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
• VAPP or prolonged immunodeficiency-associated VDPV (iVDPV) infection/shedding after OPV in severe antibody deficiency

They do not replicate and therefore provide less antigen amplification than most live vaccines. Priming doses, adjuvants and later boosters may be needed to establish and maintain protective responses. This is a tendency, not a fixed dose count.

• 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
• Immediate action: Any suspected acute flaccid paralysis is a public-health emergency. Notify and investigate without waiting for confirmation.
• Diagnosis/surveillance: Collect two stool specimens at least 24 hours apart, ideally within 14 days of paralysis onset, and send through the designated reference laboratory pathway. Respiratory/CSF samples may supplement investigation, but CSF is often negative; “culture CSF” is not the core surveillance instruction. ^[18]
• Treatment: Supportive; monitor respiratory, bulbar and autonomic function. Some survivors of respiratory paralysis require prolonged or lifelong ventilation.
• Sequelae: Post-polio syndrome in up to 50% of survivors — new/progressive muscle weakness years later

The clinical slide correctly highlights the importance of cVDPV, particularly type 2, but case proportions change over time and should not be memorized as a permanent statistic. OPV does not reliably “immunize contacts”: secondary spread can contribute to population immunity, but prolonged spread in under-immunized communities is exactly how cVDPV emerges. ^[16]

The Global Polio Eradication Initiative was launched in 1988 (inspired by smallpox eradication).

Current checkpoint (2026): wild poliovirus types 2 and 3 are eradicated; only WPV1 remains. Afghanistan and Pakistan remain the two endemic countries. All countries remain vulnerable to importation, and cVDPV outbreaks remain a separate endgame problem. ^[19][20]

Challenges: reaching every child in conflict/inaccessible areas; sustaining high routine IPV/OPV coverage; sensitive AFP and environmental surveillance; rapidly stopping cVDPV outbreaks; and safe containment and ultimately coordinated withdrawal of live poliovirus materials/vaccines after eradication.

Eradication requires human as the only reservoir, effective intervention to halt transmission, effective surveillance, and social feasibility ^[1]

How to read the lecture's “polio endgame” fork: the slide contrasts accepting infection while preventing paralysis with interrupting transmission. It is a provocative teaching contrast, not current eradication policy. The eradication goal requires interruption of all poliovirus transmission through high coverage, OPV/IPV strategies matched to epidemiology, surveillance, outbreak response, containment and eventual coordinated OPV withdrawal. ^[20]

• Virus/transmission: A highly contagious airborne virus; R₀ is often estimated around 12–18, but is setting-dependent.
• Presentation: Incubation → prodrome (fever, cough, coryza, conjunctivitis, ± Koplik spots) → cephalocaudal maculopapular exanthem → recovery. Modified measles may be attenuated or atypical.
• First actions: Mask, immediately isolate using airborne precautions, and notify public health. Infectiousness extends from about 4 days before to 4 days after rash onset; immunocompromised patients may shed longer. ^[21]
• Diagnosis: Obtain throat or nasopharyngeal swab for RT-PCR plus serum measles IgM; urine can improve molecular yield. Routine “throat/urine viral culture” is not the modern bedside formulation. ^[21]
• Treatment: Supportive and treat complications. WHO recommends two age-appropriate doses of vitamin A for suspected measles in children under 5, at diagnosis and the next day. There is no approved routine antiviral treatment; ribavirin evidence is limited to case reports/small series. ^[22]
• 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)

MMR is a live attenuated trivalent vaccine; MMRV is tetravalent. Two measles-containing doses are used in the HKCIP. 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

The slide contrasts an Italian third-dose study in seronegative healthcare workers with a US study showing little gain in already seropositive adults. The safe takeaway is not “everyone needs a third measles dose.” A negative commercial IgG result does not automatically negate documented age-appropriate vaccination; current CDC guidance, for example, accepts documentation of two properly spaced MMR doses as evidence of measles immunity even when a later IgG is negative/equivocal. Routine third-dose MMR is not a general measles recommendation. ^[23]

Modified breakthrough disease can be milder because immune memory may constrain replication, but it remains contagious and still requires the same immediate isolation/public-health response.

