5.1 Routine Immunization Schedule & Vaccine Science

The WHO Expanded Programme on Immunization (EPI)

The World Health Organization (WHO) Expanded Programme on Immunization (EPI), launched in 1974, is the framework that Arab League and Gulf Cooperation Council (GCC) ministries of health adapt into national immunization schedules. The ABHS Part 1 written exam expects candidates to know both the WHO-model EPI schedule (the reference standard used across most lower- and middle-resource Arab countries, e.g. Jordan, Egypt, Yemen) and the higher-resource Gulf/UAE-style schedule that substitutes inactivated components on a calendar-month timetable. Both share the same underlying principle: protect the infant against the highest-burden diseases before the age at which natural exposure risk peaks.

Birth Dose Vaccines

At birth, up to three vaccines are typically due:

• Bacillus Calmette-Guérin (BCG) vaccine — a single live attenuated dose given intradermally, protecting against severe childhood tuberculosis (TB meningitis, miliary TB) in TB-endemic countries. Most Arab League countries remain BCG-endemic and give it at birth or first health contact.
• Hepatitis B vaccine, birth dose — given within 24 hours of birth regardless of maternal hepatitis B surface antigen (HBsAg) status, because perinatal transmission risk is highest immediately after delivery and the birth dose alone prevents most mother-to-child transmission.
• Oral poliovirus vaccine (OPV), "zero dose" — given at birth in polio-endemic or high-importation-risk countries; WHO-model EPI programs that still use OPV give this dose in the first 2 weeks of life.

Completing the Hepatitis B Series

The full hepatitis B primary series is three doses: birth dose within 24 hours, then doses 2 and 3 with the pentavalent/hexavalent visits at 6 and 14 weeks (WHO model) or at 2 and 6 months (Gulf calendar-month schedules). A common exam trap is treating the birth dose as a standalone vaccine and forgetting that two additional doses are still required for full infant protection. If the birth dose was missed, the series can be started at any age using the same minimum-interval rules as catch-up immunization — it does not need to restart from birth if the infant is already older.

The Primary Series: 6, 10, and 14 Weeks (WHO Model)

The WHO-model EPI schedule clusters most primary-series doses into three infant visits at 6, 10, and 14 weeks of age. Gulf/UAE programs often use a parallel 2-, 4-, and 6-month calendar-month timetable instead. The first two visits align reasonably (6 weeks ≈ 2 months; 10 weeks ≈ 4 months), but the WHO third visit at 14 weeks (~3.5 months) is not interchangeable with a Gulf 6-month visit — a common exam trap is treating them as the same anchor:

The diphtheria-tetanus-pertussis (DTP) component is bundled with hepatitis B and Haemophilus influenzae type b (Hib) as a combination "pentavalent" vaccine in most WHO-model programs; UAE/Gulf programs commonly use a hexavalent product (diphtheria-tetanus-acellular pertussis [DTaP]-HepB-IPV-Hib) that also builds in inactivated polio, replacing the OPV doses entirely. On the exam, a stem specifying "acellular pertussis" points to a Gulf/high-resource hexavalent context; "whole-cell pertussis" points to the WHO-model/pentavalent context.

Polio component — key update: WHO's current position is that at least one dose of IPV plus at least two doses of bivalent OPV (bOPV), separated by 4-8 weeks, are needed in the routine schedule. As of March 2025, WHO no longer recommends a routine IPV booster after a complete 3-dose primary series that begins at or after 6 weeks of age — a change from earlier position papers that candidates who trained on older review material may not have absorbed.

Measles-Containing Vaccine and the Second Year of Life

• Measles-containing vaccine (MCV), dose 1 — 9 months. In high-transmission settings (the WHO-model default for most Arab League EPI programs), the first measles-containing dose is given at 9 months rather than 12 months, because maternal antibody wanes enough by 9 months to allow seroconversion while still protecting against measles exposure as early as possible.
• MCV dose 2 — 15-18 months, given as measles-mumps-rubella (MMR) vaccine in most national programs, with a minimum interval of 4 weeks from dose 1. This second dose is not a "booster" in the classic sense — it primarily protects the roughly 10-15% of children who fail to seroconvert after dose 1, rather than boosting waning immunity.
• Higher-resource Gulf schedules that delay MCV1 to 12 months (rather than 9 months) do so because lower community measles transmission reduces the urgency of early protection; they then give MCV2 at 4-6 years rather than 15-18 months.

Varicella and Other Boosters

Varicella (chickenpox) vaccine is a live attenuated vaccine not universal across WHO-model EPI (WHO does not consider it a priority for resource-limited national programs) but is routine in UAE/Gulf schedules: dose 1 at 12 months, dose 2 at 4-6 years. DTaP/DTP boosters are typically given at 15-18 months and again at 4-6 years (school entry) in most Arab national schedules, with a tetanus-diphtheria-acellular pertussis (Tdap) dose added in adolescence.

Live vs. Inactivated Vaccines

This distinction drives contraindication logic tested throughout the immunization content area:

• Live attenuated vaccines contain a weakened but replicating organism: BCG, OPV, MMR, varicella, and most rotavirus vaccines. They generally produce durable immunity from fewer doses but can theoretically cause disease in a severely immunocompromised host and are contraindicated in pregnancy.
• Inactivated/subunit vaccines contain a killed organism, protein subunit, polysaccharide, or conjugate: IPV, DTaP/DTP, hepatitis B, Hib, PCV, and injectable influenza. They cannot replicate or cause the target disease and are safe in immunocompromised hosts, but typically require multiple doses and boosters to sustain protection.
• Rule for non-simultaneous live vaccines: if two live injectable vaccines (e.g., MMR and varicella) are not given on the same day, they must be separated by at least 4 weeks (28 days). This rule does not apply to inactivated vaccines, which can be given at any interval before or after another vaccine.

Catch-Up Scheduling Principles

A common ABHS-style stem presents a child who missed scheduled visits and asks how to proceed. Core principles:

1. Never restart a series because of a delay — count doses already validly given and resume from the next dose, regardless of how long the gap was.
2. Use minimum intervals, not just minimum ages, when catching up: for example, the minimum interval between DTP doses 1 and 2 is 4 weeks even in a catch-up schedule.
3. Live vaccines can be given simultaneously at any catch-up visit; if not simultaneous, apply the 4-week live-vaccine spacing rule above.
4. Doses given before the minimum age or before the minimum interval are not counted as valid and must be repeated.

Key Minimum Intervals (Catch-Up Reference)

Exam trap: a stem describing an unimmunized 4-year-old is not a cue to "start over" with the infant schedule — it is a catch-up-schedule question testing whether the candidate knows the accelerated minimum-interval timeline used for older, previously unvaccinated children.