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Human Development & Aging
Immunizations across lifespan
Core Principle of Immunizations
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Vaccines work by presenting antigens to the immune system in a controlled manner, generating memory B and T cells without causing disease.
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Active immunity results from either natural infection or vaccination; passive immunity comes from maternal antibodies or immunoglobulin administration.
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Live attenuated vaccines trigger both humoral and cell-mediated immunity; killed/inactivated vaccines primarily stimulate antibody production.
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The goal is to achieve herd immunity — when enough of the population is immune, disease transmission is interrupted, protecting those who cannot be vaccinated.
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Board pearl: Live vaccines are contraindicated in immunocompromised patients but provide longer-lasting immunity than inactivated vaccines.
Types of Vaccines and Immune Responses
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Live attenuated: MMR, varicella, rotavirus, intranasal influenza, yellow fever — replicate in host, generate strong cellular and humoral immunity.
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Inactivated/killed: polio (IPV), hepatitis A, rabies, injectable influenza — cannot replicate, require multiple doses for immunity.
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Subunit/recombinant: hepatitis B, HPV, acellular pertussis — contain purified antigens, very safe but may need adjuvants.
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Toxoid: tetanus, diphtheria — inactivated bacterial toxins that generate antitoxin antibodies.
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Conjugate: Hib, pneumococcal, meningococcal — polysaccharide linked to protein carrier to enhance T cell response in young children.
Birth to 2 Months: Foundation of Protection
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Hepatitis B: first dose within 24 hours of birth (especially critical if mother HBsAg+), prevents vertical transmission.
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At 2 months, begin primary series: DTaP, IPV, Hib, PCV13, rotavirus (RV1 or RV5).
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Rotavirus vaccine must start before 15 weeks and complete by 8 months — strict age limits due to intussusception risk.
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Board pearl: Premature infants receive vaccines based on chronological age, not adjusted age, with same doses as full-term infants.
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Maternal antibodies provide some protection initially but wane by 6 months, necessitating early vaccination.
Primary Series Completion (2-6 Months)
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DTaP, IPV, Hib, PCV13 given at 2, 4, and 6 months (though Hib and IPV may skip 6-month dose depending on formulation).
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Hepatitis B second dose at 1-2 months, third dose at 6-18 months — minimum intervals must be maintained.
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Rotavirus: 2 or 3 doses depending on vaccine type (RV1 = 2 doses, RV5 = 3 doses).
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Board distinction: Unlike other vaccines, rotavirus has maximum age limits — cannot start series after 15 weeks.
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Simultaneous administration of multiple vaccines does not decrease immunogenicity or increase adverse events.
12-15 Month Boosters and New Vaccines
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MMR first dose at 12-15 months — not given earlier because maternal antibodies interfere with response.
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Varicella first dose at 12-15 months — can be given as MMRV combination.
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Hepatitis A: two doses starting at 12 months, separated by 6-18 months.
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Hib and PCV13 boosters complete primary series protection.
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Board pearl: MMR and varicella are live vaccines — space other live vaccines by 4 weeks or give simultaneously.
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DTaP fourth dose at 15-18 months maintains pertussis immunity.
Preschool Boosters (4-6 Years)
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DTaP fifth and final childhood dose.
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IPV fourth and final dose.
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MMR second dose — not a booster but ensures primary immunity in the 5% who don't respond to first dose.
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Varicella second dose — completes two-dose series.
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Board clue: A 5-year-old starting kindergarten without documentation needs all age-appropriate vaccines — use catch-up schedule.
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Annual influenza vaccine recommended starting at 6 months of age.
Adolescent Platform (11-12 Years)
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Tdap replaces DTaP — single dose provides tetanus, diphtheria, and pertussis boosters.
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Meningococcal conjugate (MenACWY) first dose at 11-12 years, booster at 16 years.
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HPV series: 2 doses if started before age 15, 3 doses if started at 15 or older.
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Board pearl: HPV vaccine prevents 90% of cervical cancers and is recommended for all adolescents, not just females.
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Annual influenza vaccine continues throughout life.
Human Papillomavirus (HPV) Vaccine Details
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Covers oncogenic strains (16, 18 cause 70% of cervical cancers) and wart-causing strains (6, 11).
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9-valent vaccine (HPV9) is current standard, covering 9 strains total.
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Recommended for all individuals through age 26; shared decision-making for ages 27-45.
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Board distinction: Number of doses depends on age at initiation — 2 doses if started before 15th birthday, 3 doses if started at 15 or older.
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Most effective when given before sexual debut but still beneficial after sexual activity begins.
Adult Immunizations (19-64 Years)
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Tdap once if not received in adolescence, then Td booster every 10 years.
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Annual influenza vaccine for all adults.
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Varicella: 2 doses for adults without evidence of immunity (birth before 1980 considered evidence of immunity).
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MMR: 1 or 2 doses for adults born in 1957 or later without evidence of immunity.
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Board pearl: Healthcare workers need documented 2 doses of MMR regardless of birth year.
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Hepatitis B series for at-risk adults: healthcare workers, multiple sexual partners, injection drug users, chronic liver disease.
Pregnancy Considerations
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Tdap during every pregnancy (ideally 27-36 weeks) to provide passive pertussis protection to newborn.
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Influenza vaccine (inactivated) during any trimester of flu season.
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Live vaccines (MMR, varicella) contraindicated during pregnancy — screen and vaccinate postpartum if non-immune.
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Board pearl: Pregnancy is not a contraindication to inactivated vaccines; live vaccines should be avoided.
