CFB PAE03 Immunization
Immunization is the process of inducing protective immunity against infectious diseases through the administration of vaccines (active immunization) or preformed antibodies (passive immunization).
Immunization — CFB PAE03 (Prof. Yu Lung Lau)
This lecture is a cornerstone of paediatric and public health medicine. It covers the Hong Kong Childhood Immunisation Programme (HKCIP), the biological basis of different vaccine types, the evidence pathway for vaccine authorization (immunogenicity → efficacy → effectiveness), and disease-specific vaccination strategies for polio, measles, HPV, influenza, and pneumococcal disease. It concludes with vaccination principles in immunocompromised and special paediatric patients — a classic exam discriminator.
Why this matters clinically: Every doctor in Hong Kong must know the HKCIP schedule, understand why certain vaccines are or aren't included, counsel parents on immunization, and make safe decisions about vaccinating immunocompromised patients. This is tested repeatedly in MCQ, SAQ, minicase, and OSCE formats.
Learning Objectives (from lecture) [1]
1. State the schedule of the Childhood Immunization Programme in Hong Kong 2. Understand how vaccines are approved based on immunogenicity, efficacy, and effectiveness data (HPV, influenza, pneumococcal) 3. Recognize the public health goal of immunization with examples of polio and measles — each infectious disease requires a different vaccination strategy 4. Know the principles of vaccination in immunocompromised patients
Active immunization — antigen induces host response Passive immunization — immunoglobulins (IG) [1]
| Feature | Active | Passive |
|---|---|---|
| Mechanism | Host generates own immune response (antibodies + T cells) | Pre-formed antibodies transferred |
| Onset | Delayed (days–weeks) | Immediate |
| Duration | Long-lasting (memory cells) | Temporary (weeks–months) |
| Examples | All vaccines | IVIG/SCIG, monoclonal Abs, convalescent plasma |
Why the distinction matters: Passive immunization is used when immediate protection is needed (e.g., post-exposure HBV prophylaxis with HBIG, COVID-19 monoclonal antibodies for immunocompromised) or when the patient cannot mount their own response (e.g., XLA patients on IVIG replacement). [1][2]
Passive immunization subtypes [1]:
- Monoclonal antibodies (e.g., Evusheld for COVID-19)
- Convalescent/hyperimmune plasma (e.g., COVID-19, Zoster, HBV — HBIG)
- IG replacement (e.g., IVIG or SCIG for XLA patients)
Why XLA patients need IVIG
X-linked agammaglobulinaemia (XLA) = no B cells (BTK mutation) = no antibody production. These patients cannot respond to vaccines, so they need passive immunization with IVIG/SCIG for life. This is a classic exam link between immunodeficiency and immunization. [3]
2. Vaccine Types — Classification
This is one of the most frequently tested classification systems. Know the mechanism, clinical notes, and key examples for each type.
| Type | How It Works | Key Examples | Clinical Notes |
|---|---|---|---|
| Live attenuated | Genetically attenuated pathogen; most must replicate to be effective | OPV, BCG, Chickenpox, MMR, Monkeypox, flu nasal spray | Risk of vaccine-strain disease in immunocompromised (e.g., BCGitis, VDPV2). Interference from circulating antibody (no response in seropositive hosts or patients on IVIG/SCIG) |
| Inactivated whole pathogen | Dead/inactivated pathogen; cannot replicate | Whole-cell pertussis, IPV, HAV, IM flu, CoronaVac | No risk of vaccine-strain disease. Minimal Ab interference. Generally require 3–6 doses |
| Protein subunit | Contains a subunit of pathogen only (usually receptor-binding protein) | Acellular pertussis, Hib, PCV, NovavVax (COVID), diphtheria toxoid, tetanus toxoid | Minimal Ab interference |
| Polysaccharide | Pure polysaccharide capsule antigens | PPSV23 | T-cell independent; poor response in < 2 years; no immune memory |
| Nucleic acid (mRNA/DNA) | Host cells produce antigen from delivered nucleic acid | BNT162b2, mRNA-1273 (COVID); ZyCoV-D (DNA, COVID) | Risk of myocarditis with mRNA vaccines |
| Viral vector | Subunit gene delivered via a replication-incompetent viral vector | ChAdOx1, Ad26.COV2.S (adenoviral vector, COVID) | Risk of vaccine-induced thrombocytopenic thrombosis (VITT) |
Live attenuated vaccines: risk of evolution leading to vaccine-strain disease in immunocompromised hosts (e.g., BCGitis), and in some cases transmissible (e.g., VDPV2) [1]
Inactivated vaccines: no risk of vaccine-strain disease. IPV has replaced OPV. Generally require 3–6 doses [1]
Key Exam Discriminator: Live vs Inactivated
Students commonly confuse which vaccines are live. Remember: BCG, MMR(V), OPV, varicella, flu nasal spray = LIVE. Everything else in the HKCIP schedule is inactivated or subunit. Live vaccines are contraindicated in immunocompromised patients — this is tested in almost every exam cycle.
