Vaccines and immunobiology — formative SAQs

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]