Paeds SAQs · clinical-pharmacology-and-therapeutics
Vaccines and immunobiology — formative SAQs
Two MedVellum formative short-answer questions on vaccines and immunobiology in children: the live attenuated versus inactivated vaccine distinction with the four-week spacing rule and the contraindications in significant immunocompromise and pregnancy, and a catch-up plan for a child with an incomplete immunisation record built on minimum intervals with the rotavirus age limits and the premature-infant chronological-age rule, plus the recognition of anaphylaxis after a vaccine. The marks and timing support transparent self-assessment. They are not an official board format or pass standard.
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Target exams
SAQ 1 — Live versus inactivated vaccines and the spacing rule
Question 1 — 10 formative marks; suggested time 15 minutes [1]
A twelve-year-old girl newly diagnosed with juvenile idiopathic arthritis is about to start methotrexate. Her immunisation record shows the second dose of measles-mumps-rubella is overdue and her varicella immunity is uncertain. The team must complete her vaccination before immunosuppression begins. [2]
- Classify childhood vaccines into live attenuated and inactivated families with named examples, and explain why the distinction matters for contraindications. (3 marks)
- State the absolute contraindications to live vaccines. (2 marks)
- State the rule for spacing two injectable live vaccines, and what to do if the rule is broken. (3 marks)
- Describe how you would recognise and manage anaphylaxis after a vaccine. (2 marks)
Full-credit answer — SAQ 1
Reveal full-credit answer for SAQ 1
1. Live attenuated versus inactivated
Childhood vaccines fall into two families. The live attenuated vaccines contain a weakened organism that replicates in the host and generates durable memory — measles-mumps-rubella, varicella, the oral rotavirus vaccine, BCG, yellow fever, and oral typhoid. The inactivated vaccines cannot replicate and cannot cause infection, and present fixed fragments (toxoid, subunit, conjugate, recombinant, killed) usually with an adjuvant — diphtheria-tetanus-acellular-pertussis, inactivated polio, Haemophilus influenzae type b, hepatitis B, pneumococcal conjugate, meningococcal, human papillomavirus, and inactivated influenza. The distinction matters because a replicating live vaccine can harm a host who cannot contain it, which is why live vaccines are largely contraindicated in significant immunocompromise and in pregnancy while inactivated vaccines are safe almost everywhere. [1] [2]
2. Absolute contraindications to live vaccines
The absolute contraindications to live vaccines are significant immunocompromise — primary immunodeficiency, HIV with severe immunosuppression, malignancy on chemotherapy, high-dose corticosteroids, recent transplant, or biologic immunosuppression — and pregnancy, because a replicating vaccine organism can cause uncontrolled vaccine-strain disease in the immunocompromised host or harm the fetus. A severe allergic reaction to a prior dose or a vaccine component is a contraindication to that vaccine in particular. [2] [11]
3. The four-week spacing rule
Two injectable live vaccines are given either on the same day or at least four weeks apart. This is because the immune response mounted to the first can interfere with the replication and take of the second if they are given too close together. If they are given less than four weeks apart, the second dose does not count and must be repeated. The oral rotavirus vaccine is an exception because it replicates in the gut and does not interfere with injectable live vaccines. [11]
4. Anaphylaxis after a vaccine
Anaphylaxis is rare but declares itself within minutes of the injection — rapid onset of airway, breathing, or circulation compromise, often with skin changes. 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, with oxygen, fluid, and airway support to follow, and observation and admission afterwards. [1]
SAQ 2 — Building a catch-up plan
Question 2 — 10 formative marks; suggested time 15 minutes [1]
A four-year-old boy has recently arrived from overseas with an incomplete and partly undocumented immunisation record. You cannot confirm which doses he has received. His mother also asks whether his younger sibling, born at thirty weeks and still followed in clinic, needs any special vaccination plan. [1] [3]
- State the core principle of catch-up vaccination, and whether the series is restarted. (2 marks)
- Describe how you would use minimum intervals to build a catch-up plan for this child. (4 marks)
- State the rotavirus age limits and the consequence of a breach, and explain why they exist. (2 marks)
- Explain how the premature sibling should be vaccinated. (2 marks)
Full-credit answer — SAQ 2
Reveal full-credit answer for SAQ 2
1. The catch-up principle
A child who has fallen 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. When the record is absent or unreliable, the child is assumed non-immune and a full catch-up plan is built from the start of the schedule, with serology used selectively where it would change the plan. [1]
2. Minimum intervals and the catch-up plan
The catch-up plan is built on the minimum intervals between doses of the same vaccine — commonly four weeks — while respecting the minimum age for the first dose of each vaccine. For a four-year-old with an unconfirmable record I would assume non-immune, begin the scheduled primary series at the minimum age-appropriate points, and space each subsequent dose at the minimum interval until the series is complete, applying the live-vaccine same-day-or-four-weeks rule throughout and documenting every dose given. The plan is written down and shared with the family and the primary-care record so the next visit is unambiguous. [1] [11]
3. The rotavirus age limits
The oral rotavirus vaccine has strict upper age limits: the first dose is given before fifteen weeks of age, and the whole course is completed by twenty-four weeks. These limits exist because the small risk of intussusception after rotavirus vaccination rises with age, and the benefit-risk balance shifts unfavourably beyond them. A breach — a first dose at or after fifteen weeks, or a course completed after twenty-four weeks — is a recognised error; the dose is generally not given once the limit is passed, and the situation is documented. [5]
4. The premature sibling
The premature sibling is vaccinated by chronological age, not a corrected age, with the same full doses and no reduction. This is because the premature infant's immune response is adequate for routine vaccination and the risk of vaccine-preventable disease is if anything higher, so deferring is the wrong instinct. The hexavalent vaccine is immunogenic and safe in preterm infants, and where the infant is still an inpatient at the scheduled age the oral rotavirus vaccine can be given in the hospital setting with infection-control measures for other vulnerable babies. [3]
References
- [1]Wiley CC Immunizations: vaccinations in general Pediatrics in review, 2015.PMID 26034255
- [2]Miller K; Leake K; Sharma T Advances in vaccinating immunocompromised children Current opinion in pediatrics, 2020.PMID 31790029
- [3]Omeñaca F; Vázquez L; Garcia-Corbeira P; Mesaros N; et al Immunization of preterm infants with GSK's hexavalent combined diphtheria-tetanus-acellular pertussis-hepatitis B-inactivated poliovirus-Haemophilus influenzae type b conjugate vaccine: a review of safety and immunogenicity Vaccine, 2018.PMID 29336924
- [5]Koch J; Harder T; von Kries R; Wichmann O Risk of intussusception after rotavirus vaccination Deutsches Arzteblatt international, 2017.PMID 28468712
- [11]Michel R; Berger F; Ravelonarivo J; Dussart P; et al Observational study on immune response to yellow fever and measles vaccines in 9 to 15-month old children. Is it necessary to wait 4 weeks between two live attenuated vaccines? Vaccine, 2015.PMID 25843268