From first principles: a live attenuated vaccine contains a weakened pathogen that replicates in the host to stimulate a strong, broad immune response (similar to natural infection). In an immunocompromised host, the attenuated pathogen can replicate unchecked → actual disease. Classic examples:
- BCGitis/BCGosis in SCID patients
- Vaccine-derived paralytic polio (VDPV) in XLA patients given OPV
Because the pathogen is dead, it cannot replicate and amplify the immune signal. The immune response is weaker and primarily humoral. Multiple doses are needed to achieve adequate seroconversion and booster the memory response.
This is the single most testable table in the lecture. Memorize it. [1]
| Age | Vaccine |
|---|---|
| Newborn | BCG vaccine; Hepatitis B vaccine (1st dose) |
| 1 month | Hepatitis B vaccine (2nd dose) |
| 2 months | DTaP-IPV (1st dose); PCV13 (1st dose) |
| 4 months | DTaP-IPV (2nd dose); PCV13 (2nd dose) |
| 6 months | DTaP-IPV (3rd dose); Hepatitis B vaccine (3rd dose) |
| 12 months | MMR (1st dose); PCV13 (booster); Varicella vaccine (1st dose) |
| 18 months | DTaP-IPV (booster); MMRV (2nd dose) |
| Primary 1 | DTaP-IPV (booster) |
| Primary 5 | 9-valent HPV vaccine (1st dose) |
| Primary 6 | dTaP-IPV (booster); 9-valent HPV vaccine (2nd dose) |
Key details from the lecture:
- MMR first dose given at 12 months as MMR; varicella vaccine given to other arm to reduce risk of febrile seizure [1]
- Second MMR dose moved up to 18 months since 2018 (from Primary 1/6 years) as MMRV to maximize protection before pre-school [1]
High Yield: Vaccines NOT in HKCIP
Haemophilus influenzae type b (Hib) vaccine — very low incidence in HK [1] Rotavirus vaccine — no deaths in HK [1]
These are in the private sector schedule but NOT in the universal public programme. The reason is always public health cost-effectiveness: if the disease burden in HK is low, the money is better spent elsewhere. Hib vaccine and rotavirus vaccine are commonly given in Western countries and in private practice in HK. [4]
Why is Hepatitis A vaccine NOT in HKCIP?
Same principle: HAV has low endemicity in HK, no chronicity, low mortality. Cost-effectiveness analysis doesn't justify universal vaccination. It's recommended for travelers to endemic areas. [5]
4. Polio — Disease and Vaccination Strategy
- Virus: Human enterovirus C
- Presentation: > 90% asymptomatic or mild. Only 0.1% develop acute flaccid weakness (variable: one muscle → quadriplegia + respiratory failure), with viral meningitis and muscle pain
- Diagnosis: Viral isolation from stool, CSF, or throat
- Treatment: Supportive. Lifelong mechanical ventilation for respiratory failure. Monitor CVS for bulbar involvement
- Sequelae: Post-polio syndrome in up to 50% of survivors — new/progressive muscle weakness years later
| Feature | OPV (Live Attenuated) | IPV (Inactivated) |
|---|---|---|
| Use | Developing countries | Developed countries (including HK) |
| Advantages | Replicates and transmits → immunizes unvaccinated contacts | No risk of vaccine-strain disease |
| Disadvantages | Reversion → VDPV (vaccine-derived poliovirus); risk of vaccine-associated paralytic poliomyelitis especially in XLA | Needs more doses (3+3 IPV) |
| Schedule | 3 OPV + 1 IPV | 3+3 IPV |
Emergence of circulating vaccine-derived polioviruses (mostly serotype 2) now cause the majority of paralytic polio worldwide [1]
The Global Polio Eradication Initiative was launched in 1988 (inspired by smallpox eradication).
