5.2 Contraindications, Adverse Events & Special Populations

Contraindications vs. Precautions

The ABHS exam draws a hard distinction between a true contraindication (a condition that makes vaccination unacceptably risky, so the vaccine must not be given) and a precaution (a condition that raises the risk of an adverse reaction or reduces vaccine effectiveness, so vaccination is usually deferred but is not absolutely forbidden). Confusing the two is one of the most common trap patterns in immunization stems.

True Contraindications

• Anaphylaxis after a previous dose, or a known severe allergy to a vaccine component (e.g., egg protein in some influenza/yellow fever formulations, gelatin, neomycin) is an absolute contraindication to that specific vaccine.
• Severe combined immunodeficiency (SCID) and other severe primary or acquired immunodeficiencies are contraindications to all live vaccines — BCG, OPV, MMR, varicella, and rotavirus. Classic case pattern: an infant develops disseminated BCG disease ("BCG-osis") after the birth dose; the diagnosis to suspect is SCID, and this same child must never receive OPV or MMR.
• Encephalopathy within 7 days of a pertussis-containing vaccine, not attributable to another identifiable cause, is a contraindication to further doses of the pertussis component (the child can still receive diphtheria-tetanus vaccine without pertussis).
• Pregnancy is a contraindication to live vaccines such as MMR and varicella because of theoretical risk to the fetus. Inactivated vaccines are not only permitted but recommended: Tdap is given in each pregnancy (ideally at 27-36 weeks gestation) to protect the newborn through transplacental antibody, and inactivated influenza vaccine is recommended in any trimester during influenza season because pregnant women have higher risk of severe influenza and the vaccine also protects the newborn in the first months of life.

Precautions (Not True Contraindications)

• Current moderate-to-severe acute illness, with or without fever, is a precaution: defer vaccination until the illness resolves, mainly so a vaccine reaction is not confused with worsening of the underlying illness. A mild illness — low-grade fever, common cold, mild diarrhea — is explicitly NOT a reason to withhold vaccination; this is one of the most heavily tested "myth vs. fact" points in pediatric board exams.
• Recent receipt of an antibody-containing blood product (e.g., immune globulin, packed red cells, whole blood) is a precaution specifically for live injectable vaccines (MMR, varicella): passive antibody can blunt the immune response, so these vaccines are typically deferred for an interval that depends on the product and dose (commonly 3-11 months).
• A family history of an adverse event after vaccination, or a family history of seizures, is not a contraindication to any vaccine.
• Prematurity alone is a precaution managed by chronological-age dosing (see below), not a contraindication.

Common Minor Adverse Events vs. True Vaccine-Injury Patterns

Exam trap: a stem describing fussiness and a low-grade fever the evening after a routine DTaP/pentavalent visit describes an expected, self-limited adverse event, not a contraindication to future pertussis-containing doses. Only encephalopathy within 7 days, or a documented anaphylactic reaction, stops further pertussis-containing vaccine.

Rotavirus-Specific Contraindications and Age Limits

Beyond the general live-vaccine rules above, rotavirus vaccine has product-specific constraints that appear frequently in vignettes:

• History of intussusception is a contraindication to further rotavirus doses (and often to starting the series, depending on national guidance).
• Severe combined immunodeficiency (SCID) or other severe immunodeficiency is a contraindication because rotavirus is a live oral vaccine.
• Age limits: the first dose must be given by 15 weeks of chronological age, and the series must be completed by 32 weeks (8 months) — doses started or completed outside these windows are not valid. A stem describing a healthy 4-month-old who has never received rotavirus is still eligible; a 9-month-old presenting for dose 1 is not.

Vaccination in Preterm Infants

Preterm and low-birth-weight infants are vaccinated according to chronological age since birth, not corrected/adjusted gestational age — with one dose-timing exception for hepatitis B:

• If the infant is medically stable and weighs 2,000 g or more, the hepatitis B birth dose can be given at birth as usual.
• If the infant weighs under 2,000 g and the mother is hepatitis B surface antigen (HBsAg)-negative, the first hepatitis B dose is delayed until 1 month of chronological age or hospital discharge, whichever comes first, because low-birth-weight infants respond less reliably to a birth-timed dose.
• If the mother is HBsAg-positive, or her status is unknown, every infant — regardless of birth weight — must receive hepatitis B vaccine plus hepatitis B immune globulin (HBIG) within 12 hours of birth; this exception is not weight-dependent.
• All other EPI vaccines (pentavalent/hexavalent, OPV/IPV, PCV, rotavirus, MCV) are given on the standard chronological-age schedule at full, undivided doses — prematurity is never a reason to split or reduce a dose.

Vaccination in Immunocompromised and HIV-Exposed Children

• Live vaccines are generally withheld from children with severe immunosuppression, but the exact rule differs by vaccine and by HIV status:
  • HIV-exposed, HIV-uninfected infants follow the standard schedule, including live vaccines, once infection has been excluded.
  • HIV-infected children who are not severely immunosuppressed (adequate CD4 percentage/count for age) can still receive MMR and varicella, because the benefit of protection against measles and varicella outweighs the small theoretical risk.
  • Severely immunosuppressed HIV-infected children should not receive live vaccines (MMR, varicella, and especially BCG and OPV).
• BCG at birth is withheld from infants known to be HIV-infected because of the risk of disseminated BCG disease, even though WHO still recommends it for HIV-exposed infants whose infection status is not yet confirmed, in high-TB-burden countries.
• Rotavirus vaccine is recommended for HIV-exposed and HIV-infected infants regardless of clinical or immunologic status in high child-mortality settings, since the benefit of preventing severe rotavirus gastroenteritis outweighs the theoretical risk.
• Inactivated vaccines (hepatitis B, pentavalent/hexavalent, IPV, PCV) are safe and recommended in all immunocompromised children, though the immune response may be reduced — which is why some immunocompromised children require extra doses or higher-dose regimens (e.g., a higher-dose hepatitis B series).