Branching cross-examination

This is a MedVellum formative viva. It is not an official RACP, MRCPCH, ABP, ACGME or RCPSC station, mark scheme, duration or pass standard. Release each update only after the candidate states the principle, the rule, and the safeguard. [1] [11]

Candidate brief

You are the paediatric registrar in a general paediatrics clinic seeing a family for routine vaccination. Speak as you would when vaccinating a premature infant and building a catch-up plan for an older sibling. State the rule and the safeguard at each step. This is one continuous case. Each escalation branch leads to the next update. [1]

Question 1 — The premature infant at two months

Stimulus update. The younger sibling was born at thirty weeks and is now two months chronological age. The parent asks whether vaccination should wait until the baby is bigger. Question: When and how do you vaccinate this infant? [3]

Consultant-level model answer. "A premature infant is vaccinated according to chronological age, not a corrected age, with the same full doses and no reduction. The immune response is adequate and the risk of vaccine-preventable disease is high, so deferring is the wrong instinct. The scheduled vaccines due at two months are given at two months chronological age, and the oral rotavirus vaccine can be given in the hospital setting where the infant is still an inpatient. There is no dose reduction for prematurity." [3]

Probing follow-up. "The mother had hepatitis B. Does anything change?" A strong answer is: "A baby of a hepatitis B surface antigen-positive mother receives the hepatitis B vaccine and hepatitis B immunoglobulin within twelve hours of birth regardless of gestation, so this should already have happened, and I would confirm it in the record." [3]

Common weak answer. "Wait until corrected age or give smaller doses." This is the classic error — premature infants are vaccinated by chronological age with full doses. [3]

Escalation branch. If the candidate states the chronological-age rule with full doses, release the older sibling scenario in Question 2. [3]

Question 2 — The four-year-old with an incomplete record

Stimulus update. The older sibling is four, recently arrived from overseas, and the immunisation record is incomplete and partly undocumented. Question: How do you build a catch-up plan? [1]

Consultant-level model answer. "A child who is behind, or whose record cannot be confirmed, does not restart the series. Every dose already documented counts, and the gaps are filled using the minimum intervals between doses — commonly four weeks — while respecting the minimum age for the first dose of each vaccine. When the record is unconfirmable I assume non-immune and build a full plan from the start of the schedule, applying the live-vaccine same-day-or-four-weeks rule throughout, and I write the plan down and share it with primary care." [1]

Probing follow-up. "Should you check antibody levels first?" A strong answer is: "Serology is selective — it can confirm immunity when the record is uncertain or guide the plan in an immunocompromised child, but it should not delay routine inactivated vaccines, and a catch-up plan can proceed on minimum intervals without waiting for results." [1]

Common weak answer. "Start the whole series again from scratch." This ignores the no-restart principle and over-vaccinates the child. [1]

Escalation branch. If the candidate states the no-restart and minimum-interval principles, release the live-vaccine rule in Question 3. [11]

Question 3 — The live-versus-inactivated distinction and the spacing rule

Stimulus update. The catch-up plan will include live vaccines such as measles-mumps-rubella and varicella. Question: How do live and inactivated vaccines differ, and how do you space two live vaccines? [11]

Consultant-level model answer. "Live attenuated vaccines — measles-mumps-rubella, varicella, rotavirus, BCG, yellow fever — replicate in the host and generate durable memory, but are largely contraindicated in significant immunocompromise and pregnancy. Inactivated vaccines cannot infect and are safe almost everywhere, but usually need multiple doses. Two injectable live vaccines are given either on the same day or at least four weeks apart; if they are given less than four weeks apart, the second dose does not count and must be repeated, because the response to the first can interfere with the take of the second. The oral rotavirus vaccine is an exception as it does not interfere with injectable live vaccines." [1] [11]

Probing follow-up. "Why is a live vaccine contraindicated in immunocompromise?" A strong answer is: "Because a replicating vaccine organism in a host who cannot contain it can cause uncontrolled vaccine-strain disease, which is why significant immunocompromise and pregnancy are absolute contraindications to live vaccines." [2]

Common weak answer. "Give them a week apart, it will be fine." This breaks the four-week rule, and the second dose will not count. [11]

Escalation branch. If the candidate states the spacing rule and the contraindication, release the rotavirus question in Question 4. [2]

Question 4 — The rotavirus age limits

Stimulus update. The family asks whether the infant can have the rotavirus vaccine a little late, as they missed the early appointment. Question: What are the rotavirus age limits and why do they exist? [5]

Consultant-level model answer. "The oral rotavirus vaccine has strict upper age limits: the first dose before fifteen weeks of age, and the whole course completed by twenty-four weeks. These exist because the small risk of intussusception after rotavirus vaccination rises with age, so the benefit-risk balance shifts unfavourably beyond the limits. A first dose at or after fifteen weeks, or a course completed after twenty-four weeks, is a recognised error; once the limit is passed the dose is generally not given." [5]

Probing follow-up. "How would intussusception present if it occurred?" A strong answer is: "Intermittent abdominal pain, vomiting, and the eventual passage of redcurrant-jelly stool, typically within a week of the dose; the child is managed along the standard paediatric pathway with fluid resuscitation, imaging, and surgical or pneumatic reduction, and the vaccination link is documented and reported." [5]

Common weak answer. "Rotavirus can be given at any age." This ignores the strict age limits and the intussusception rationale. [5]

Escalation branch. If the candidate states the age limits and the rationale, release the anaphylaxis question in Question 5. [5]

Question 5 — Recognising anaphylaxis after a vaccine

Stimulus update. Immediately after a vaccine, one of the children becomes flushed, wheezy, and floppy. Question: What is happening, and what do you do? [1]

Consultant-level model answer. "This is anaphylaxis until proven otherwise, and it is the rare event I must be ready for with every vaccination. The immediate actions are to call for help, stop the vaccination process, position the child flat with legs raised, and give intramuscular adrenaline into the anterolateral thigh at the weight-appropriate dose, repeating as needed. Oxygen, fluid, and airway support follow, and the child is observed and admitted. Every vaccination setting must have an adrenaline dose chart and a clear anaphylaxis plan." [1]

Probing follow-up. "Why the anterolateral thigh?" A strong answer is: "Because intramuscular adrenaline into the anterolateral thigh gives the fastest and most reliable absorption in a child, and it is the standard site for emergency adrenaline in paediatric anaphylaxis." [1]

Common weak answer. "Give an antihistamine and observe." This under-treats anaphylaxis; the first-line treatment is intramuscular adrenaline. [1]

Escalation branch. If the candidate treats the event as anaphylaxis and gives intramuscular adrenaline, close the viva by asking the one principle they most want to carry forward. [1]

Closing principle

Consultant-level model answer. "Safe vaccination rests on three pillars — screen, give on time, catch up. Every child is screened for immunocompromise, pregnancy, severe allergy, and the interval since any live vaccine or blood product. Every vaccine is given on time at the right age with full doses, with two injectable live vaccines spaced same-day-or-four-weeks and rotavirus held to its strict age limits. And a child who is behind is caught up with minimum intervals and never restarted. The classic failures are the live vaccine given to an immunocompromised or pregnant child, the second live vaccine counted within four weeks of the first, and the rotavirus dose given past its age limit." [1] [2] [11]