Challenges:
- Circulating vaccine-derived polioviruses (cVDPV2) — the very tool (OPV) used to eradicate polio is now the main source of paralytic polio
- Vaccine hesitancy in developed countries; low IPV coverage in Israel, US, UK
- Wild polio persistence in war-torn Afghanistan and Pakistan
| Level | Definition | Example |
|---|---|---|
| Eradication | Wipe pathogen off the face of the Earth. Requires: human as only reservoir, effective intervention, effective surveillance, social feasibility | Smallpox (achieved) |
| Elimination of infection | No local transmission in a defined area | Measles in HK (since 2016) |
| Elimination of disease | No effect on human life in a defined area | — |
| Control | Reduce infection/disease to acceptable level with ongoing measures | Most VPDs currently |
Eradication requires human as the only reservoir, effective intervention to halt transmission, effective surveillance, and social feasibility [1]
Polio endgame options [1]:
- Allow poliovirus circulation and eliminate paralytic polio with IPV-only approach, OR
- Design safer, highly immunogenic vaccines and ensure global coverage
5. Measles — Disease and Vaccination Strategy
- Virus: Measles virus. A leading cause of mortality in children under 5. R₀ = 16 (thought to be the most infectious pathogen)
- Presentation: Usually symptomatic. Four stages: incubation → prodrome → exanthem → recovery. Modified measles in breakthrough infections
- Diagnosis: Serum measles IgM, throat/NPS culture, urine viral culture
- Treatment: Supportive. Vitamin A as needed; may consider ribavirin in some
- Complications (30%): Diarrhoea, secondary infections (due to measles-induced lymphopenia), neurological syndromes:
- Encephalitis
- ADEM (acute disseminated encephalomyelitis)
- SSPE (subacute sclerosing panencephalitis — years later)
- Giant cell pneumonia (immunocompromised)
Live attenuated tetravalent viral vaccine, 2 doses as primary series First dose at 12 months as MMR (varicella given to other arm to reduce febrile seizure risk) Second dose moved up to 18 months since 2018 as MMRV
Protection rates [6]:
- 1st dose: 93% effective against measles
- 2nd dose: 97% effective against measles
- 10–30% become seronegative by age 30 due to lack of lifetime exposure to circulating measles
- 3rd dose in seronegative HCW → seroconversion (Italian study)
- 3rd dose in seropositive adults → does NOT increase antibody (US study)
- Breakthrough infections are mild ("modified") as T cell responses remain
Why breakthroughs are mild: Even when antibody levels wane, memory T cells persist. These can't prevent infection (that requires mucosal antibodies), but they rapidly control viral replication, preventing severe disease.
- WHO SAGE recommended measles eradication in 2010 but no formal goal committed
- HK has achieved elimination status since 2016 (verified by WHO)
- Endgame options:
- Allow measles circulation, eliminate complications with 2-dose approach, OR
- Authorize boosters and ensure global coverage
R₀ = 16 and Herd Immunity Threshold
The herd immunity threshold = 1 − 1/R₀. For measles with R₀ = 16, this is 1 − 1/16 = 93.75%. This means > 93% of the population must be immune to prevent sustained transmission. This is why measles is among the hardest diseases to eliminate — even small pockets of unvaccinated individuals lead to outbreaks.
This is the framework for understanding how vaccines are approved. Examinable for HPV, flu, and pneumococcal vaccines. [1]
| Level | Phase | Endpoint | Sample Size | Design | Key Notes |
|---|---|---|---|---|---|
| Immunogenicity | 2, 3, or 4 | Established/likely correlate of protection | 50–1000 | Placebo-controlled RCT or single-arm | Can use immunobridging: compare to age group with established VE |
| Efficacy | 3 | Clinical disease/infection | 1000+ | Placebo-controlled RCT | Cannot detect rare outcomes; short follow-up; healthy pop only |
| Effectiveness | 4 | Common AND rare outcomes | Very large (whole pop) or 1000+ | Observational (cohort, test-negative, case-control) | Can detect waning, VOCs; subject to bias |
Immunobridging [1]
Given an immune marker is presumed or proven to correlate with protection, vaccine age extension may be authorized based on comparison of immunogenicity to an age group with established efficacy
Why this matters: For paediatric age groups (e.g., HPV vaccine in 11–12 year olds), you cannot ethically or practically run efficacy trials (they're too young to get HPV infection or cancer). Instead, you show their antibody response is non-inferior to the adult group where efficacy was proven. This is immunobridging.
