Vaccines produce active acquired immunity: the immune system responds to an antigen and forms memory B and T cells without the child having the disease. Passive immunity is borrowed antibody — from the mother across the placenta or in breast milk, or from immune globulin (e.g., HBIG, rabies immune globulin); it works immediately but fades within weeks to months. When enough people are immune, transmission falls and those who cannot be vaccinated are also protected (community or herd immunity).

Conjugate vaccines (Hib, PCV, meningococcal) link a polysaccharide to a protein so that infants under 2 years, who respond poorly to plain polysaccharides, can make lasting immunity.

Screening before every vaccination

• Is the child sick today? (mild illness, with or without low fever, is not a reason to delay)
• Allergies — especially severe allergic reaction (anaphylaxis) to a previous dose or a vaccine component (e.g., gelatin, neomycin, yeast)
• Reactions after previous vaccines
• Immune status: cancer, chemotherapy, high-dose corticosteroids (2 mg/kg/day or more, or 20 mg/day or more of prednisone, for 14 days or longer — wait at least 1 month after stopping before live vaccines), HIV, congenital immunodeficiency (including severe combined immunodeficiency, SCID), biologic drugs; immunosuppressed household members
• Recent blood transfusion, immune globulin, or IVIG (delays MMR and varicella)
• Pregnancy (adolescents)
• Seizures, brain or nerve problems; history of Guillain–Barré syndrome
• Previous intussusception (rotavirus)
• Current immunization record, including doses given elsewhere

Contraindications vs. precautions

Not contraindications (commonly mistaken): mild illness, low-grade fever, current antibiotics, recovery from illness, prematurity, breastfeeding, a pregnant or immunosuppressed household contact (for most vaccines), family history of seizures or of adverse events, local reactions to a previous dose, and egg allergy for MMR or influenza vaccines. After DTaP, fever within 48 hours, persistent crying of 3 hours or more, a seizure within 3 days, or a collapse-like episode after a previous dose are no longer listed as precautions in current US guidance, but they should be reported and discussed with the provider.

• Immunization record (paper, registry, or electronic) — the main "test"; count only documented doses
• Serology is not routinely needed; it is used after hepatitis B vaccination in infants of HBsAg-positive mothers and in selected immunocompromised children
• Tuberculin skin test and MMR: give on the same day, or delay the skin test 4–6 weeks after MMR (measles vaccine can suppress the reaction)

Schedules

• Schedules are national and change periodically — always check the current official schedule: CDC/ACIP in the US (the AAP also publishes its own schedule) and the DOH National Immunization Program (Expanded Program on Immunization) in the Philippines. See Country Notes
• Typical pattern: hepatitis B from birth; a primary infant series of DTP-containing vaccine, polio, Hib, and pneumococcal vaccine in the first 6 months; rotavirus orally in early infancy; measles-containing vaccine around 9–15 months; boosters in the second year and at school entry; Tdap/Td, HPV, and meningococcal vaccines around early adolescence; influenza yearly from 6 months where recommended

Timing rules

• Do not restart an interrupted series — continue where it stopped
• Respect minimum ages and intervals; doses given 5 or more days too early are not counted
• Multiple vaccines may be given at the same visit; separate injection sites by at least 2.5 cm (1 in)
• Preterm infants are vaccinated by chronologic age (not corrected age) with full doses. Exception: a hepatitis B birth dose given to an infant under 2,000 g is not counted in the series (HBsAg-negative mother: an infant under 2,000 g gets the first dose at 1 month or discharge, whichever comes first; HBsAg-positive mother: vaccine + HBIG within 12 hours at any weight; unknown status: vaccine within 12 hours, plus HBIG within 12 hours if under 2,000 g or within 7 days if 2,000 g or more and the mother tests positive)
• Influenza, first season, age 6 months–8 years: 2 doses at least 4 weeks apart, then 1 dose each year
• Rotavirus: first dose no later than 14 weeks 6 days of age; all doses by 8 months (in the US); not restarted if the infant spits up a dose
• Immunosuppressed children: no live vaccines during immunosuppression; inactivated vaccines are safe but may be less effective

Listed in priority order.