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Rh-negative mothers receive RhoGAM at 28 weeks and within 72 hours of delivery if baby is Rh-positive.
Immunocompromised Patients
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Live vaccines contraindicated: MMR, varicella, rotavirus, live influenza, BCG, oral polio.
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Inactivated vaccines generally safe but may have reduced efficacy.
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Pneumococcal: both PCV13 and PPSV23 recommended with specific timing.
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Board distinction: HIV+ patients with CD4 >200 can receive MMR and varicella; avoid if CD4 <200.
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Household contacts should receive all vaccines including MMR and varicella but avoid live influenza vaccine.
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Consider passive immunization with immunoglobulin for exposures.
Elderly Adults (≥65 Years)
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Pneumococcal: PCV13 followed by PPSV23 at least 1 year later (or PPSV23 alone based on shared decision).
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Zoster vaccine: recombinant zoster vaccine (RZV/Shingrix) preferred — 2 doses for all adults ≥50 years.
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Annual high-dose or adjuvanted influenza vaccine preferred over standard dose.
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Tdap once if not previously received, then Td every 10 years.
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Board pearl: Recombinant zoster vaccine is not live and can be given to immunocompromised patients.
Travel Vaccines
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Yellow fever: required for entry to certain countries, given at designated centers, contraindicated in immunocompromised.
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Typhoid: indicated for travel to endemic areas — oral live vaccine or injectable polysaccharide.
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Hepatitis A: all travelers to endemic areas if not previously vaccinated.
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Japanese encephalitis: for travelers spending ≥1 month in endemic rural areas during transmission season.
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Board pearl: Yellow fever vaccine certificate is valid starting 10 days after vaccination.
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Meningococcal vaccine required for Hajj pilgrimage and sub-Saharan Africa travel.
Catch-Up Immunization Principles
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Use minimum intervals between doses rather than restarting series — "never restart, just continue where left off."
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Minimum intervals: 4 weeks between live vaccines, varies for inactivated vaccines.
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For unknown vaccination status, serologic testing can confirm immunity for some vaccines (hepatitis B, varicella, measles).
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Board pearl: When in doubt, vaccinate — extra doses are safer than being unprotected.
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Document all administered vaccines and provide patient with updated record.
Vaccine Adverse Events and Contraindications
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Common mild reactions: local pain, low-grade fever — not contraindications to future doses.
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Severe allergic reaction (anaphylaxis) to vaccine or component — only absolute contraindication.
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MMR can be given to egg-allergic patients (grown in chick embryo fibroblast, not egg).
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Board distinction: Influenza vaccine contraindicated only in severe egg allergy with anaphylaxis.
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VAERS (Vaccine Adverse Event Reporting System) for monitoring; VICP (Vaccine Injury Compensation Program) for rare serious events.
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Guillain-Barré syndrome history: avoid influenza vaccine only if GBS occurred within 6 weeks of previous flu vaccine.
Special Populations and Risk Groups
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Asplenia (functional or anatomical): pneumococcal, meningococcal, Hib vaccines critical — give before elective splenectomy.
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Cochlear implants: increased risk of pneumococcal meningitis → pneumococcal vaccination.
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Complement deficiency: meningococcal vaccines (both MenACWY and MenB).
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Chronic liver disease: hepatitis A and B vaccines.
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Board pearl: Sickle cell disease causes functional asplenia → needs same vaccines as surgical splenectomy.
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Men who have sex with men: hepatitis A and B, HPV through age 26.
Passive Immunization
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Immunoglobulin provides immediate but temporary protection through antibody transfer.
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Hepatitis B immunoglobulin (HBIG): newborns of HBsAg+ mothers, needlestick exposures.
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Varicella-zoster immunoglobulin (VariZIG): exposed immunocompromised or pregnant patients.
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Rabies immunoglobulin (RIG): bite wounds from potentially rabid animals.
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Tetanus immunoglobulin (TIG): contaminated wounds in unvaccinated patients.
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Board pearl: Passive and active immunization can be given simultaneously at different sites.
Herd Immunity Thresholds
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Measles: most contagious vaccine-preventable disease (R0 = 12-18) requires ~95% immunity for herd protection.
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Pertussis: ~92-94% coverage needed due to waning immunity.
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Polio: ~80-86% coverage can interrupt transmission.
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Rubella: ~83-85% coverage protects pregnant women from congenital rubella syndrome.
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Board pearl: Herd immunity threshold = 1 - (1/R0) where R0 is basic reproduction number.
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Vaccine hesitancy threatens herd immunity, leading to outbreak clusters in under-vaccinated communities.
Board Question Stem Patterns
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Newborn whose mother is HBsAg+ → hepatitis B vaccine and HBIG within 12 hours.
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2-month-old former 28-week premie → vaccinate according to chronological age with full doses.
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Healthcare worker exposed to varicella with no vaccination history → check titers immediately.
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Pregnant woman exposed to rubella → check immunity status, cannot vaccinate until postpartum.
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5-year-old with unknown vaccination status starting school → begin catch-up schedule.
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Adult with HIV and CD4 count 150 → avoid all live vaccines.
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67-year-old asking about shingles vaccine → recommend recombinant zoster vaccine (2 doses).
One-Line Recap
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Immunizations create active immunity through controlled antigen exposure, following age-based schedules from birth (hepatitis B) through elderly years (pneumococcal, zoster), with live vaccines contraindicated in immunocompromised patients, special populations requiring additional vaccines, and catch-up schedules using minimum intervals to protect those with delayed or unknown vaccination status.
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