7. HPV Vaccine
- Bivalent (Cervarix) — types 16, 18
- Quadrivalent (Gardasil) — types 6, 11, 16, 18
- Nonavalent (Gardasil 9) — types 6, 11, 16, 18, 31, 33, 45, 52, 58
They vary in cost and ability to prevent just cancers, or also anogenital warts (types 6 and 11 cause warts; types 16 and 18 cause ~70% of cervical cancers)
- 3 doses (0, 1, 6 months) when authorized for adults
- 2 doses (0, 12 months) when authorized for 11–12 years
- 1 dose? — under active investigation
Early vaccination before puberty minimizes the chance of pre-existing HPV infection, improving efficacy Younger immune systems are stronger, may require fewer doses Longer prime-boost interval may be more immunogenic
Key limitation: Efficacy cannot be directly demonstrated in adolescents because:
- They are too young to catch HPV infection
- Cancers take years/decades to develop
→ Hence the use of immunobridging to authorize paediatric use.
The pivotal study showed that 2 doses (0, 6 months) in 9–14 year old girls and boys achieved antibody GMTs that were non-inferior (and in fact significantly higher) than 3 doses (0, 2, 6 months) in 16–26 year old women for all 9 HPV types.
- HPV vaccine cost is too high for less-resourced countries
- Evidence from Costa Rica Vaccine Trial: immunogenicity of 1 dose higher than natural infection and durable for at least 7 years
- Australian National Vaccination Register shows comparable effectiveness vs CIN2+ with 1 dose as with 3 doses
- WHO SAGE recommended 1-dose HPV vaccine for young women and girls aged < 21 years in April 2022 [1]
Exam Focus: HPV Dose Reduction Logic
The exam loves asking WHY we can reduce doses. Answer framework:
- Younger immune systems → stronger response to fewer doses
- Immunobridging shows non-inferiority
- Cost savings allow wider global coverage
- Real-world effectiveness data supports equal protection
- Primary 5: 9-valent HPV vaccine 1st dose
- Primary 6: 9-valent HPV vaccine 2nd dose
8. Influenza Vaccine
- Inactivated influenza vaccines (≥ 6 months)
- Live attenuated, nasal-spray influenza vaccines (≥ 2 years) — LAIV
- Recombinant haemagglutinin influenza vaccines (≥ 18 years)
- Flu vaccines contain 2 strains of flu A and 1/2 strains of flu B
- Different compositions recommended for Northern and Southern hemispheres
- Based on mice immunogenicity data only
- WHO convenes consultations in February each year for Northern hemisphere (almost a year in advance)
- COVID-19 pandemic disrupted flu transmission → possible extinction of some lineages → harder to predict dominant strains
| Feature | LAIV (Nasal Spray) | IIV (Intramuscular) |
|---|---|---|
| Route | Intranasal — delivered to site of infection | IM |
| Pre-2013–14 data | Greater immunity against mismatched strains, immediate outbreak protection, better T cell responses | Standard |
| Post-2013–14 US data | IIV was more effective than LAIV4, particularly against influenza A(H1N1)pdm09 | Preferred |
Children aged 6–24 months Asthma or wheezing Immunodeficiency (or close contact with immunodeficient person) Aspirin use Chronic neurological/lung/heart/blood/endocrine/kidney/liver/metabolic disorders Recent receipt of flu antivirals
Why asthma/wheezing is a contraindication for LAIV: The live attenuated virus replicates in the respiratory tract. In children with reactive airways, this can trigger bronchospasm and exacerbation.