• WHO SAGE recommended measles eradication in 2010 but no formal goal committed
• Hong Kong was verified as having interrupted endemic transmission in 2016 and maintained elimination through 2025, but imported and epidemiologically linked local cases continue to occur. Elimination does not mean “no cases.” ^[24][25]
• The slide's “allow circulation versus authorize boosters” fork is not present policy. Sustaining elimination requires approximately 95% two-dose coverage, rapid case isolation and laboratory confirmation, contact/outbreak response and closing immunity gaps. A routine third dose is not a substitute for high two-dose coverage.

• 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

The bivalent product targets two oncogenic types; quadrivalent/nonavalent products also include types 6 and 11, which cause most anogenital warts. The clinic advertisements and HK$ prices shown on slide 27 are 2023 historical examples, not current cost evidence.

• The pivotal nonavalent immunobridging trial compared 2 doses at 0 and 6 months or 0 and 12 months in girls and boys aged 9–14 with 3 doses at 0, 2 and 6 months in young women. Antibody GMT non-inferiority supports bridging; it does not directly measure cancer prevention.
• Current Hong Kong programme: eligible female students receive 9vHPV in Primary 5 and Primary 6. This remains a two-dose public programme. ^[10]
• WHO position: 2 doses at least 6 months apart can be used at licensed ages; an alternative single-dose schedule may be used for ages 9–20 where supported for the product. Immunocompromised people, including those with HIV, should receive at least 2 and, where possible, 3 doses. ^[26]

Early vaccination before puberty minimizes the chance of pre-existing HPV infection, improving efficacy Younger adolescents generate higher antibody titres than young adults, allowing fewer doses through immunobridging Longer prime-boost interval may be more immunogenic

Why initial adolescent authorization uses a bridge rather than a cancer endpoint:

1. The programme targets vaccination before likely exposure, so infection endpoints are initially uncommon
2. Precancer/cancer endpoints take years to 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 generally numerically 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's 2022 position permits an alternative single-dose schedule for ages 9–20 for products with supporting evidence; this does not replace Hong Kong's current two-dose HKCIP schedule. ^[26]

The early one-dose data included post-hoc trial subgroups and observational registry comparisons, so confounding was a real limitation; subsequent evidence synthesis supported WHO's alternative schedule. “Comparable” should not be read as proof that every product/population has identical titres or durability.

• Primary 5: 9-valent HPV vaccine 1st dose for eligible female students
• Primary 6: 9-valent HPV vaccine 2nd dose for eligible female students

HPV vaccines are prophylactic, not therapeutic: they do not clear an established infection or treat an existing lesion, but previous exposure to one type does not remove benefit against other vaccine types or future susceptible-type infection. Vaccinating boys and men can provide direct protection and reduce transmission, although Hong Kong's universal school offer described above remains female-only as of this update.

• Inactivated influenza vaccines (≥ 6 months)
• Live attenuated, nasal-spray influenza vaccines (≥ 2 years) — LAIV
• Recombinant haemagglutinin influenza vaccines (≥ 18 years)

• Influenza vaccine composition is updated for the expected season and differs between Northern and Southern Hemisphere recommendations.
• Selection is not based on mice immunogenicity alone. WHO integrates global GISRS virological and epidemiological surveillance, antigenic and genetic characterization, human serology, animal antisera, antiviral susceptibility and candidate-vaccine-virus availability.
• The February Northern Hemisphere decision creates manufacturing lead time and therefore unavoidable mismatch risk.
• B/Yamagata has not been confirmed in natural circulation since March 2020. Hong Kong's 2026–27 recommendation is therefore trivalent only: A(H1N1)pdm09, A(H3N2) and B/Victoria antigens. ^[12]

