About This Page
This is a clinician-written, evidence-based summary aligned to the USMLE Step 2 CK Content Outline. It is intended for medical students preparing for USMLE Step 2 CK. Management reflects current ACC/AHA, USPSTF, and APA guidelines. Always cross-reference with UpToDate, institutional protocols, and clinical judgment.
The Bottom Line
- Use the CDC/AAP schedule: birth HepB, 2-month series, 12-15 month live vaccines, 4-6 year boosters, and adolescent Tdap/HPV/meningococcal vaccines
- Inactivated vaccines can generally be given to immunocompromised patients; live vaccines are contraindicated in severe immunodeficiency and pregnancy
- Rotavirus has strict age limits: do not start too late and do not give after the maximum age limit
- Catch-up vaccination does not require restarting a series, regardless of how long the delay has been
- Step 2 CK often tests live vaccine contraindications, post-exposure prophylaxis, and which vaccine is due at a given age
Overview
The pediatric immunization schedule is a core public health intervention and a high-yield Step 2 CK topic. The CDC schedule, endorsed by the AAP and other US organizations, includes age-based routine vaccination, minimum intervals for catch-up, and risk-based recommendations for special populations. Questions often present a child with delayed vaccination, an immunocompromised household member, pregnancy, HIV, asplenia, or international travel and ask for the next best vaccine decision.
Epidemiology
Vaccine programs have dramatically reduced diseases such as measles, rubella, Haemophilus influenzae type b disease, polio, and invasive pneumococcal disease. However, under-immunization, vaccine hesitancy, travel, and outbreaks allow recurrence of measles, pertussis, varicella, and other vaccine-preventable illnesses. Infants, immunocompromised children, and children too young for specific vaccines rely heavily on herd protection.
Clinical Features
Symptoms
Most children are asymptomatic and present for routine preventive care or school requirements
Mild fever, local pain, swelling, or irritability after vaccination
History of anaphylaxis to a vaccine component or prior dose
Moderate or severe acute illness at the visit may justify deferring non-urgent vaccination
Immunocompromising condition, pregnancy, or recent blood product receipt affects live vaccine decisions
Wound exposure, animal bite, or outbreak exposure may require tetanus, rabies, hepatitis B, measles, or varicella post-exposure prophylaxis
Signs
Normal examination at preventive visit; vaccines can be given with mild URI or low-grade fever
Prior injection-site erythema and soreness are common and not a contraindication to future doses
Urticaria, wheeze, hypotension, or angioedema after prior dose suggests anaphylaxis
Absent spleen, complement deficiency, cochlear implant, or CSF leak increases risk of encapsulated organisms
Immunosuppression from chemotherapy, transplant, high-dose steroids, or severe primary immunodeficiency changes vaccine selection
Investigations
First-line
Review immunization recordConfirm vaccine type, dose number, dates, and minimum intervals; do not rely only on caregiver recall when documentation is needed
Age-based scheduleApply routine schedule for birth, 2, 4, 6, 12-15, 15-18 months, 4-6 years, and 11-12 years
Catch-up scheduleUse minimum ages and intervals; a delayed series is continued, not restarted
Contraindication screenAsk about anaphylaxis, pregnancy, immune suppression, encephalopathy after pertussis vaccine, and relevant vaccine components
Second-line
Serologic testingSelective use for hepatitis B immunity, varicella immunity, or uncertain records in specific circumstances; usually not needed for routine catch-up
Risk-based vaccine assessmentAsplenia, complement inhibitor use, chronic lung/heart/liver disease, diabetes, immunocompromise, travel, and outbreak status may add vaccines or alter timing
Post-exposure prophylaxis decisionDetermine timing and immune status for measles, varicella, hepatitis B, tetanus-prone wounds, and rabies exposure
Specialist
Allergy/immunology consultationSevere immediate reaction or complex vaccine component allergy may need supervised vaccination or alternative strategy
Infectious diseases or public healthComplex immunocompromise, outbreak management, rabies exposure, or uncertain international vaccination documentation
1
Routine infant schedule
- Birth: hepatitis B vaccine within 24 hours for medically stable infants >=2000 g; add HBIG if maternal HBsAg-positive
- 2 months: DTaP, IPV, Hib, PCV, rotavirus, hepatitis B if due; RSV prevention depends on season and eligibility
- 4 months: repeat DTaP, IPV, Hib, PCV, rotavirus
- 6 months: DTaP, IPV/HepB if due, Hib/PCV depending product, influenza annually from 6 months, COVID per current schedule
2
Toddler and preschool schedule
- 12-15 months: MMR, varicella, HepA, Hib and PCV boosters as due
- 15-18 months: DTaP booster
- 4-6 years: DTaP, IPV, MMR, and varicella boosters
- Annual influenza vaccination for all children >=6 months unless contraindicated
3
Adolescent schedule
- 11-12 years: Tdap, HPV series, MenACWY
- HPV before age 15 usually 2 doses; starting at >=15 years or immunocompromised requires 3 doses
- MenACWY booster at 16 years; MenB is shared clinical decision-making for many healthy adolescents and recommended for certain high-risk groups
4
Live vaccine rules
- MMR, varicella, live attenuated influenza, and rotavirus are live vaccines
- Avoid live vaccines in severe immunodeficiency and pregnancy
- Household contacts of immunocompromised patients can usually receive MMR and varicella; avoid contact with vesicular varicella vaccine rash if it occurs
- MMR and varicella can be given on the same day; if not same day, separate live injectable vaccines by at least 4 weeks
5
Catch-up principles
- Never restart a vaccine series solely because time has elapsed
- Use minimum intervals and age limits from the CDC catch-up schedule
- Rotavirus has strict maximum ages for first and final doses
- Unknown or unreliable records: vaccinate when safe; extra inactivated vaccine doses are usually preferable to leaving a child unprotected
Complications
- Vaccine-preventable infection: Measles, pertussis, varicella, pneumococcal disease, meningococcal disease, and Hib can be severe in under-immunized children
- Outbreak transmission: Under-immunization threatens infants and immunocompromised contacts
- Anaphylaxis: Rare but requires epinephrine and future vaccine allergy assessment
- Febrile seizure: Can occur after fever-producing vaccines but does not usually contraindicate further immunization
- Missed catch-up: Delayed vaccine series can leave prolonged susceptibility if clinicians incorrectly restart or defer vaccines
USMLE Step 2 CK Exam Tips
- 1Catch-up immunization: continue the series; do NOT restart from dose 1
- 2Live vaccines are contraindicated in pregnancy and severe immunodeficiency; inactivated vaccines are generally safe but may be less effective
- 3MMR and varicella are given at 12-15 months, with boosters at 4-6 years
- 4Tdap, HPV, and MenACWY are the classic 11-12 year adolescent vaccines
- 5HepB at birth plus HBIG if mother is HBsAg-positive; do not wait for outpatient follow-up
- 6Rotavirus has age cutoffs — it is the vaccine most commonly withheld if the child presents too late
- 7Mild illness or low-grade fever is not a contraindication to vaccination
- 8Prior anaphylaxis to a vaccine or component is a true contraindication to that vaccine
practicetest your knowledge on immunization scheduleApply what you've learnt with USMLE Step 2 CK-style questions from the iatroX Q-Bank — pediatrics and beyond.
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