9. Pneumococcal Vaccine
- Pneumococcus is a leading cause of death in children aged 1–59 months globally
- Clinical spectrum: pneumonia, meningitis, empyema, necrotizing pneumonia, septic shock
-
92 different serotypes of pneumococci
| Vaccine | Serotypes Covered |
|---|---|
| PCV7 | 4, 6B, 9V, 14, 18C, 19F, 23F |
| PCV10 | + 1, 5, 7F |
| PCV13 | + 3, 6A, 19A |
| PCV15 | = PCV13 + 22F, 33F |
| PCV20 | = PCV15 + 8, 10A, 11A, 12F, 15B |
- Meningitis: 10A, 15B, 19F, 23F
- Empyema: 1, 3, 5, 7F, 8, 19A
- Necrotizing pneumonia: serotype 3
- Septic shock: serotypes 3, 19A
- Increased case-fatality: serotypes 3, 6B, 9N, 11A, 16F, 19F, 19A
Serotype 3 vaccine failure in Hong Kong [1] — Despite being covered by PCV13, serotype 3 continues to cause significant IPD. This is a key exam point.
Serotype 3 antibody response better with PCV15 and worse with PCV20 compared to PCV13 [1]
Conjugate vaccines work by linking polysaccharide capsule antigens to a carrier protein → this converts the immune response from T-cell independent (polysaccharide alone = poor response in < 2y, no memory) to T-cell dependent (protein-conjugate = robust response even in infants, immunological memory, mucosal IgA → reduced nasopharyngeal colonization).
Two modes of protection:
- Direct protection against invasive disease (via opsonizing antibodies)
- Direct protection against colonization (reduces carriage → herd effect)
Heavy disease burden PCV10 or 13 similar impact 2+1 or 3+0 schedule similar impact Serotype replacement & vaccine failure (serotype 3) Surveillance of IPD (with serotype testing) is a must before & after PCV vaccination Major barrier for using PCV is vaccine price
- IPD decreased dramatically during COVID-19 pandemic (due to mask-wearing, social distancing)
- IPD increasing since end of pandemic measures
- Serotype 3 still dominant
Asplenic patients are at high risk for overwhelming post-splenectomy infection (OPSI) from encapsulated organisms. Required vaccines:
- PCV13 + PPSV23 (repeat Q5y)
- Hib vaccine
- Meningococcal ACWY vaccine
- Annual influenza vaccine
- Penicillin V prophylaxis (especially in children)
10. COVID-19 Vaccines
| Feature | mRNA (BNT162b2, mRNA-1273) | Inactivated (CoronaVac) |
|---|---|---|
| Antigen | Spike protein only | Whole virus (S, N, M, E) |
| Antibody response | Higher anti-S; stronger neutralizing antibodies | Lower anti-S; but generates anti-N, anti-M |
| T cell response | S-specific CD4 > CD8 | S + N + M + E peptide responses; broader T cell repertoire |
| Myocarditis risk | Yes (especially young males) | Lower |
| VITT risk | No (that's adenoviral vector) | No |
- Children > adults (for same dose)
- mRNA > inactivated (for anti-S antibodies)
- Vaccination > infection (for antibody response)
- 3 doses > 2 doses
- T cell longevity > antibody (T cells persist longer than circulating antibodies)
- 30+ mutations in BA.1 Spike protein → neutralization weak after 2 doses, improved by dose 3
- T cell responses remain preserved despite Omicron mutations (because T cell epitopes are more conserved)
- N-specific and M-specific T cells (from inactivated vaccines) partly reduced against Omicron
- Similar to flu vaccine strategy: bivalent wild-type/BA.5 mRNA vaccines authorized for Fall/Winter 2022
- Concerns: no BA.5 variant-specific human data; unknown clinical superiority over original booster; SARS-CoV-2 is not influenza (evolves differently)
- XBB became dominant by end of 2022, not BA.5
Do we vaccinate to prevent infections OR severe diseases of SARS-CoV-2? Annual vaccination only for immunocompromised? [1]
11. Vaccination in Immunocompromised Patients
This is the highest-yield clinical application of this lecture. It bridges immunology and paediatrics.