There is no timeless “IIV always beats LAIV” rule. For 2026–27, Hong Kong recommends IIV, RIV and LAIV within their age/contraindication ranges and describes LAIV protection as broadly comparable to IIV. Choose by age, product licence, contraindications, availability and preference. ^[12]

LAIV is licensed from age 2 to 49 years, so age under 2 is outside its indication. It is generally contraindicated with:

• severe allergy to a vaccine component or prior influenza-vaccine dose;
• salicylate therapy in children/adolescents;
• age 2–4 with asthma or wheeze in the previous 12 months;
• immunocompromise;
• being a close contact/caregiver of a severely immunosuppressed person requiring a protected environment;
• pregnancy; or
• influenza antiviral use within the previous 48 hours.

The lecture's broad boxes (“any asthma”, “any chronic disease”, “any immunodeficient contact”) overstate the contraindications. Older children with asthma require assessment, while many chronic-risk patients should receive an appropriate non-live influenza vaccine rather than no vaccine. Antiviral deferral can also be product/drug-specific, so check the current insert/guideline.

Routine seasonal dosing (2026–27): one dose for people aged ≥9 years and for children under 9 who have previously received seasonal influenza vaccine; two doses ≥4 weeks apart for a child under 9 receiving it for the first time. Annual vaccination is recommended for everyone aged ≥6 months without a contraindication. ^[12]

• 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

Complicated community-acquired pneumonia (slides 42–43): the serial films progress from bilateral/lower-zone air-space disease (day 7) to a left pleural effusion (day 10), worsening bilateral pleural/parenchymal disease (days 12–16), then a septated/loculated right pleural collection on ultrasound (day 20), followed by radiographic improvement (days 26 and 55).

• Chest radiography shows the evolving distribution and size of opacity/effusion; ultrasound characterizes fluid and loculations, which matters for drainage planning.
• The day-20 honeycomb/septated appearance is the visual clue to a complicated parapneumonic effusion/empyema rather than a simple free-flowing effusion.
• Radiographic recovery lags clinical recovery: the near-normal day-55 film illustrates why a persistent opacity early in convalescence is not automatically treatment failure.

Fulminant pneumococcal meningitis (slide 44): a previously healthy 3-year-old had coryza/fever, transient partial response to oral cephalosporins, then sudden neck stiffness and collapse on day 6, dying within hours despite ICU antibiotics. The montage links diffuse cerebral oedema on CT with Gram-positive lancet-shaped diplococci, alpha-haemolytic growth and optochin/bile-based identification of Streptococcus pneumoniae.

The clinical lesson is time-critical recognition: sudden deterioration, meningism, altered consciousness or shock demands immediate sepsis/meningitis management. Obtain cultures and CSF when safe, but do not delay empiric antibiotics for imaging or lumbar puncture. Exact empiric therapy must follow current local resistance and meningitis guidance.

• 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

The slide labels persistent serotype 3 disease as “vaccine failure.” A safer formulation is lower/less consistent effectiveness against serotype 3 than against many other vaccine serotypes. Continued cases do not mean PCV provides no benefit. Likewise, the slide's comparison of serotype-3 antibody responses after PCV15/PCV20/PCV13 is an immunogenicity comparison, not proof that one product has superior clinical effectiveness against serotype-3 disease.

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 under age 2, no memory) to T-cell dependent (protein-conjugate = robust response even in infants, immunological memory and reduced vaccine-type nasopharyngeal carriage).

