolerates freezing; the vaccine vial monitor shows cumulative heat exposure; the shake test detects freeze damage to adsorbed vaccines (DTP, HepB, TT).
• Coverage surveys use cluster sampling (classically 30 clusters of 7 children).

Rules for giving vaccines

• True contraindications: anaphylaxis to a previous dose or component; live vaccines in pregnancy (MMR, varicella, yellow fever) and severe immunodeficiency. Not contraindications: mild illness, low-grade fever, prematurity (vaccinate by chronological age), antibiotics, breastfeeding, family history of fits.
• Two live injected vaccines (MMR and varicella) are given on the same day or at least 4 weeks apart, because the interferon response to the first can block the second. OPV and rotavirus can be given at any interval.
• After immunoglobulin or IVIG, delay MMR and varicella (for several months after high-dose IVIG): the passive antibodies neutralise the live vaccine virus.
• Hepatitis B vaccine: three doses (0, 1, 6 months) IM into the deltoid (anterolateral thigh in infants). Buttock injection often lands in fat, where antigen is poorly presented, causing non-response (the same is true of rabies vaccine).
• Checking HBV immunity: measure anti-HBs 1–2 months after the course. Below 10 mIU/mL is a non-response: check HBsAg (and anti-HBc) to exclude infection, then repeat the full course and retest; persistent non-responders need HBIG after any exposure.
• Asplenia: pneumococcal, Hib and meningococcal ACWY and B vaccines 2 weeks before elective splenectomy (or 2 weeks after emergency surgery), annual influenza and antibiotic prophylaxis.
• Vaccine reactions: local Arthus (type III) reaction when a toxoid booster such as diphtheria or tetanus is given to someone with high antitoxin levels; immune thrombocytopenia (petechiae, large platelets) a few weeks after MMR, usually self-limiting.

Passive immunisation

• Anti-D details: at 28 weeks it covers silent fetomaternal bleeds of late pregnancy. After a large bleed (abruption, trauma), the Kleihauer–Betke test measures fetal cells in maternal blood to decide whether more anti-D is needed. Give within 72 hours; late is better than never (up to about 10 days).
• Rabies vaccine and immunoglobulin must be given at different sites and never in the same syringe: the antibody would neutralise the vaccine antigen. Vaccine goes into the deltoid, not the buttock.

Post-exposure prophylaxis

• Tetanus: clean minor wound: toxoid if the course is incomplete or the last dose was over 10 years ago. Tetanus-prone wound: toxoid if over 5 years; add tetanus immunoglobulin if the primary course is incomplete or unknown.
• Rabies (WHO): category I (touching, licks on intact skin): none; category II (nibbles, minor scratches without bleeding): washing plus vaccine; category III (bites or scratches breaking skin, licks on broken skin or mucosa, bat contact): washing for 15 minutes, immunoglobulin plus vaccine; delay suturing.
• HIV: antiretroviral PEP within 72 hours (ideally within hours), for 28 days.
• Hepatitis B: vaccine, with HBIG if the person is non-immune and the source is HBsAg positive.

Hospital-acquired infection

• Commonest sites: urinary tract (catheter-associated, classically the commonest), surgical site, pneumonia (ventilator-associated), central line bloodstream, *C. difficile*.
• Organisms: *S. aureus* (including MRSA; the usual cause of wound infection after clean surgery such as cholecystectomy), *E. coli* and *Klebsiella* (ESBL, carbapenemase), *Pseudomonas* (a leading cause of hospital-acquired and ventilator pneumonia), *Acinetobacter*, *C. difficile*, VRE, *Candida auris*, norovirus.
• Hand hygiene is the single most important measure; standard precautions apply to every patient: PPE by risk, respiratory hygiene, safe injections, sharps safety, cleaning, waste and linen handling.

• Care bundles: central lines (full barrier insertion, chlorhexidine, daily review, early removal); catheters (avoid, remove early); ventilators (head up 30–45°, sedation holds, oral care); surgical site (clippers not razors, normothermia, glucose control, timely prophylaxis). Plus surveillance, audit, antimicrobial stewardship and staff vaccination (HBV, influenza, MMR, varicella).

Sharps and waste

• Needlestick: let it bleed, wash with soap and water (no scrubbing or sucking), report, assess the source. Transmission risk: HBV about 30% (HBeAg positive), HCV about 3%, HIV about 0.3%.
• Never recap; discard sharps at the point of use into rigid boxes filled no more than three-quarters.
• Segregate waste by colour (infectious for incineration, sharps, cytotoxic, general) under Pakistan's hospital waste rules. Blood spills: 10,000 ppm hypochlorite.

Remember Live vaccines need a working immune system and no competing antibody; conjugates need T-cell help.

Exam trap Live vaccines (MMR, varicella, yellow fever, BCG) are not given in pregnancy; a non-immune pregnant contact of chickenpox gets VZIG or aciclovir.

Exam trap Immunoglobulin neutralises live and rabies vaccines: separate sites, separate syringes, and delay MMR after IVIG.

How FCPS asks about this

• Conjugate vaccines (Hib, PCV, meningococcal): recruit T-cell help and memory, so they work under 2 years.
• OPV advantage: gut IgA blocks transmission; IPV protects the individual only.
• Hepatitis B non-responder: anti-HBs below 10 mIU/mL; check HBsAg and repeat the course; buttock injection into fat explains failure.
• Vaccine spacing: MMR and varicella same day or 4 weeks apart; delay MMR after IVIG.
• Anti-D: 28 weeks for silent bleeds; Kleihauer test after large bleeds.
• Rabies: category III needs RIG into the wound plus vaccine in the deltoid at a different site.
• Hospital infection: hand washing is the mainstay; TB, measles and varicella need airborne precautions.