| Immune Defect | Live Vaccine Rules |
|---|---|
| T cell defects (e.g., SCID) | Avoid ALL live vaccines |
| HIV | Avoid all if CD4 < 200/µL |
| B cell defects (e.g., XLA) | Avoid OPV, live bacterial vaccines |
| Phagocytic defects (e.g., CGD) | Avoid live bacterial vaccines (e.g., BCG) |
| Post-HSCT | Avoid ALL live vaccines for 24 months |
| Post-rituximab | Wait 6–12 months |
| Post-organ transplant | Wait 3–12 months |
| Immunosuppressives | Individual assessment |
Except for imminent outbreak (e.g., COVID-19, flu): vaccinate now [1]
- Repeated doses or higher doses may be needed (e.g., 3+1 for COVID)
- Post-HSCT patients need complete revaccination as their immune system is rebuilt
- Post-organ transplant: timing depends on 3–12 months post-transplant
IG replacement interferes with response to live vaccines (the circulating antibodies neutralize the live attenuated virus before it can replicate and stimulate immunity) Live vaccines generally not required anyway as patients are passively immunized by IVIG/SCIG Give inactivated flu and COVID-19 vaccines as IG replacement cannot neutralize new/circulating viruses
Avoid live vaccines and HPV vaccines during pregnancy
For patients with coagulopathies/thrombocytopenia:
- Schedule IM injections shortly after clotting factor replacement
- Use 23g or finer needle
- Apply firmer pressure for 10 minutes
- Consider subcutaneous administration for IM vaccines
The lecture presents a classic public health model (Vaccines 2004):
| Stage | What Happens |
|---|---|
| 1. Pre-vaccine era | Disease is prominent; no vaccine |
| 2. Increasing coverage | Disease declines; vaccine uptake rises |
| 3. Loss of confidence | Disease is rare → people worry more about vaccine side effects → coverage drops |
| 4. Resumption of confidence | Outbreak occurs → public sees disease is real → coverage rises again |
| 5. Eradication | If sustained coverage → disease eradicated → vaccination stops |
This cycle explains vaccine hesitancy — when disease is rare (thanks to vaccination), people forget the disease and focus on vaccine side effects. This is a universal phenomenon seen with MMR-autism scares, HPV vaccine controversies, and COVID-19 vaccine hesitancy.
While not the focus of this lecture, HBV vaccination is part of the HKCIP and is heavily tested.
| Aspect | Detail |
|---|---|
| Vaccine type | Recombinant subunit (HBsAg only, yeast-derived) — first subunit vaccine ever [8] |
| HKCIP schedule | 0, 1, 6 months [1] |
| Efficacy | ~95% develop anti-HBs [8] |
| Booster needed? | No — anamnestic (memory) response even if anti-HBs falls to undetectable [8] |
| Neonatal prevention | HBIG + HBV vaccine within 24h of birth for babies of HBsAg+ mothers → 95% protection [8][9] |
14. Exam Intelligence
- HKCIP schedule — especially what's IN vs NOT IN (Hib, rotavirus NOT included)
- Live vs inactivated — which vaccines are live? Contraindications in immunocompromised
- Immunobridging — why can we give HPV vaccine to 11–12 year olds without efficacy data?
- Serotype 3 vaccine failure for pneumococcus
- OPV vs IPV and VDPV
- Eradication vs elimination vs control definitions
- Vaccination in immunocompromised — what to avoid, what to give, timing
- Waning measles immunity and implications for HCW
| Trap | Correct Answer |
|---|---|
| "Hib vaccine is in HKCIP" | No — very low incidence in HK |
| "All live vaccines are contraindicated in HIV" | Only if CD4 < 200/µL |
| "IVIG interferes with all vaccines" | Only with live vaccines |
| "Booster needed for HBV if anti-HBs declines" | No — anamnestic response protects |
| "PCV13 fully covers serotype 3" | Vaccine failure for serotype 3 in HK data |
| "Measles is eradicated" | Only eliminated in HK (no local transmission), not eradicated globally |
| "LAIV is better than IIV in children" | Post-2013 US data shows IIV > LAIV4 especially for H1N1pdm09 |
- MCQ: "Which of the following vaccines is contraindicated in a child with SCID?" → All live vaccines
- SAQ: "Describe the HK childhood immunization programme" → Table format
- SAQ: "Explain why HPV vaccine can be authorized for adolescents without efficacy data" → Immunobridging
- Minicase: Child with recurrent infections found to have XLA → asked about vaccine modifications
- MCQ: "Which pneumococcal serotype is associated with vaccine failure despite PCV13?" → Serotype 3
-
List the vaccines given at 12 months of age in the HKCIP.