Two modes of protection:

1. Direct protection against invasive disease (via opsonizing antibodies)
2. Reduced vaccine-type nasopharyngeal carriage → less transmission → indirect/herd protection

• Global burden graph (2000–2015): estimated pneumococcal deaths in children aged 1–59 months fell as PCV coverage increased. It is compelling population-level context, but the simultaneous trends alone do not prove all of the mortality decline was caused by PCV; nutrition, access to care, antibiotics and other interventions also changed.
• Vaccine-probe meta-analysis: the estimated PCV effect was much smaller for nonspecific clinical pneumonia (~8%) than for chest-radiograph-positive pneumonia (~36%). Specific endpoints enrich for pneumococcal disease; endpoint definition changes the apparent vaccine effect.
• Hong Kong monthly IPD graph (2007–June 2018): cases showed winter/spring peaks. Interpretation across 2014–15 is complicated by the addition of PCR and transition to statutory notification; a taller bar can reflect better ascertainment as well as more disease.
• Age/serotype panels: serotype 19A incidence fell most clearly in children under 2; serotype 6A became uncommon in children; serotype 3 remained prominent and appeared to rise in some child age groups. These are historical ecological surveillance data, not randomized product comparisons.
• Proportion chart: PCV7-type proportion fell while the PCV13-additional category (substantially serotype 3) occupied a larger share. A proportion can rise even without a rise in absolute incidence, so always pair proportional with incidence data.
• Pandemic slide: IPD fell during COVID-19 measures and rose after relaxation. Reduced respiratory-virus circulation/contact and altered healthcare/testing likely contributed; the graph does not isolate one cause.

Heavy disease burden PCV10 and PCV13, and 2+1 versus 3+0 schedules, had broadly similar population impact in the WHO evidence reviewed in the lecture; choice still depends on local epidemiology and programme goals Serotype replacement and persistent serotype 3 disease limit programme impact Surveillance of IPD (with serotype testing) is a must before & after PCV vaccination Major barrier for using PCV is vaccine price

The 2023 lecture observation was that IPD fell during pandemic restrictions, then rose as measures ended, with serotype 3 still prominent. Treat this as a dated epidemiological snapshot rather than a permanent trend.

Asplenic/hyposplenic patients are at high risk of overwhelming infection with encapsulated organisms. They need a risk-based plan covering pneumococcus, meningococcus, Hib and annual influenza, plus education and any locally indicated antibiotic prophylaxis. Do not memorize the old “PCV13 + PPSV23, repeat every 5 years” line as universal: product sequence and revaccination depend on age, risk and prior doses; current Hong Kong programmes now use PCV15 in relevant groups. ^[13]

• mRNA remains in the cytoplasm and provides a transient template; it does not need to enter the nucleus or integrate into DNA.
• Neutralizing anti-spike antibody is most relevant to blocking infection, while memory B cells can rapidly expand after re-exposure.
• Antibody also supports Fc-mediated functions: NK-cell ADCC, complement deposition and opsonophagocytosis.
• T-follicular helper cells (CD40L and cytokine help) drive affinity maturation, plasma cells and memory B cells; CD4/Th1 responses coordinate immunity and CD8 cells kill infected cells.
• Whole-virus inactivated vaccine exposes N/M/E as well as spike, so antigen breadth is wider; this does not automatically mean stronger or more protective T-cell immunity than mRNA vaccination.

The slide's inequalities (“children > adults”, “mRNA > inactivated”, “vaccination > infection”, “3 doses > 2”) summarize selected 2023 studies. They are not universal laws: response depends on dose, age, product, interval, variant, infection history, immune compromise and time since exposure. The durable principle is that circulating neutralizing antibody wanes faster than immune memory, while booster benefit is greatest when risk of severe outcome is high.

The BA.1/BA.2/BA.5/XBB phylogeny on the 2023 slide is now historical. It showed that spike evolution markedly reduced neutralization after two ancestral doses, a third exposure improved titres, and many T-cell epitopes remained cross-reactive. Non-neutralizing Fc functions were variably retained. Antigenic drift usually erodes infection-blocking antibody more readily than broad T-cell recognition, helping explain why protection from infection is shorter-lived than protection from severe disease. This principle persists even though the named variants no longer describe the current antigen decision.