1. Be ready for anaphylaxis before vaccinating — epinephrine, airway equipment, and a protocol in the room; it usually starts within minutes
2. Screen for contraindications and precautions every visit; check the record and the right vaccine, dose, route, site, and patient
3. Informed consent and education — give the required vaccine information (in the US, the current Vaccine Information Statement before each dose)
4. Cold chain — refrigerate most vaccines at 2–8 °C (36–46 °F); never freeze liquid inactivated vaccines; varicella and MMRV are kept frozen; protect MMR from light; use a stand-alone unit with a digital data logger and do not store vaccines in the door; check expiry and diluent
5. Correct administration technique — site, needle, and route matched to age (see "Administration technique" below)
6. Positioning and syncope prevention — adolescents are seated or lying down during vaccination and observed for 15 minutes; if faintness occurs, lay flat and raise the legs
7. If anaphylaxis occurs (hives plus breathing difficulty, facial or lip swelling, hypotension):
   1. Call for help and give epinephrine 1 mg/mL IM into the anterolateral thigh immediately — 0.01 mg/kg, maximum 0.3 mg in children (0.5 mg in adolescents and adults); autoinjector 0.1, 0.15, or 0.3 mg by weight — this is the first-line drug and must not be delayed
   2. At the same time, open and support the airway and give oxygen; position supine with legs raised (or sitting up if breathing is difficult)
   3. Repeat epinephrine every 5–15 minutes if needed; IV fluids, antihistamines, and bronchodilators are only adjuncts; transfer for observation (biphasic reactions can occur hours later)
   4. Epinephrine commonly causes pallor, tremor, anxiety, and tachycardia; in anaphylaxis there is no absolute contraindication — the danger of withholding it is greater. Never give the 1 mg/mL concentration IV push
8. Document: vaccine, manufacturer, lot number, expiry, dose, route, site, date, information given, and the administrator; update the registry and the family's record
9. Report clinically significant adverse events (in the US, to VAERS)

Administration technique

• IM at 90°; do not aspirate (not needed for vaccines and adds pain); inject quickly
• Subcutaneous vaccines (e.g., MMR, varicella in many products): 45°, 5/8 in needle, thigh in infants, upper outer triceps from 12 months
• BCG is intradermal; oral vaccines (rotavirus, OPV) are given slowly into the cheek pouch
• Give the most painful vaccine last; comfort measures: breastfeeding or sweet solution in infants, holding upright, distraction, topical anesthetic if time allows

• Common reactions: soreness, redness, swelling, fussiness, low fever for 1–2 days. Use a cool compress on the site; give acetaminophen or ibuprofen (ibuprofen not under 6 months) for discomfort after vaccination rather than routinely beforehand, dosed by weight
• BCG: a small bump appears in 2–3 weeks, may become a pustule or small ulcer, and heals over weeks to a few months with a scar — keep clean and dry; do not squeeze, rub, or apply ointment
• Rotavirus: vaccine virus is shed in stool for a short time — wash hands after diaper changes
• MMR may cause fever or rash 1–2 weeks later; varicella vaccine may cause a few spots
• Keep a personal record; it is needed for school, travel, and emergencies
• Vaccines do not cause autism; delaying vaccines leaves the child unprotected during the highest-risk months

• Anaphylaxis (minutes to hours) — epinephrine first
• Syncope in adolescents (risk of head injury)
• High fever or febrile seizure (e.g., after MMR or combination vaccines) — usually benign but evaluate
• Intussusception after rotavirus vaccine (highest in the week after the first dose): episodes of inconsolable crying with drawing up the legs, vomiting, "currant jelly" stools
• Encephalopathy within 7 days of pertussis vaccine
• BCG: large abscess, spreading lymph node swelling, or disseminated infection in an immunocompromised infant
• Vaccine-strain illness after live vaccines in severely immunocompromised children

• Live vaccines: MMR, varicella, rotavirus, BCG, OPV — contraindicated in pregnancy and severe immunodeficiency
• The only contraindication for all vaccines: anaphylaxis to a prior dose or component; moderate or severe illness is a precaution; mild illness is not
• Egg allergy is not a contraindication to MMR or influenza vaccine
• DTaP contraindication: encephalopathy within 7 days of a previous dose
• Rotavirus contraindications: SCID and prior intussusception; start by 14 weeks 6 days
• Preterm infants: chronologic age, full dose
• First influenza season 6 months–8 years: 2 doses 4 weeks apart
• Infants: vastus lateralis; older children: deltoid; no aspiration; most painful last
• Live injectables: same day or 28 days apart
• Anaphylaxis: IM epinephrine 0.01 mg/kg (max 0.3 mg child) in the thigh — first
• Syncope: seated vaccination, 15-minute observation
• Schedules change — always check the current CDC/ACIP or DOH schedule