- MMR (1st dose), PCV13 (booster), Varicella (1st dose)
-
A 2-year-old with SCID is due for vaccinations. Which vaccines should be avoided and why?
- Avoid all live vaccines (BCG, MMR, OPV, varicella, LAIV). SCID = absent T cell immunity → live attenuated pathogens replicate unchecked → vaccine-strain disease (e.g., BCGitis, disseminated varicella).
-
Explain why the HPV vaccine was authorized for 11–12 year olds without efficacy data against cervical cancer.
- Immunobridging: 2-dose regimen in 9–14 year olds showed antibody GMTs non-inferior (higher) than 3-dose regimen in 16–26 year old women where efficacy was established. Direct efficacy cannot be shown as adolescents are too young for HPV infection/cancer.
-
Name two vaccines NOT included in the HKCIP and explain why.
- Hib vaccine (very low incidence in HK); Rotavirus vaccine (no deaths in HK). Public health cost-effectiveness analysis does not justify universal programme inclusion.
-
What is the difference between eradication and elimination? Give an example of each.
- Eradication: permanent zero worldwide (smallpox). Elimination: no local transmission in a defined area (measles in HK since 2016). Eradication requires human as only reservoir, effective intervention, surveillance, and social feasibility.
-
A healthcare worker is found to be measles-seronegative. What is the recommended action?
- Give a 3rd dose of measles vaccine. Evidence shows seronegative HCW seroconvert after a 3rd dose (Italian study). Breakthrough infections in seropositive individuals are mild due to persistent T cell memory.
-
Describe how PCV works and explain serotype replacement.
- PCV conjugates polysaccharide capsule to carrier protein → T-cell dependent response → memory + mucosal IgA → reduces invasive disease AND nasopharyngeal colonization. Serotype replacement: as vaccine serotypes are eliminated from carriage, ecological niche is filled by non-vaccine serotypes, maintaining some IPD burden.
High Yield Summary
HKCIP Schedule: BCG + HepB at birth → DTaP-IPV + PCV13 at 2, 4, 6mo → MMR + Varicella + PCV13 booster at 12mo → MMRV + DTaP-IPV booster at 18mo → HPV at Primary 5–6. NOT included: Hib (low incidence) and Rotavirus (no deaths in HK).
Vaccine Types: Live attenuated (BCG, MMR, OPV, varicella, LAIV) → contraindicated in immunocompromised. Inactivated/subunit → safe but need more doses.
Authorization Pathway: Immunogenicity → Efficacy → Effectiveness. Immunobridging allows age extension without efficacy trials.
Polio: OPV → VDPV risk; IPV → safe but more doses. Eradication challenges: cVDPV2, hesitancy, war zones.
Measles: R₀ = 16; MMR at 12mo + MMRV at 18mo; waning immunity → 10–30% seronegative by 30; breakthroughs mild due to T cell memory.
HPV: 9-valent (Gardasil 9) in HKCIP; 2 doses for adolescents authorized via immunobridging; 1-dose recommended by WHO SAGE for < 21y.
Pneumococcus: PCV13 in HKCIP; serotype 3 vaccine failure; serotype replacement is ongoing; surveillance essential.
Immunocompromised: Avoid live vaccines (specifics vary by defect); may need extra doses; time with IVIG; revaccinate post-HSCT.
Active Recall - Immunization
[1] CFB (PAE03) Immunization.pdf — Prof. Yu Lung Lau, Department of Paediatrics and Adolescent Medicine, HKU [2] Jerry's immunodeficiencies.pdf — XLA section [3] Jerry's immunodeficiencies.pdf — Primary immunodeficiencies table (BTK, IVIG replacement) [4] Adrian Lui Pediatrics Notes.pdf — HKCIP schedule (p.18) [5] Block A - Jaundice after raw oysters_ acute hepatitis.pdf — Hepatitis A vaccine not in HKCIP (p.15) [6] MBBS Final MB (Pediatrics) (Felix PY Lai).pdf — MMR vaccine protection rates (p.51) [7] Maksim Surgery Notes.pdf — Post-splenectomy vaccination (p.153) [8] Block A - I am a hepatitis B carrier.pdf — HBV vaccine details (p.74) [9] MBBS Final MB (Medicine) (Felix PY Lai).pdf — HBV prevention (p.754)
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