The slide's bivalent ancestral/BA.5 product and its initial reliance on limited human BA.5 data were a Fall 2022 regulatory case study. It illustrates the trade-off between waiting for perfect variant-specific clinical data and updating quickly enough for a moving target; post-authorization effectiveness and safety surveillance remain essential.

Current checkpoint (2026–27): Hong Kong's scientific committees list LP.8.1, XFG or NB.1.8.1 as potential antigen choices. The programme's primary objective is preventing severe disease, hospitalization and death, not guaranteeing sterilizing immunity. Current free boosters are targeted to higher-risk groups: residential-care-home residents; community-dwelling adults ≥65; adults 50–64 with comorbidities; immunocompromised people ≥6 months; pregnant women; and healthcare workers, generally ≥180 days after the last dose or infection. This is a current programme rule, not a permanent annual schedule. ^[14][27]

An outbreak does not make a contraindicated live vaccine safe. During imminent exposure, urgently use specialist/public-health pathways: a suitable non-live vaccine, passive prophylaxis, antimicrobial prophylaxis, cocooning and/or exposure avoidance as appropriate.

• Non-live vaccines are safe in altered immunocompetence but can be less effective; some schedules use extra doses or revaccination after immune recovery. Do not carry the lecture's “3+1 COVID” example forward without checking the current product and programme.

• HSCT recipients generally require systematic revaccination regardless of pre-transplant records because immune memory may be lost.
• Inactivated vaccine series often begin months after HSCT; live vaccines are a much later, conditional decision. Solid-organ transplant and CAR-T/B-cell-depleting regimens have distinct schedules. Use the transplant/immunology protocol rather than a single “3–12 month” rule.

• Antibody-containing products most importantly interfere with measles- and varicella-containing vaccines; the interval after IG/blood product is dose/product-specific (roughly 3–11 months in CDC tables). If MMR/varicella is given first, wait ≥2 weeks before IG when clinically possible.
• Yellow fever, rotavirus, oral Ty21a and LAIV are exceptions to that antibody-spacing rule; non-live vaccines generally require no spacing from IG.
• In severe antibody deficiency on replacement IG, both effectiveness and safety/need must be assessed individually. “IVIG passively immunizes against everything” is false; seasonal influenza and evolving SARS-CoV-2 are examples where replacement product may not supply reliable protection.

• Most live vaccines (e.g., MMR, varicella and LAIV) are contraindicated during pregnancy.
• Defer remaining HPV doses until after pregnancy; no intervention is required after inadvertent HPV vaccination, and pregnancy testing is not required before HPV vaccination. ^[28]
• Pregnancy is an indication—not a contraindication—for seasonally appropriate IIV/RIV, and for other recommended non-live maternal vaccines under the current local programme.

For patients with coagulopathies/thrombocytopenia:

• Confirm that IM administration is reasonably safe with the clinician familiar with the bleeding risk; if factor replacement is used, schedule soon after it.
• Use a 23-gauge or finer needle.
• Apply firm pressure without rubbing for at least 2 minutes and counsel about haematoma risk. ^[29]
• Give a vaccine by its recommended route. Do not routinely convert an IM vaccine to subcutaneous administration: it may reduce immunogenicity and increase local reactions.

1. HKCIP schedule — especially what's IN vs NOT IN (Hib, rotavirus NOT included)
2. Live vs inactivated — which vaccines are live? Contraindications in immunocompromised
3. Immunobridging — why can we give HPV vaccine to 11–12 year olds without efficacy data?
4. Persistent serotype 3 disease / lower PCV effectiveness and cautious interpretation of surveillance graphs
5. OPV vs IPV and VDPV
6. Eradication vs elimination vs control definitions
7. Vaccination in immunocompromised — what to avoid, what to give, timing
8. Waning measles immunity and implications for HCW

• 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 remains problematic despite inclusion in PCV13/15?" → Serotype 3